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2 The visible biosphere.

1. Soil and bioclimate

1.1 The atmosphere on the Earth's surface today

The visible biosphere, as we see it today, is the result of a very long process, as we have seen in the previous chapters. But this result shows itself and is organized according to prevailing atmospheric conditions. In effect, history allows us to understand the past, but not to explain the present. The present results from combining the factors inherited from the past in accordance with the possibilities allowed by present-day conditions.

We cannot even begin to understand the present-day biosphere without considering the processes that have shaped the oxygenated layer (21% oxygen, 78% nitrogen, as basic components) suitable for the maintenance of life or, without taking account of the even more fascinating string of events, that have caused life to be organised in the form of the species existing today. These issues have been covered above.

However, our perception of the present, our only immediate and obvious reality, is not limited to the schematic and frequently inductive or deductive ideas to which we can reduce the past. The present-day biosphere is all around us, diverse, effervescent and alive. we ourselves form part of it and we intervene--in no small way--in the processes that take place in it. A summary description, for all that it leaves out details, still requires a lengthy consideration of factors and elements. This is in fact the object of this work.

The arrangement and functioning of the different elements of the biosphere are conditioned more than anything by the climate, and also by the soil. For some time the human species has also played an important organising role (or disorganising role, according to one's point of view) and has left its mark in all the biomes it has occupied. All these factors have combined to shape the biosphere we know.

1.2 Bioclimatic factors

The distribution of climates on the planet is related to the rotation and orbit of the Earth. Rotation determines the difference between day and night while the orbit gives rise to important differences in the amount of solar energy received per unit area, not because the distance between the Sun and the Earth changes significantly during the year but because of the inclination of the axis of the planet's rotation in relation to the plane which contains its orbit around the Sun. The main factors which determine the climate are very much influenced by latitude, even though other parameters such as the degree of continentality or altitude tend to vary them. Each zone, then, on the planet represents a combination of climatic variables which basically depend on its location.

Climatic variables

The climate is an essential conditioning factor of the biosphere's landscapes, especially in the case of terrestrial ecosystems. The climate acts as a whole, but it can be broken down into factors to allow us to studt the effect of each one on the others. The most important climatic factors are temperature, humidity and atmospheric pressure, even though the last of these only has an influence on landscapes as far as it affects the distribution of the first two.

Radiation and temperature

Solar radiation passes through the air without heating it up and falls on the Earth's surface. This heats up and as a result emits long-wave radiation at the same time as it gives off heat through conduction to the layers of air which come into contact with it. Nevertheless, this can produce a flow of important latent heat associated with the changes in the state of the water (melting, evaporation).

As a consequence of the inclination of the Earth's axis of rotation, solar radiation strikes each zone on the planet at a different angle, depending on the distance from the equator (latitude) and the time of year. This means that the lower latitudes receive the radiation almost perpendicularly while the higher ones receive solar radiation at an oblique angle and so they heat up much less than the former. What is more, the low latitudes receive the same radiation throughout the year, contrasting with the high latitudes where the angle of incidence of solar radiation changes in a very markedly, giving rise to much colder periods than in the other latitudes. This phenomenon is called seasonality.

Despite these difficulties, the extreme temperatures recorded at different points on the planet vary within relatively narrow bands because of the efficient mechanism for the transport of heat through masses of air and water. Thus, apart from phenomena related to geothermometry, temperatures go from extremes of 176[degrees]F to 126[degrees]F (80[degrees]C to -88[degrees]C). The temperature on most of the planet, though, varies little (104[degrees]F and 14[degrees]F [40[degrees]C and -10[degrees]C]), an ideal environment for living beings.

Humidity and precipitation

The availability of water, relayed to atmospheric humidity and precipitation, is another important climatic factor. Also important are, the changes in the state of the water, associated with it's circulation cycles which imply quantitatively important transfers of heat. We have to bear in mind that the calorific capacity of the air is around 0.20 cal/g while the heat involved in water melting and evaporating is 79.7 and 540 cal/g respectively. This means that the hydrological cycle leads to the transport of large quantities of heat without there necessarily being important changes in temperature.

Both atmospheric humidity and precipitation are easily measurable. The first is usually expressed as relative humidity, which is the actual vapour pressure expressed as a percentage of the saturation vapour pressure which would be possible at the same air temperature. Precipitation is measured in litres per square metre (l/m2), equivalent to millimetres of rain. The average or total values have a limited value as climatic variables as their distribution in time is fundamental in defining the climate. Variation in humidity and precipitation over time is very irregular, contrasting with temperature which is closely linked to seasonal variations in the amount of sunshine associated with latitude. The spatial and temporal distribution of precipitation is closely related to atmospheric and oceanic circulation.

Atmospheric and oceanic circulation

Atmospheric and oceanic circulation implies the transport of great quantities of heat and water between very distant points on the planet. In fact as regards the balance of radiation received and given off by the Earth's surface at different latitudes, we can see a clear surplus of heat at low latitudes and a negative balance at the high ones. In other words, at high latitudes more heat is lost than is received through radiation. These differences are compensated by the active transport of heat associated with oceanic currents and air masses. Atmospheric circulation is determined by these differences in warming, which means that air rises in zones near the equator and is replaced by colder and denser air from high latitudes. However, a single convection cell is not formed, but rather a descent of partially cooled down air takes place at medium latitudes. This apparently simple model of circulation is in reality complicated by the appearance of a deflecting force (the Coriolis force) derived from the movement of the Earth's rotation and which tends to divert paths towards the right in the northern hemisphere and in the opposite direction in the southern hemisphere.

Furthermore, zones dominated by the rising or falling of air become respectively zones of low and high pressure. This complicates the circulation model even further. Low pressures appear in the equatorial zone, in a band at middle to high latitudes and above the large continental masses, when they heat up intensely because of the strong sunshine. High pressures, on the other hand, have a tendency to form above oceans in subtropical latitudes and also above continents which have cooled down during the cold seasons. The effect of these zones of high and low pressure on atmospheric and oceanic circulation is considerable.

Wind has a tendency to go from high to low pressures, which is the same as saying that the direction of its movement is perpendicular to the isobars (lines which join points which are at the same pressure). The Coriolis force, however, diverts this trajectory by 90[degrees], giving rise to a wind which moves parallel with the isobars. The friction of the wind over the planet's surface also helps to divert the wind, giving rise to models of cyclonic circulation (low pressure) and anticyclonic (high pressure), which in low-pressure areas become centers of convergence and rising of masses of air, and in high-pressure ones become centers of descent and divergence of cold air. This has important climatic effects as the rise and consequent cooling of masses of air is associated with the formation of clouds and precipitation, due to the condensation of humidity derived from the drop in temperature. On the other hand, in the case of the high ones, the descent of masses of air leads to warming by compression, giving rise to dry air, in other words, stable weather without precipitation.

Low pressures or storms formed at medium and high latitudes also lead to the mixture and interaction of polar masses of air with others of tropical origin. This contact, or boundaries, between different masses of air gives rise to so-called fronts, which are nothing more than contact surfaces between these masses of air in which condensation of water vapour becomes spectacularly active, which eventually leads to precipitation. The distribution of pressures affects, then, not only atmospheric circulation but also the distribution of precipitation on the planet.

Oceanic circulation, which also transports large quantities of heat from low latitudes to high ones, derives from the gradients of density and temperature of masses of water and the frictional drag caused by prevailing winds. Thus, warm currents are formed in an equator-pole direction and cold currents in the opposite direction. The prevailing atmospheric circulation, adjusted by the distribution of the centers of high and low pressures, also contributes to the frictional drag of masses of water. Finally, the Coriolis force diverts these currents in the directions just indicated, giving rise to more or less closed circuits which have a tendency to turn clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere.

The buffering effect of climatic gradients derived from oceanic currents is evidently much more important in continental zones near the coasts, while it is negligible in the interior of the large continental land masses. Atmospheric circulation, on the other hand, has effects which are felt in all the planet's zones.

Latitude and altitude

We could imagine that the distribution of climates follows a pattern that is closely associated with latitude, which would give rise to parallel homogeneous bands, from the equator to the poles. In reality, however, the distribution of climates follows a considerably more complicated scheme because of the effect that the variable proportions between seas and continents at different latitudes has over it.

We have already mentioned the effect of continental and oceanic masses on the distribution of low and high pressure areas, or rather, in relation to atmospheric and oceanic circulation. Furthermore, these masses also have direct effects on temperature and rainfall. In the interior of a large continental mass, temperatures becomes much lower in winter and higher in summer, increasing the thermal oscillation. Parallel with this, we find a decrease in precipitation over the continents at medium and high latitudes as a consequence of the blocking effect of continental anticyclones on depressions. Continentality, then, is associated with drier climates, which are colder in winter and warmer in summer than those at similar latitudes in coastal areas. This means that the isotherms, or lines which join points with equal temperatures, are not parallel with the equator but are deformed by the effect of the continents. The unequal distribution of the continental masses in both hemispheres gives rise to much more oceanic climates, in other words, mild in the southern hemisphere and with a much more marked degree of continentality in the northern hemisphere. In fact, one part of this distortion can be attributed to the oceanic currents which, as warm waters are carried towards higher latitudes, moderate the climate of the coastal regions. In contrast, cold currents coming from high latitudes provoke climatic cooling in the zones they affect. The North Atlantic current, which heats up the British and Scandinavian coasts, and the Humboldt and Benguela currents, which cool down the southwestern coasts of Africa and South America respectively, are good examples of these effects.

Furthermore, within continents we still find the effects or distortions caused by altitude. Temperature decreases by some 1.1[degrees]F (0.6[degrees]C) for each 328 ft (100 m) of altitude and it is for this reason that mountain climates are colder than lower areas at the same latitude. This cooling down of the air facilitates the condensation of vapor and the formation of clouds and fogs which make mountain climates more humid.

An increase in altitude has often been compared to a displacement towards higher latitudes, both in terms of climatic effects and effects on the landscape, but this tends not to be so in the majority of cases. This simple scheme might be valid when dealing with modest differences in altitude, but it loses validity in cases of altitudinal gradients of thousands of meters. To begin with, above the level of condensation, the mountain climate becomes simultaneously drier and colder. Furthermore, the altitudinal gradient is not associated with a strong variation in the length of the day or night, that is, the total quantity of radiation received and its seasonal variation. All in all this means that high mountains have climatic conditions which are quite different from those of flat lands at higher latitudes, although there are some similarities.

The distribution of climates over the Earth depends, then, on latitude, continentality and altitude, but is also affected by the proximity of cold or warm oceanic currents. The variety of landscapes in the biosphere and their complex distribution are a consequence of this climatic distribution.

Types of climate

Many models for the definition and classification of climates have been tried, taking different climatic variables as their basis. Thus we can find classifications based on temperature, precipitation, or a combination of both. The most accurate and useful classifications as regards the interpretation of landscapes, are those that take into account both these variables and their distribution or variation throughout the year. The classifications derived from this methodology, such as the well-known Koppen-Geiger system, are of an empirical-quantitative nature; in other words, they start from measured variables and apply simple quantitative criteria like the value of the mean annual temperature, of thermic oscillation or of the annual rainfall. These systems are useful for the definition of large climatic units, but not so good for detecting more subtle variations which are often of great importance in modifying the landscape.

This sort of classification is based essentially on the climatic requirements of certain vegetation types, and divides the planet into the following climatic regions: the equatorial zone (high rainfall, hot temperatures and daily thermal oscillations which are greater than seasonal ones); the tropical zone (a perceptible seasonal oscillation of temperatures and rainfall, increasing during the warm season); the arid subtropical zone (temperatures which show a clear seasonal oscillation and very weak rainfall); the transition zone (with winter rains, a hot and dry summer and moderately cold and humid winters); the temperate zone (moderately cold summer and cold winters, but abundant rainfall of cyclonic origin throughout the year. The length of winter and the greater or lesser continentality determine the definition of some sub-zones); and the arctic zone (short, cold summer without nights, a long, cold and dark winter and weak rainfall spread out throughout the year).

Much more flexible classification methods have been developed which arrive at a much more accurate definition of the bioclimate, in other words of the conditions of cold and heat and humidity and aridity which affect living beings. It is clear that the same precipitation can give rise to biological conditions of dryness or humidity depending on the temperatures. Furthermore, the combination of the distribution of precipitation and temperature is also a factor which defines the bioclimate as the fact that the hottest or coldest seasons coincide with the rainy seasons is not without effect.

The phenomenon of microclimates

The microclimate or precise climatic conditions which organisms can tolerate at a specific location can be very different from the macroclimate which affects a particular zone. This depends not only on the size of the organisms but also on their position within the landscape. For example, small organisms which live deep down in the soil have a much more humid environment and with thermal oscillations which are much less marked than those which live on the soil's surface. Analogically, sunny areas will have much warmer and drier conditions than shady ones, converting a cold climate in a warm microclimate or a humid climate into a dry microclimate.

There are an infinite number of examples and they refer both to the aforementioned situation in space and to the vital strategy and the biological cycle of the organisms under consideration, as an organism can avoid unfavourable periods if there is a very short and reduced life cycle in those periods when the climatic conditions are less extreme. We will be able to comment on this when we analyse certain cases of adaptations to extremely dry or cold climates.

1.3 Edaphic factors

Plant growth requires a medium in which the roots can develop. This is only possible if the medium contains spaces which allow the roots to penetrate and then room for them to grow. In general this does not happen in rock which is normally a compact, hard and massive material, and when rock outcrops on the surface of an area, the growth of vegetation is impeded. Root penetration allows the plants to take root, but the system of spaces which allows the roots to penetrate must also contain air, so that the roots and other living organisms can breathe, and water, which transports nutrients in solution. The natural medium for plant growth is soil, because it satisfies these conditions. This is a functional conception of soil: throughout this chapter other concepts will be presented which will give an understanding, not only of soil's importance to life and to the biosphere, but also of its formation and its importance as a natural resource, and the need for its conservation.

From rocks to soil

Igneous and metamorphic rocks, which are formed in the heart of the lithosphere, have been subjected to either high temperatures or high pressure, or both simultaneously. When the materials lying on top of the rock break down and the rock breaks through to the surface, the conditions will be completely different to those which were present at its formation. The pressure of the material which lay above it, or lithostatic pressure, will have ceased, whilst at the same time the temperature will be much lower and there will be great differences in temperature during the day and during the course of the year. Contact with the atmosphere--which one must remember is an oxidising medium--and with water, will, however, be the most important factors in accelerating the transformation of rock at the earth's surface into more stable structures, under the conditions which characterise the biosphere. The changes brought about by these external factors, which lead to the formation of new materials, are called weathering processes.

Although the final product of weathering can not yet be called soil (those derived from compact rocks like granite are often called saprolites, whilst those derived from less compact rocks, such as argillite [lutite], are often called regoliths which are often considered to include the soil), they nevertheless blend into and interact with soil forming (or edaphogenic) processes. Weathering does not end where edaphogenesis begins, but rather the two processes interact and take part in soil formation. The classical view of weathering in rocks and minerals established a division which, although pedagogically successful, was nevertheless a little inconsistent. Weathering was divided into three types: mechanical, chemical and even biological. The problem is one of scale in the first two cases, whilst in the last instance the role of certain agents is highlighted. Weathering processes, on the level of rock or large minerals causes materials to break down (physical weathering), whereas on the level of crystalline structure, this process takes place at a molecular or submolecular level (chemical weathering). In the first case, fragmentation could be considered a mechanical process where chemical or mineralogical changes in composition are not involved, whereas compositional change does occur when fragmentation takes place at a molecular or submolecular level.

Mechanical

The elimination of lithostatic pressure on a rock massif allows a degree of expansion. When it expands, the rock splits along a series of dilatation planes or expansion joints which are subparallel and perpendicular to the direction of pressure release. These planes occur every 3-10 ft (1-3 m) and can affect a mass of up to 66 ft (20 m) in depth. This type of process is typical in massive igneous rocks, in residual reliefs like "inselbergs," and explains the existence of large amounts of sandy soil above the granite, the formation of which is favoured by the passage of water along the fracture systems. The formation of typical granite landscapes, in which superimposed blocks can be seen, is related to this type of weathering by expansion. It should be pointed out, however, that although fragmentation favors the circulation of water throughout the rock mass, this circulation will also have repercussions at the molecular and submolecular level. Processes on one level or another act slowly and together, reinforcing each other's actions in the transformation of rocks and their mineral constituents.

Rocks that already lay on or near the surface are affected by heat transfer and temperature variations. The fact that rocks have a low heat conductivity and that the minerals which compose them have differing dilatation coefficients, suggests the existence of a process of chemical disintegration. The existence of such a process, called thermoclasty (insulation weathering), has been called into question because it has not been possible to reproduce it in the laboratory unless humidity is present: in other words, for such a process to be effective the existence of prior hydration or hydrolysis would seem to be necessary. Thermoclasty does not appear to explain spheroidal weathering in concentric layers (exfoliation or onionskin weathering), common in rocks such as basanite. These instead seem to be due to the action of water. As regards heat transfer, and specifically in relation to physical change in water held in the rock, we find fragmentation due to the formation of ice (gelivation or frost-wedging). The more often the freeze-thaw cycle is repeated the more effective is the process. This process is more active in subarctic areas than in arctic regions, where thaws only occur once a year.

Rock can also be subjected to internal pressures from animals or the activity of organisms which live within it (biomechanical processes), or by the growth of salt crystals, which can cause the rock to fragment (haloclasty). These processes can be important in arid or semi-arid areas, as the phreatic layers can be saline, and also because the rocks themselves can contain a certain amount of salts which, on dissolving, precipitate inside the rock or on its surface, showing themselves as a white bloom (efflorescence). Alveolate weathering processes which operate on clays in semi-arid regions, such as the so-called "tafoni" of Corsica and Sardinia, can also be related to this type of crystal growth, as can the differentiated thermal expansion of salts held in the rock. In other environments, such as coastlines, these mechanisms can explain the rapid breakdown of schists.

Weathering by dissolving action is important in areas with gypsum (CaSO4 x 2H2O). In general this process effects those soluble products resulting from weathering which can be transported vertically in solution (this is referred to as loss by leaching), or laterally along the surface. Both processes can be considered as part of the weathering system.

Chemical weathering

Fragmentation at the level of crystalline structure is due to exothermic chemical reactions, since weathering is a spontaneous process in nature. One of the principal processes is hydration, or combination with water molecules, which causes an increase in volume, thereby forming a new mineral. The change from anhydrite (CaSO4) to gypsum (CaSO4 x 2H2O) is a good example. Oxides of iron also play a part in processes of this type. The reaction of the mineral with water, or to put it in a more generic way, the reaction of the mineral with H+ ions from various sources, a process known as hydrolysis, is the most important weathering process, because it affects all of the silicates and these are the principal rock-forming minerals. H+ ions are able to enter into the crystalline structure and displace other ions such as, for example, the potassium ions (K+) in orthoclase feldspars (KAlSi3O8), which then go into solution. The difference in size between the ions makes the structure unstable and liable to collapse.

The weathering of carbonated rocks requires the presence of carbon dioxide (CO2) in the water to make its dissolution possible. Limestone, no matter how white it may appear nor how pure it may seem, will always contain silicates (clays) as impurities. These impurities are the materials left from which the soil will be formed once the carbonates have been lost by successive leaching of the bicarbonates into which the chalk has been transformed. The weathering of a rock with carbonic acid anhydride is called carbonation. The rendzinas and terra rossa originated in this way. The processes of oxidation-reduction (redox) also play a part in weathering, because rock can contain elements in a state of reduction. The iron of a biotite for example becomes unstable on contact with the atmosphere or with water and has a tendency to change, albeit slowly, from ferrous iron (Fe2+) to ferric iron (Fe3+) with an increase in volume which causes tensions within the rock of which the affected mineral forms part. The presence of chelating substances, which are liable to form coordination complexes with metallic ions, rather than causing the crystalline structure to collapse, weaken it, since they extract elements previously liberated by hydrolysis.

The result of weathering is a set of components, derived from the rock, some of which have been transformed to varying degrees, some of which are capable of reorganizing themselves in situ, thereby giving rise to minerals with new structures, and, finally, other, soluble, elements which can be transported by water. All of this shows just how complex the weathering system--or the process of transforming rocks into soil--really is.

Soil forming processes

A weathering process can be linked to a action-reaction model. The processes acting upon the rock will be of one type or another depending on the environmental conditions under which the system develops. Therefore the responses, that is to say the mineral components of soil in different parts of the biosphere at any given time, can also be completely different. There is not necessarily, therefore, a one-to-one correspondence between rock and regolith (or saprolite), nor between regolith (or saprolite) and soil. It should be remembered that weathering acts upon any type of material whatsoever, not just on in situ rocks, but also on transported materials and on soil itself. For that reason, by analogy, soil forming processes act upon a regolith or a saprolite, on transported material, or on previously existing soils. It is usual to talk about the mother rock in the first cases and parent material in the others. Not all the soils are the same, because the mother rock and the parent materials from which they are formed are not the same, and also because the weathering and soil formation systems evolve differently according to environmental conditions. The factors which control the system vary in importance according to geographical area, geomorphological position, and the stage reached in the soil forming process.

Since the beginning of soil science the principal factors involved in soil formation have been identified as climate, mother rock or parent material, living organisms (the vegetation and the soil biota), geomorphological position, and weather. This leads to a view of soil as a natural system whose origin is linked to the environment in which it was formed. Soil is a result of the continuous action of the climate and of living organisms on the parent material or mother rock, which occupies a particular place in the landscape. Its formation depends upon the length of time these factors have been at work and on their intensity.

In the humid tropical zone, for example, the processes of weathering and soil formation are usually very active because of the high temperatures (the rate of reaction doubles every 18[degrees]F [10[degrees]C]). Meanwhile the high rainfall means that the soil system loses its soluble elements by leaching as soon as they are liberated, thereby favouring the continuation of the reactions, and the final products will be the only ones that are possible under these conditions--clays poor in silicon (kaolinites), oxides of iron and aluminium (bauxites), etc. They are all acidic compounds, poor in bases and not very suitable for the sustaining natural vegetation or agriculture. On the other hand, if an area in a tropical region has poor drainage, leaching is slowed down and the products of weathering are resynthesised into new mineral components which are very different from the original ones. Silicon is not lost through leaching, and there might be bases like magnesium, and also possibly iron. Under these conditions the formation of silica-rich clays, such as smectites, is possible. The resultant soils, such as the vertisols, will be rich in swelling clays. These are fertile soils, though difficult to work. Likewise, it is the environmental conditions in which weathering and edaphogenesis take place which explain why the biosphere's mantle of soils is the way it is. These also explain why some areas are highly efficient and productive whilst others make life difficult for both plants and men, who, whether they know it or not, act upon extremely fragile ecosystems, the degradation of which could become irreversible.

Soil formation, then, is the combined result of processes which affect the weathering the rock and its component minerals; processes of decomposition of incorporated organic material; edaphic processes which affect all the constituents of the system; and processes of degradation (erosion, amongst others). The resulting soil is divided into horizontal layers called horizons, each of which have their own characteristics. The upper horizons are called the A horizons and they are generally darker, looser and richer in humus than the lower layers. The B horizons are layers where many of the products removed from the A horizon accumulate. They are generally more compact than the A horizons and often have more vivid coloring. They are often structured and enriched, due to the leaching action of water coming down through the upper layers, with humus, iron, clay, or calcium (illuviation), depending on the composition of the upper layers. Finally, C horizons are those layers that rest on the mother rock or the parent material and which have been produced directly by weathering of the latter.

Fresh organic material, which comes mainly from plant remains

and, to a lesser degree, from the soil fauna, is rapidly transformed by the activity of organisms (animals and micro-organisms) which live in the soil. It was Darwin who, in 1881, drew attention to earthworms as being particularly effective in the processes that take place in soil. More specifically, they play a part in forming edaphic structure because by mixing mineral components and organic matter with secretions from their digestive tracts, through which a large amount of soil passes over the course of a year. Over a year an active earthworm population could process up to 40 Mg of soil. Earthworms, however, cannot tolerate acid soil conditions with a pH of less than 5, so that mixing of mineral components and organic matter does not take place in acid soils. Furthermore, in acid soils bacteria do not operate either, whereas such conditions are favorable to fungi. As a result organic material is not broken down quickly in acid soils and so it accumulates on the soil's surface, giving rise to an acidic leaf litter which is not very biologically active and which constitutes a mor-type 0 horizon (an 0 horizon being the name given to horizons which lie on top of the actual soil).

The processes affecting organic material in the soil are, firstly, the mineralisation of simple compounds such as carbon dioxide, water and ammonium, nitrate, sulphate and phosphate ions, etc; secondly, the transformation of nitrogenous substances (ammonification, nitrification, immobilization and denitrification); and lastly, humification. This latter process, controlled by environmental conditions, leads to the formation of humus, which may be "mull" in biologically active media with many bases, "moder," where conditions are intermediate, or "mor," in acid soils.

Soils and vegetation

The suitability of a soil for plant growth is determined by a series of characteristics. One of those characteristics on its own does not determine whether or not the soil is fertile: for example, roots can only take advantage of a balanced nutrient content if the soil has a suitable structure. Likewise, adequate porosity will be useless if there are problems of toxicity caused by a certain element.

Amongst the physical characteristics of soil, texture is important, texture being defined as the relative proportion by weight of different sized mineral particles in the soil. Texture, or granulometry, refers to those mineral particles measuring less than 2 mm (terra fina). These are divided into three classes, sand, silt and clay, although the limits between each class vary according to classification criteria. The results of granulometric analysis are represented in triangles named according to texture, in which soils are divided into textural classes. This classification indirectly provides information about other aspects of the soil: for instance, a sandy soil is, in principle, more permeable and aerated than an clayey soil, while a soil with a looser or more balanced texture holds more water available for plants.

The structure of the soil is the product of the arrangement of elemental particles into aggregates, separated by planes of weakness. These aggregates separate into individual pieces when a clod of earth is carefully broken up. The structure affects the characteristics of the largest spaces in the soil (macropores): a good aggregation helps root penetration, the aeration of the soil, and the capacity of water to infiltrate, whilst hindering erosion. The classical description of structure is based on the shape and size of the aggregates (prismatic, blocky, platy, crumb, etc.) and on their stage of development (weak, moderate, strong).

Porosity is the ratio of the total volume of pores to the total volume occupied by the soil. The movement of air and water in the soil, as well as root behavior, not only depend on the porosity of the soil, but also on the characteristics of the pores themselves, such as their shape, diameter, and their continuity or degree of connection between them. Thus, two soils with the same porosity values might behave completely differently depending on whether or not the interstitial voids are vesicles (unconnected cavities), or holes created by the soil's fauna, or fissures which are in contact with the exterior atmosphere and which can take water and nutrients to the roots.

The characteristics of the pores have a decisive influence on the properties of soil in relation to water. These properties include: infiltration capacity, which is the ability of a soil surface to absorb water; hydraulic conductivity (permeability), which is a measure of the soil's capacity to permit fluids to flow through it; and hydric potential, which measures the amount of energy with which water is retained within the pores. This latter factor is important because plant roots only have a limited ability to take in water. Thus, in a soil which has just been watered the roots will not have too much difficulty in absorbing water, but, as the water drains deeper into the soil, or evaporates, or is absorbed by the plants, the remaining water is retained around the soil particles with more energy, causing the hydric potential to become more and more negative until it reaches a certain level below which the roots will not be able to extract more water. The quantity of water retained by a soil between these two energy levels is traditionally called the field capacity (field moisture capacity) and the wilting point, is the available water capacity which in general reaches its optimum level in loosely textured soils.

In order to grow, plants need a certain amount of nutrients which they have to extract from the soil. The principal nutrients are nitrogen, phosphate and potassium. Smaller quantities of other elements are also essential for plant growth. When these elements are present they are absorbed by the roots in the form of ions in the soil solution. These elements come from a number of different sources; they might be the result of the mineralisation of organic material, or of the alteration or dissolving of the minerals which released them, or they might be the result of the activities of micro-organisms. These complex, interrelated processes determine the availability and mobility of nutrients at any given moment according to the characteristics of the soil. These characteristics include water content, the pH of the soil, and its redox potential, among others. One of the properties of soil which most affects the mobility of ions is the base (cation) exchange capacity: some soil components, such as clays and organic material, carry negative electric charges on their surface and these attract cations present in the soil solution. The absorbed cations are balanced with the cations in solution (forming a nutrient reserve). They also play a regulatory role in preventing ionic imbalance in the soil.

Soil nomenclature

The useful properties of soils are what actually interest its users who are not normally interested in their nomenclature. Nevertheless, the classification of soils allows us to order our knowledge of them. It makes it possible to generalise about one area by using experience acquired in another part of the biosphere, as long as the soils are similar, a fact which will be reflected in their having the same denomination. This is the main reason for classifying soils.

One of the first difficulties we come across in classifying soils is deciding on the criteria to use. It is necessary to define and choose a taxonomic system which is capable of expressing soil characteristics and which can be understood by other users. Systems based on the intrinsic properties of soils are, when applied, more objective, and are better when they are based on morphometry. Genetic classifications based on the genesis of the soil are more speculative since it is necessary to first of all infer their genesis before they can be established. There are two systems which give an overall view of the soils of the biosphere. The first of these is the Soil Taxonomy, and the other is the Soil Units System developed by FAO-UNESCO.

Soil Taxonomy

A system developed by the U.S. Department of Agriculture was presented to the scientific community at the International Soil Science Congress held in the United States in 1960. This came to be known informally as the Seventh Approximation and since then it has been revised a number of times to bring it up to date. The current version was published in 1975: Soil Taxonomy--a Basic System of Soil Classification for Making and Interpreting Soil Surveys, generally called simply Soil Taxonomy. (See also "Soil classification" in volume 11.)

From its introduction the popular names used to designate soils in different countries were not used, though these names are still used today in other classifications. The Soil Taxonomy introduced a new nomenclature based on Greek and Latin roots, which is self-explanatory for a series of soil characteristics and environments. For instance, talking of an Australian soil called quartzipsamment it is possible to tell just from the, apparently cryptic, name and without ever having seen the soil itself, that it is a sandy soil (psammos, sand). Because it is a sandy soil we know that it will have a low water and nutrient storage capacity, a high level of hydraulic permeability and there will be a risk of wind erosion. We furthermore know that only the A and C horizons will be present, because it is an entisol (the order is indicated by the termination -ent, the element which forms the order); that its potential fertility is very low, given that all sand is formed from quartz (indicated by the prefix "quartz"); that plants, apart from specially adapted ones, will have difficulty living there; and that in order to be cultivated a nearby water source, fertilization and irrigation will be necessary.

Soil Taxonomy establishes six hierarchical levels of homogeneity which increase as they pass from the lowest categories, and it is capable of classifying soils on the level of plots. The levels are: order, suborder, great group, subgroup, family and series. The different categories are defined by the system according to the presence or absence of diagnostic soil horizons--the epipedon that forms at the surface, and the endopedon (or subsurface diagnostic horizon) that originates below the surface, and humidity and temperature regimes and other diagnostic characteristics. The system's main drawback is that it places humidity and temperature regimes very high on the scale and, in many parts of the world, this information is simply not available. Another drawback is that the system requires laboratory analysis. This is, at the same time, an advantage because the information given by a soil cartography based on this system is much fuller.

FAO Soil Units

At first the FAO (1971) did not intend to develop a soil classification system: the idea was simple to draw up a list of Soil Units to be used on maps that it was then producing with UNESCO. This is a compromise scheme which uses many of the Soil Taxonomy's concepts (for example, diagnostic horizons), although in labelling the soils many names from European classifications are used. It was conceived as a method of designating soils on a worldwide scale, which are then represented on small-scale maps. As it is not a classificatory system it is not hierarchical and it only establishes two categories. The higher of these is sometimes equivalent to the order in the Soil

Taxonomy, whilst at other times it is the same as a group. The lower category is formed by intermediate soils or by soils with special horizons. The main advantage of this system compared with the Soil Taxonomy is its simplicity, although the latest revision by the FAO in 1989 made it considerably more complex and brought the two systems closer together. Nevertheless the FAO system still does not require as much information as the Soil Taxonomy, nor does it utilise humidity regimes and temperature in its classifications. In the following study of the soils of the biosphere the FAO system will be used because the maps that have been produced on a world scale have used this system. Wherever possible, however, correlations with the Soil Taxonomy will be made.

Edaphic models

By looking at soils on a global scale and comparing their distribution with the distribution of weathering-edaphogenic systems, climatic variation, vegetation and soils (the Strakhov diagram), we know that the processes of edaphogeneisis do not take place at random. It is therefore possible to establish edaphic models for the biosphere's main environment types. Although some of the processes outlined here are broadly associated with the world's principal climate types, they are not necessarily restricted to one climate type and can be found in other environments when the circumstances are right.

Edaphic model for well-drained, temperate and humid areas

In these areas edaphogenesis is controlled by the soil's udic regime which is dominated by percolation because precipitation exceeds evapotranspiration. Leaching affects the carbonates and other soluble products resulting from weathering. The level of decarbonisation reached depends on the type of parent material and the climate. The release of iron oxides by weathering allows these to combine with clays and organic material to form insoluble complexes. This process is called brunifaction and gives the soil a characteristically brown colour. It is typical of these environments which are characterised by a moderate degree of weathering, light acidification, the development of soils with an A BW C profile and "mull" type humus. These, according to the FAO system, are cambisols (equivalent to inceptisols in the Soil Taxonomy). If the climate is Mediterranean the oxides of iron may rehydrate give the soil a reddish colour--a process called rubifaction.

In more humid regions and with parent material which is poorer in bases, acidification progresses more rapidly, the organic material gives rise to moder humus, and the argiles disperse and can initiate a process of illuvation in which the clays migrate for purely physical reasons, transported in suspension from the upper part of the profile and deposited at a certain depth. The result is the formation of horizons of accumulated illuviated clays which are called argillic endopedons. In the FAO system these types of soil are called luvisols, and they are equivalent of alfisols in the Soil Taxonomy. Under conditions in which the bases are more thoroughly leached and where vegetation of resiniferous plants predominates the clays can become unstable, the collapse of crystalline structure can release iron and aluminium, and the soil can become acid. Organic material evolves slowly into mor-type humus whose components are able to form soluble or pseudosoluble chelates with the iron and aluminium, which can then move through the whole profile. This process is called podzolization and the resulting soil will be a podzol with an O A E Bh ir type profile (equivalent to a spodosol in the STS system).

Edaphic model for arid and semiarid areas

Soils in arid and semiarid areas are characterized by the presence of components that are highly susceptible to weathering as long as there is sufficient water. In these soils percolation is never predominant and is sometimes absent (xeric or arid regimes). Under these conditions carbonates, chalk, or highly soluble salts may be present.

Carbonate accumulation processes are frequent, these being brought about by solubilisation-translocation and the precipitation of calcium carbonate within the soil itself. The result is the formation of calcite nodules (producing a calcic horizon) or typical calcareous crusts (petrocalcic horizon) which are impenetrable to roots. Typical profiles are A Bk Ck or even A Bkm Ck, the soils are calcic cambisols according to the FAO system (calcixerolic xerocrepts in the Soil Taxonomy) or also xerosols, yermosols, or calcisols. Chalk is a frequent component of soil in arid and semiarid areas. It is the protagonist in processes of gypsification which are characterized by the translocation of this component, giving rise to vermiform chalk or to more generalized accumulations in the form of gypsic or hypergypic horizons. The resulting soils can be gypsic xerosols or yermosols, or gypsisols in the FAO system (gypsic xerocrepts or gypsiorthids in the Soil Taxonomy).

In certain areas the landscape might favour the accumulation of more soluble salts (NaCl, Na2SO4, MgCl2, etc.). Under these conditions the salinization processes only allow the establishment of plant communities comprising mainly halophytes (Suaeda, Salicornia, Arthrocnemum among others). In the FAO system these soils are called solonchaks (salorthids or saline soils in the Soil Taxonomy). Sodication processes occur when there is a large increase in the percentage of sodium at the cation exchange sites (CEC>15%) and these give rise to sodic soils or solonetz in the FAO system (natrixeralfs, natrustalfs and others, in the Soil Taxonomy). The formation of these soils requires conditions of greater humidity so that other cations, such as calcium and magnesium, can be leached out.

Edaphic model for tropical humid regions

In humid tropical zones the soils are subjected to intense leaching due to the high rainfall (udic regimes). The vegetation provides large quantities of organic materials which, because they decompose rapidly, assure a rapid return of nutrients, an essential factor in soils which, in themselves, are not very fertile.

Ferrugination is an edaphogenic process in these regions, characterised by the release of oxides of iron and silicates which only allows the formation of clays, like kaolinite, which are poor in silicates and are typical of ultisols. Ferralization constitutes a more advanced degree of weathering and is the origin of laterites (called ferrasols or oxisols in the Soil Taxonomy) which are characterised by having horizons which are rich in hydrated iron and aluminium oxides. They are very poor soils that can give rise to ferruginous crusts if the iron oxide horizon remains on the surface due to erosion and the soil dries out. This is the reason for their use as building materials in tropical regions.

In those cases where the parent rock is a sandstone or a quartzite, which are poor in bases, leaching is so effective that the soil which develops is known under the FAO system as a tropical podzol (spodosol in the Soil Taxonomy).

Edaphic model for poorly drained areas

In closed depressions and in places where there is no exterior drainage, water accumulates and the phreatic layer breaks through or is very near to the surface of the soil. In these environments the soil is saturated with water (aquic moisture regime) and the respiration both of roots and micro-organisms, exhausts the dissolved oxygen to produce reducing environments (negative redox potentials). Under these conditions the process known as gleyzation or gleying can take place. This process involves the reduction of chemical compounds in the soil, such as the change of iron from ferric (Fe3+) to ferrous (Fe2+). Since the latter is soluble, the soil takes on green or gray tones (mottled) caused by the absence of iron. The resulting soils are hydromorphic and are classified as gleysols in the FAO system (aquents in the Soil Taxonomy).

Under anoxic conditions the organic material decomposes very slowly, and in some extreme cases the accumulation of plant remains can give rise to organic soils called histosols. An example of these are the peat bogs that arise in water-saturated and base-poor environments and that provide perfect conditions for the growth of Sphagnum.

The Earth's edaphic mantle

The map of soils produced by the FAO-UNESCO (1971-1981) is the only one available which gives an overall view of the distribution of different soil types over the earth's surface. It is on a 1:5,000,000 scale. The map, which is the result of correlating existing information about soils from different countries, groups together very diverse data. It has brought together, under a single unified legend, maps of different scales, of different observational density and different classifications. Although the legend was later modified (FAO 1989), the map itself has not been modified, and it therefore shows soil distribution corresponding to the first edition of the legend (FAO 1971). There is no equivalent map based on the Soil Taxonomy classification system: only a schematic representation of orders and suborders on a worldwide basis has been produced, based on a map produced by the Soil Conservation Service of the United States Department of Agriculture. The FAO map allows us an overall view of the world's soils.

In tropical zones, where high temperatures and rainfall predominate, the harsh climatic conditions provoke a high degree of chemical interaction, which is reflected in the fact that the majority of soils in these areas are ferralsols, acrisols, and nitosols, with vertisols, andosols, cambisols, luvisols, arenosols, and litosols occupying a smaller surface area. The great deserts are almost totally made up of xerosols and yermosols which are characterized by a low level of development and by the accumulation of salts, carbonates or chalk.

The representative soils in the central areas of North America, Asia and, to a lesser extent, South America and northern Africa, are greyzems, chernozems, phaeozems and kastanozems (mollisols in the Soil Taxonomy). These have a surface horizon which is rich in organic material and a notable degree of biological activity. They are considered to be the best agricultural soils and they support the great, fertile meadowlands of the continental interiors, such as the Russian steppes, the American prairies, and the Argentinian pampas.

In wetter, colder regions such as northern Europe and Asia, luvisols, podsols and podzoluvisols are found, in which clays and/or organic materials have migrated to deeper horizons. They broadly correspond to the taiga zones.

Mediterranean regions are mostly occupied by regosols, cambisols, alfisols and xerosols, which reflect the particular characteristics of the Mediter-ranean climate, such as a warm dry season, the different geological and geomorphological characteristics, as well as the conditions which existed in the past.

The youngest soils, or soils which have undergone few edaphogenetic processes, are represented by regosols, fluvisols and arenosols (entisols in the Soil Taxonomy). They are common in the principal mountain chains (the Himalayas, the Alps, and the Andes) and throughout the great fluvial valleys, since the soils form on top of recent geological deposits. They are also found in deserts due to the extreme dryness.

2. Bioclimatic and edaphic domains: biomes

2.1 The biome concept

Each little zone of the biosphere is unique and unrepeatable. We can, however, compare different fragments in a search for regularities that allow us to make a classification of the major kinds of landscape, overlooking the differences due to local factors, whether past or present. Macroclimatic conditions lend themselves particularly to subdividing the biosphere into large units as the climate is the primary factor which influences all others, both edaphic and biotic.

To arrive at a definition and classification of the major units of the biosphere, we must add to the concept of a climatic zone or domain, another which encompasses biotic aspects. It is of little use to use fauna for this end, since although the distribution areas of most vertebrates are well known, the great mobility of these species and their tendency not to be confined to one region, gives them a character which is quite independent of climatic conditions. Vegetation, on the other hand, is particularly suitable due to its immobility and its clear dependence on climatology. We usually use the concept of formation, based more on physiognomy and phenology than on specific composition, which means that the same formation can stretch through different biogeographical regions as it is more a product of the climate than of evolutionary history. Despite the greater variability of soils, which are not useful for making a general classification because of the great influence of historical climatic conditions or of non-climatic factors, like the composition of the parent rock, the concept of plant formation is also related to edaphic, and even to geomorphological characteristics, to the extent that these are an expression of present-day climatic conditions as well as historical ones. So, we tend to classify the Earth's landscapes in large units or biomes starting from the concept of climatic domain and of formation, together with aspects of morphology, behavior and biological rhythm throughout the year, before arriving at a differentiation which can be considered natural.

Biomes are very large units, occupying areas of between thousands and millions of square kilometers. Within each large biome we can often find large areas with distinguishing features which clearly differentiate them from the rest and which are caused especially by special edaphic or climatic conditions. It is for this reason that we use the word orobiomes to refer to mountain areas influenced by a climate different from the general one, and pedobiomes to refer to developed fragments in peculiar edaphic conditions, normally in azonal or intrazonal soils.

The classification of the Earth's landscapes as large units or biomes is complex, especially when defining the boundaries, as no matter how much information we have on the features which define each biome, we cannot ignore the transitional band which separates two different biomes and which can be considered from various different points of view. We must not forget that these transitional zones are active and dynamic interphases which can change with the passing of time, above all when human activity leads to modifications which can shift to one side or another, of what was previously midway between the two. Intensive and extensive human activity has greatly complicated what was an already a varied panorama of the biosphere, whether by direct exploitation of the plant cover, through elimination and replacement by crops and grazing lands, irrigation, the introduction or elimination of species, or irreversible environmental degradation like erosion. However, the difficulties in defining the frontiers of the great biomes does not mean that a classification of this type is any less useful or interesting, since it represents a very understandable synthesis obtained by looking at the biosphere from a distance so as to grasp the broad similarities rather than the small differences.

2.2 The world's great biomes

Among all the possible classifications of the world's great biomes, we have opted for one which considers nine large units, some of which strictly correspond to a biome while others are sufficiently broad as to justify subdivision into areas, which can be clearly differentiated. In addition, some isolated systems with peculiar characteristics such as islands, caves, or marine systems, are treated separately.

Tropical forests: humid tropical lands

We usually distinguish two basic types of rain forest: lowland rain forests and the montane cloud forests. These latter are situated in areas which do not have a high rainfall but are influenced by clouds and mists of orographic origin which give them the necessary moisture. The rain forests and cloud forests are characterised by being evergreen and by the luxuriance of their living forms and their great taxonomic diversity. The difference between the two types of forest lies in the fact that the second has a relatively simpler structure and is much richer in epiphyts. The enormous vigor of the vegetation is reflected in the height which the trees can reach, up to 197 ft (60 m) tall, and the high values of phytomass per surface unit area, between 500 and 1,000 t/ha. The dominant leaves are of average size, soft and not very tough and contain a large number of stomata which facilitate transpiration. Their evergreen nature is achieved through a combination of truly perennial-leaved species with other deciduous ones but which behave asynchronically, even reaching a total lack of synchronism between individuals of the same species or between parts of the same individual.

As for taxonomic richness, we could say that the rain forests are the richest biomes in terms of species in the world. One very important reason for this is the long-term climatic stability in these regions; environmental conditions, which are very suitable for the development of life, have changed little since the Pleistocene. Great diversification, combined with their role as refuge for particular evolutionary lines which are in regression, has given rise to this explosion of diversity. The structure of the whole is very complex and is characterised by a lack of dominance of one particular species, a fact which is very evident when we look at the vegetation. The plants are arranged in differentiated strata, and there can be as many as five of them--three tree strata, one of shrubs and one herbaceous. In the tree layer, we can easily find up to 40 different species per hectare (1 hectare=2.5 acres) and in some cases there can be as many as 100, generally belonging to different families.

Most soils are Tertiary, even though we can also find recent volcanic soils and alluvial soils. The latosols are dominant, well leached and rich in iron and aluminium sesquioxides. Decomposition of the dead leaves is very intense and mineralized elements are rapidly absorbed through the roots which only occupy the upper layers of soil. The nutrient cycle is strongly controlled by the living fraction, which accumulates almost all the ecosystem's reserves. This combination of features means that the tropical forests are apparently very stable systems but with a low resilience; in other words, they have only a small capacity to resist modification or alteration or to return to their initial state. The forests have been called green deserts because of their deceptive lushness. They are not very suitable for exploitation and their replacement by agricultural crops uncovers the poor soils which rapidly become degraded, thereby impeding the system's later recovery.

The equatorial rain forests, which are the Earth's most developed and complex systems because of the favourable climatic conditions and their long-term stability, have a potential area of some 20 million square kilometers distributed along a belt around the equator. They are divided into three groups, one in the American continent, another in the African and a third fragmentary one in Malaysia.

Savannahs: the dry tropical lands

Under the umbrella term of savannahs, which has no precise geobotanical meaning, a whole range of tropical environments are considered, which go from deciduous and evergreen forests to dry semi-desert herbaceous formations, via all sorts of intermediary formations with a shrub layer or a distinct tree layer. In general, we can make a first distinction between the savannah woodlands and the humid savannahs of South America, and the deciduous savannah woodlands and the dry savannahs which stretch throughout all continents in the area between the two tropics. As a whole, though, they are very varied physiognomically, including grassy plains with palm trees and acacias, spiny scrub of low trees, cactus formations, little groups of trees in grasslands, and semi-deciduous forests.

Climate has a role to play, undoubtedly very important, but edaphic factors are often the cause of the existence of a savannah or a savannah woodland rather than a closed wood, as water availability can be strongly influenced by the soil's characteristics and structure. We usually differentiate between climatic savannahs, edaphic savannahs and flooded savannahs. In the first case, the determining factor is the combination of rainfall and temperature; in the second, the dryness is caused by a sandy or stony soil which hardly retains the rain water, necessary, a priori, to maintain a deciduous forest; in the third case, the savannahs are found on soils with variable humidity, going from flooding to dryness. This panorama, already complex in itself, becomes even more difficult to interpret when human activity is present. Deforestation can give rise to the so-called anthropogenic savannahs, but in certain cases the process is reversed as the excessive amount of pasture in certain savannah formations provokes the development of woodland as the trees are more able to compete for water than are herbaceous plants.

As far as the flora is concerned, the wealth of species is not very great. Legumes and grasses are the most diversified families in these environments. There is a certain amount of convergence in the morphological characteristics of tree and shrub plants in these formations: corky barks, xeromorphic leaves and thorns on the trunks. The general structure of these systems is not very complex and the phytomass oscillates between 10 and 160 t/ha.

As regards faunistic richness, the most notable are the African savannahs as opposed to the American or Australian ones. They are, in general, areas with large herbivores and carnivores, many of which migrate in search of water and food. The highest levels of the food chains are occupied by prestigious animals, whether herbivores (gazelles, giraffes and African zebra) or carnivores (South American wolves, Australian dingoes, lions and African leopards); also of note are the large flightless birds (cassowaries and emus in Australia, rheas in South America and ostriches in Africa).

Deserts: the dry lands

The Earth's dry lands are found in tropical and subtropical areas and are covered, for the most part, by deserts, semideserts and a whole range of transitions between these and the dry savannah-like formations or Mediterranean sclerophyllous formations. Some desert areas are, however, far distant from the subtropical domain and enter into the zones of great winter cold, so that in general terms we can make a distinction between hot deserts and semideserts, already mentioned, and cold deserts and semideserts, which are transitional between the temperate and arctic zones. The characteristic common to all deserts, whether cold or hot, is the rainfall which is erratically distributed and is considerably less than the potential evapotranspiration.

The main characteristic of deserts is the lack or scarcity of vegetation, which leads to a landscape dominated by geology. In deserts, the vegetation is restricted to areas where humidity builds up, giving rise to a so-called contracted vegetation. In contrast, in the semideserts, the vegetation is distributed evenly but is sparse, covering up to 25% of the soil, and is called diffuse vegetation.

In such extreme conditions, soils largely determine the availability of water for the vegetation. Clayey soils are much dryer than sandy ones, as the latter allow the water to penetrate more deeply, thus being protected from evaporation. This means that apparently dry and inhospitable soils which are sandy and stony are actually more humid deeper down. The salinization of soils due to the rise of salts by capillarity is a very common phenomenon. In these conditions, however, plants do not live in conditions of permanent water stress as the water supply per unit of transpiring surface area is similar in both arid regions and in humid regions. Aridity provokes an increase in the surface area of roots and a reduction in the transpiring surface area, as well as giving rise to a series of adaptations related to the water regime. Thus, in arid regions, succulent plants occur which are capable of accumulating water in special organs: poikilohydrous plants which are capable of tolerating the drying up of their vegetative parts; stenohydrous plants with a considerable capacity to regulate the loss of water through the stomata; and ephemeral plants which have a short life span and develop during the short humid periods and spend the dry seasons in the form of seed. As far as fauna is concerned, the problems caused by aridity manifest themselves in the search for food in a rather unproductive environment and in the control of water loss due to respiration and excretion. Like the vegetation, the fauna is scarce in dry environments and shows different types of adaptations, whether behavioural (nocturnal rhythm, underground habits, diapause), morphological (small volume-surface area ratio, long snouts to avoid water loss) or physiological (solid excretions, tolerance to the partial dehydration of the tissues). As in the case of plants, animals are few and far between but they do not permanently go thirsty.

Cattle-raising has been a very important factor of change in the ecology of deserts, as the effect of the grazing of domestic herbivores is much more destructive than that of wild herbivores. The great carnivores have practically disappeared (American puma, Australian marsupial wolf, African lion) and as a large majority of the existing herbivores have been domesticated, little has been left of the deserts' original energy chain.

The Mediterranean lands: temperate regions with a dry summer period

The Mediterranean areas are characterized by a dry subtropical summer and a moderately mild and rainy winter. Under these climatic conditions, sclerophyllous and evergreen woods and scrublands develop. Mediterranean areas are found between the subtropical and temperate zones, on the western coasts of the continents: the Mediterranean basin proper, California, Chile, South Africa, and Australia.

The typical vegetation of these areas, which we usually describe as sclerophyllous, is xerophytic, in other words, made up of plants which are capable of regulating transpiration by closing their stomata, so avoiding excessive loss of water during the hot, dry season. Sclerophylly tends to be associated with good root development as the plants try to reach the deep layers of soil which contain water reserves. Under the conditions of extreme aridity, scrub replaces woodland and deciduous summer trees appear, as well as small-leaved aromatic shrubs.

Resistance to cold is a remarkable characteristic of the flora of sclerophyllous woodland and scrub, as many species tolerate temperatures way below the freezing point of water without suffering any damage. This is very important since although the winters are mild, occasional long cold spells occur because of the influence of colder neighboring zones.

Fire is a very important ecological factor for Mediterranean ecosystems because of the coincidence of hot and dry weather during the summer. Adaptation to fire has been translated into different types of adaptations which go from pyrophytism (plants rich in inflammable oils, fruits that only open in the case of fire, germination favoured by the exposure of the seeds to high temperatures) to being fire-resistant (incombustible corky barks, stumps which can re-sprout) in such a way that it is very difficult to explain the working of some Mediterranean ecosystems without taking into account the recurrent fires.

The fauna of Mediterranean regions does not have any special features. Apart from the presence of some endemics, it is not very different from that of the rest of the temperate zone. The role of these areas as a winter refuge for birds from colder regions is very important, above all in the case of the Mediterranean basin. On the other hand, the intense human modification of all these landscapes has provoked the rarity or loss of all the great carnivores and herbivores from these areas.

Mediterranean systems are especially fragile. Intense human action has greatly changed these landscapes, provoking a spectacular increase in forest fires and giving rise to serious problems of erosion which could convert many Mediterranean regions into semideserts. Erosion is especially serious under these climatic conditions as the loss of soil supposes a reduction in the water holding capacity and soil fertility. Furthermore, many of the Mediterranean soils were formed in more rainy epochs than the present one and this means that presentday edaphogenesis is extremely slow or non-existent. This makes the problem of soil loss an even more serious matter.

Temperate forests: temperate lands that are neither cold nor dry

Temperate forests form in warm temperate regions, which are characterized by an abundant rainfall and ambient humidity, regularly distributed throughout the year or more concentrated during the hot season, a total absence of a dry season and a winter which is not very cold with only occasional frosts. They are typically evergreen broadleaved forests, even though needle leaf forests which grow under similar conditions could be included as well.

The most typical temperate forests are those of the Macaronesian region (the Canary islands, Madeira, the Azores and Cape Verde) and in the south east of China. We also find them in small patches in some eastern parts of the continents, (Japan, Australia, the southeast United States) as well as in some areas in the west of the American continent. In these latter zones we find, atypical examples of the valdivian forest, of giant conifer forests (sequoias) and Araucaria conifer forests.

The temperate forest is, in general, a dense woodland formation which is highly developed and has been described, in some cases, as a tropical forest outside the tropics, which gives a clear idea of its general appearance. The canopy can reach as high as 98 ft (30 m) and is made up of several different species from different families and genera, even though under difficult conditions and at the limits of their distribution area, the forest becomes poorer and the canopy tends to be monospecific, as in the case of the magellanic forests of Nothofagus or the needle-leaf Araucaria formations. Under the canopy there is an undergrowth formed of shrubs, lianas, and epiphyts. The richness of flora as a whole is notable, reaching 30-40 tree species per hectare. The phytomass is also considerable with values of as much as 200-500 t/ha. The greatest degree of development occurs in the Tasmanian eucalyptus forests which reach heights of up to 360 ft (110 m). The long duration of the vegetative period (7 to 10 months) as well as the favorable moisture conditions permit this development, combined with their evergreen character.

Woodlands: temperate lands with a cold winter and low rainfall

In the northern hemisphere, the temperate zone of humid summers and cold but not very long winters is the land of deciduous forests. These forests, which occur in the European, north American and Asiatic continents and are absent from the southern hemisphere, form magnificent changing landscapes, bright green in summer, and with yellowish or reddish autumn colouring, and bare trunks in winter.

The structure of the temperate deciduous forest is simple as it consists of a single tree layer of up to 98-131 ft (30-40 m) high with a shrubby, rather poor undergrowth and a herbaceous layer that develops in spring as it makes the most of the short favorable period before the trees grow their leaves, causing a drastic reduction in the amount of light that reaches the soil. The richness of the flora is much higher in the north American and Asiatic forests than in European. This is due to differential impoverishment of the flora caused by ice ages which was very high in the European continent because of the barriers represented by the transverse mountain ranges, and very low in other areas in which the mountain ranges are orientated north-south and permit migration and provide refuge. The tree layer, monspecific in European forests and plurispecific in the others, is a clear reflection of this phenomenon.

The fauna of temperate forests follows the rhythm established by the vegetation. With the arrival of the cold, migratory species leave, especially the birds. The small mammals, of which there are many, the reptiles and amphibians all tend to hibernate, as do many arthropods. The large mammals, which are relatively scarce, are the only ones which remain active throughout the year. In these forests, the deciduous leaf strategy derives from an adaptation to avoid the cold. Physiognomically speaking, the temperate deciduous forest is a system, with various phases which adopts a different aspect throughout the course of the year, with important consequences which are related to course of the nutrient cycles, as these pass through a phase of mass monement, a phase of storage in the layer of dead leaves and wood, and a phase of chemical transformation and mineralisation. It is for this reason that the microfauna which breaks down the system is of great importance, compared with the rather sparse presence of the surface fauna. These forests, which accumulate phytomasses of between 200 and 500 t/ha, develop on very rich and moist soils, in which the dead leaves play an extremely important part. In the areas that they occupy, they have for centuries undergone an intense exploitation, whether for their wood or for cattle grazing. Their structure, therefore, already simple in itself, has become even further simplified. In some cases, an irreversible impoverishment of the soils has led to the substitution of these forests by shrub communities, while in others they have been simply cleared to make way for crops on the fertile soils which support them.

The steppes and dry prairies

In the interior of the continents, in areas with a continental temperate climate, and far removed from the mitigating effect of the oceans, rain becomes scarce and temperatures take on more extreme values, both in summer and winter. In these dry, seasonally rigorous conditions, tree vegetation cannot develop and the landscape is dominated by herbaceous formations. This is the case of the Asiatic steppes, the north American prairies and the Argentinean pampa lands which make up the most extensive areas of this kind of formations. There is, however, some variation: thus,although in the case of the Asiatic steppes and the South American prairies, the climatic conditions are the clearest factors, in the case of the north American prairies the lack of tree and shrub vegetation is due to a combination of occasional exceptional droughts, recurrent fires, and an abundance of herbivores.

Steppe and prairie vegetation is typically herbaceous, and shrub elements only appear in zones of transition to other more wooded formations. Despite this simplicity, we can distinguish up to three sizes or layers which go from the tall grasses (up to 7 ft [2 m] high), those of average height (around 12 in [30 cm]) and tiny ones accompanied by mosses (4 in [10 cm] high). The richness and variety of flora is not inconsiderable as one can find up to 70 different herbaceous species there, sometimes dominated by grasses. The vegetation cover depends to a considerable extent on the rainfall, and for this reason it can vary considerably according to the zone and the year. The phytomass can reach values above 20 t/ha but one must bear in mind that up to 90% of this corresponds to roots which grow down in search of water.

The annual cycle is marked. When the snow melts, the soil's moisture content and the rising temperature lead to an explosive spring flowering of non-graminoid species which have spent the winter in the form of underground bulbs. At the beginning of summer, the landscape is a green meadow of grasses in active growth which end up flowering and drying out at the height of the season, not to revive until the coming spring, after having spent the winter covered with snow. In these climatic conditions, the production of organic matter is very high, both above and below ground, the latter making up the greater part of the phytomass. This gives rise to mostly chronozem soils which are very deep and rich in humus.

The fauna has to be adapted to the annual rhythm of these cold or torrid flat lands. It is a matter of searching for a balance with a herbaceous vegetation capable of producing large quantities of biomass during a short seasonal period but which, on the other hand, does not produce the same amount every year, depending on the climatic conditions. Herbivores abound here and are capable of travelling great distances in search of food, as different in aspect and strategy as those of the grasshopper, the Siberian crane or the American bison. Living in groups facilitates their defense against predators in these vast open spaces, and living in underground groups seems to be the most developed form of adaptation, adopted by animals as different as insects, reptiles and mammals. This strategy allows them to protect themselves both from predators and from the winter cold and the summer heat.

The great richness of the soils in the steppes and prairies has turned them into extensive cereal-growing areas where a large part of world production is concentrated, even though the considerable annual variability of the rainfall can give rise to important fluctuations in the production of these crops.

Boreal coniferous forests or taiga: the cold, wet lands

The cold and wet lands in the northern hemisphere, in which the polar cold and the accompanying rains and snow define the main climatic traits, are occupied by coniferous forests. This zone occupies an extensive belt of land, 50-60 degrees latitude, across the Euroasiatic and American continents. This belt is bordered in the north by the tundra and in the south by the temperate area of deciduous forest. The name taiga, originally applied to the Siberian coniferous forest, is used to describe this whole type of formation in general.

The sufficient rainfall allows for the development of tree vegetation, but the length of the winter, with monthly averages below 32[degrees]F (0[degrees]C) for more than half the year, combined with the presence of frosts almost all year round, favour needle-leaf perennials rather than any other strategy. The summer season is short and not very favourable and shedding leaves becomes an impossible luxury. The only solution is to grow leaves capable of resisting the winter colds and setting to work as soon as the favorable season starts.

The recency of this biocenosis, formed through the process of reconquering the northern lands after the glaciations, implies a lack of developed soils and a considerable poverty of species. Only a few species of trees form vast and monotonous dark green landscapes, dotted with the occasional deciduous trees in marginal zones. Where the climate becomes more continental, in the eastern Euroasiatic taiga, deciduous needle-leaf trees appear as the final and extreme form of resistance and adaptation. The shrub layer is equally poor and all over the soil we can find a carpet of cryptogams.

The taiga soils are cold, frozen and poor. The branches and evergreen canopies makes it difficult for solar radiation and snow to get through; the latter, paradoxically, acts as a protector of the soil against the cold (in direct contrast with the snow, the temperature does not usually fall below 23[degrees]F [-5[degrees]C], while the ambient temperature can drop to from -4[degrees]F [-20[degrees]C] to -22[degrees]F [-30[degrees]C]) . The soils freeze up to depths of 31 in (80 cm), and the trees have their roots in the upper layers which are the first to thaw. The low temperatures, the accumulation of water and the poor aeration make the decomposition of dead leaves difficult, which gives rise to accumulations of peat. In these conditions, being evergreen also represents a great advantage as it allows trees to retain nutrients in the phytomass, which can then recycle them and thus become less dependent on the poor supply in the soil.

The composition of the fauna is relatively varied, as the fauna of the area is joined by fauna from neighboring biomes, which are too cold in summer or too hot in winter for some species. Here we find large herbivores that eat bark or lichens (eland, reindeer, caribou), rodents (rabbits and hares), birds with powerful beaks capable of breaking pine cones and pine nuts, small mammals, birds of prey and carnivores with sought-after skins (mink, marten, lynx, wolf and fox). The bear, omnivore par excellence, makes the most of all resources. In fact, the wealth of fauna is surprising in such a cold environment in which humans, present throughout the planet, only live as hunters and shepherds, as climatic and edaphic conditions do not permit the planting of crops.

The Arctic tundra: frozen boreal lands

The large, treeless tundra region occupies the coldest lands in the northern hemisphere, above 70 degrees latitude north, in other words, above the arctic polar circle. They are not very rainy areas (less than 200 mm per year) but they are not dry because of the low temperatures in the region. During more than half the year, the average daily temperatures remain below zero and the summer temperatures do not go up much due to the heat used in melting the snow and thawing the surface soil layers. The snow, which is not very abundant, is distributed irregularly because of the strong winds that sweep it away and expose wide clearings of soil to the rigors of winter. The arctic tundra represent the last vegetation formations in the northern limits of the northern hemisphere: nearer the pole the snow and permanent frost make plant life impossible.

Deep down, the soil is permanently frozen. The vegetation only occupies the top surface layer which thaws in summer. This surface layer, which varies between 13 ft (4 m) and only 6 in (15 cm), is followed by a layer called permafrost, a heritage of the glaciations, which can be as much as 1,476 ft (450 m) thick. The soils are very poor in organic material as under conditions as hard as these, biological productivity is very low and peat does not accumulate.

These conditions are only bearable for smallish plants which have surface roots and which are active during the summer season. These are lichens and other cryptogams, with dwarf long-lived (up to 200 years), slow-growing shrubs, which are sometimes capable of producing buds at the end of the favourable period. The seeds are very small and are normally disseminated through water and wind, as zoochory or animal dispersal represent too great a waste. The extremely low probabilities of germination justify the dominance of long-lived vegetation, since a strategy like that of annual plants does not make sense in an environment of this kind. There are even aperiodic species, with a development which lasts several years and which is interrupted, no matter what stage it is in, when winter arrives. This makes them independent of the short summer. The low elevation of the sun above the horizon during summer makes the sloping and stony soils heat up much more than the flat soils, so that they become genuine flower gardens which reflect the great importance that any small improvement has in such harsh conditions.

In these zones, herbivores play a role of primary importance. These are small rodents which spend the winter underground eating the tender parts of the vegetation covered by the snow, and deer which migrate annually when the long polar night arrives.

2.3 The seas and other "outsiders"

The general scheme that divides the planet primarily into large biomes leaves out some peculiar systems that, are sometimes quantitatively or qualitatively important. They are systems in which climatic factors are less important defining factors than others. In this sort of odds-and-ends box, we can place the marine systems and all the terrestrial coastal land systems which are influenced by salinity, together with terrestrial systems characterized by their isolation and as diverse as the Antarctic continent, high mountains, islands, lakes and caves.

The Antarctic domain

The immense Antarctic continent is almost a desert as far as plant life is concerned. The greater part of the surface area is permanently covered with ice and only in some marginal lands which thaw during summer do we find mosses, lichens, terrestrial algae and no more than three species of phanerogams. The marine systems which surround this continent are very productive and it is for this reason that there is a very abundant fauna (penguins, seals, etc.) which feeds in the waters and only uses the continent as a place to rest and nest, in other words as a purely physical, solid base and not as a productive landscape into which they become integrated.

The high mountain domain

As altitude increases, the climate becomes progressively colder and more humid and it is for this reason that mountainous regions present landscapes which are very different from the flat lands that surround them. The progressive change in climatic conditions is accompanied by a progressive change in the landscape, giving rise to what is called an altitudinal zonation of vegetation. This altitudinal zonation has often been compared to a shift to more extreme latitudes, but this comparison is not entirely true, as the conditions of sunlight, linked to latitude and not altitude, continue to be the ones that correspond to the area. Furthermore, on very high mountain ranges, when the height goes over a certain level, mists and mountain clouds disappear and the climate becomes dryer. This all goes to produce a series of conditions which are quite special and cannot be associated with any other zone on the planet. At the highest points of the great mountain ranges, other very important factors appear, such as the great intensity of solar radiation and the drying effects of the constant winds.

All these factors give rise to culminant landscapes made up of a shrub and herbaceous vegetation which is very resistant to the cold, dry conditions and high levels of insolation. In each case, though, local characteristics determine a considerable part of the vegetation's appearance as outside these great common environmental traits there are a multitude of differences between the tropical high mountains and those of medium or high latitudes. Biogeographically speaking, the uppermost parts of the high mountains are like islands or archipelagoes separated by impassable seas of low lands.

Isolated systems: islands, lakes, and caves

Islands, lakes, and caves are systems which are isolated from the biomes that include them. In these three cases, even more so than in the previous one, isolation is the essential factor which allows us to understand the structure and working of these systems.

Islands present landscapes which are certainly influenced by climatology, but the structure and composition of their ecosystems depend to a large extent on factors such as the age of the island, its surface area and its distance from the nearest continents. The total number of species, the degree of speciation and endemism, the diversity of the ecosystems and the variety of the environments depend more on these factors than on climatology. Islands are true biological laboratories which have allowed us to clarify many ecological and genetic mechanisms related to the occupation of space and to the process of speciation.

Lakes are aquatic systems influenced by their catchment basins, whose characteristics determine factors as important as temperature and water composition. But as in the case of the islands, their dimensions and history are factors that have to be taken into account when explaining the characteristics of their ecosystems. The relationship between their surface area and their volume, which determines the degree of interaction between the lake system and the terrestrial systems which surround it, is also an important factor. Their very pronounced boundaries have made them the paradigm of environments and, in fact, the first studies on the functioning of ecosystems were carried out in lakes.

Caves are the last example of systems in which isolation is a determining factor. Furthermore, they are very peculiar ecological systems as they lack primary producers and depend on the import of organic material synthesised in the bright, outside world, thus giving rise to biological systems formed exclusively by decomposers and secondary consumers which are well adapted to the general scarcity of resources that are found there. But the most interesting aspect of caves, apart from their isolation, is the constancy of the environmental conditions, characterised by their extremely high humidity and temperatures which hardly vary, even though they can be found in regions with a clearly seasonal climate. This has converted them, in temperate regions, into refuges for hypogeal fauna, which was abundant in the outside world in periods of hotter and more humid climates. The speciation due to isolation gives rise to nioendics which superimpose themselves on these paleoendemics or relics of previously more extensive faunas, giving rise to a series of faunistic groupings which are basic to our understanding the history of the changes in the outside environment.

Marine systems

Marine systems include both truly aquatic, and terrestrial systems which are heavily influenced by the presence of salt water. This is the case of the ecosystems of coastal salt marshes and mangroves in which the salinity of the waters is a factor which determines the structure and functioning of the vegetation, more than climatic factors do. Certainly, the composition of the flora and other aspects of the vegetational landscape, like the dominance of herbaceous plants, shrubs or trees, are clearly influenced by the climate. Mangroves, for example, are only found on tropical coasts protected by coral reefs and subjected to the effect of tides but not to the movements of the waves, but the water's salinity is clearly the factor that determined most of the properties and characteristics of these systems.

Marine systems are a world apart. They are not independent of environmental factors like temperature, but their structure and behavior have little to do with terrestrial systems. Coastal belts, which are neither very deep nor very extensive compared with the total area of the oceans, present systems or landscapes which can be classified or arranged in the same way as terrestrial biomes. Thus we can speak of tropical coral reefs or of Mediterranean benthonic communities just as we speak of tropical forests and Mediterranean sclerophyllous forests, even though the variability of the atmospheric conditions in marine environments is not as great as in the case of terrestrial ones and we can, therefore, distinguish fewer units.

The enormous areas of ocean, beyond these narrow coastal bands, behave in quite different ways. They are filled with landscapes which we could describe as transparent to the human eye, which is only capable of distinguishing organisms of a considerable size. Marine systems are dominated by microscopic primary producers, phytoplankton, which rapidly renew themselves and provide food for a second rung which is similarly dominated by minute organisms, zooplankton. Nutrients become sedimented and are dragged along by currents over distances of the order of hundreds or thousands of kilometres, contrasting with the short journeys that they make in terrestrial ecosystems.

Marine systems are basically influenced by the availability of nutrients, often associated with the upwelling of deep waters caused by strong currents. This source of subsidiary energy which returns sedimented nutrients to the sunlit layer is what determines whether a marine system is highly productive or whether it behaves like a terrestrial desert, as happens, in practise, in the greater part of the ocean surface. At all events, they are scarcely structured ecosystems, dominated by consumers and not by primary producers as happens in terrestrial systems. We, terrestrial organisms that we are, feel uncomfortable faced with these marine landscapes, which are too empty and immense. Perhaps this very notion of landscape is not particularly useful for describing and understanding these biological systems which occupy most of the planet.

3. The biosphere, a mosaic

3.1 Diversity within biomes

The biosphere is very complex, it resists being forced into simple classification schemes that have only a few categories, especially when we examine it closely. Scale of perception is a very important factor when determining the homogenous units which we can define on the Earth's living mantle. Thus, if we move far enough away, from the point of view of an observer in a satellite or a plane, the large units we define as biomes appear satisfactorily homogenous and with clear frontiers between them. As we move closer to the Earth, the panorama becomes more complex as we gradually appreciate the marked variability in space which the biosphere presents. It is not necessary to imagine the perception of an animal with much smaller dimensions than ours, to observe that on a human scale, the biosphere appears as a multi-colored mantle in which no two pieces are the same. The biosphere is a complicated mosaic in which different norms or regularities are observed depending on the distance of the observer from the Earth.

The observation and verification of this variability leads to a definition or recognition of subordinate units in the biomes. These units can be defined according to different criteria: whether they are strictly linked to the composition of the flora or fauna, or whether they refer to variables which define the structure and the functioning of these units, as in the case of the cover, or density, the biomass, production or the rate of turnover. In the first case, we take the presence of certain species as an indication of a set of environmental conditions, while in the second we pay more attention to the system's response as a whole.

Variability within biomes

The large units which we defined earlier can easily be subdivided according to physiognomic and structural characteristics. The degree of subdivision is variable as some present a much greater homogeneity than others. Thus, forests can be subdivided into tropical rain forests and montane cloud forests. But the montane cloud forests of America are different from those of Africa, for instance, because the species that comprise them are different. Biomes, then, must not be understood as discrete and real elements but rather as methodological reductions of a continuous reality.

On the other hand, a given biome's territory can feature all kinds of geographical and edaphic unevenness (rivers, lakes, cliffs, etc.) which introduce sharp and sudden modifications into the location's ecological parameters. This leads to changes in the species composition, as well as in the structure and functioning of the ecosystems in the biome, to such an extent that at the heart of one and the same biome quite different formations can co-exist.

Thus, for both biogeographical and ecological reasons, biomes are far from homogenous, although formations within biomes tend to more similar to each other than to formations in other biomes. This is all reflected in the names commonly used to describe many of these formations. In the world of savannahs and savannah-type woods, for example, we find names such as espinal, caatinga, sertao, campo, barbacuai, miombo, and so on: the local people see these as very different areas and that is why they give them different names, but on a global scale they all formations typical of the dry tropical biome.

Microenvironments

Within the landscape we can also define lesser units associated with a finer level of structure, which depends on microclimatic conditions. Factors such as the relief make the depth of the soil and the insolation change in a very marked way within relatively small distances. This means that under certain homogenous macroclimatic conditions, variable factors like the availability of water and nutrients, average temperature, thermal variation and the availability of light, show sharp gradients. These gradients involve changes which can affect the degree of development of a type of vegetation, and can even determine more important structural changes, like the disappearance of the tree layer or the substitution of one formation for another. Under general conditions of Mediterranean climate, for example, orientation and relief determine the presence of shrub formations, or of sclerophyllous woods, under similar rainfall conditions.

These effects linked to the microclimate are of considerable importance in boundary zones or zones of transition between biomes. Under these conditions, a small variation can imply a drastic change in the landscape as a boundary or limit is crossed. The borders between biomes or formations are less definable as we get closer to them, that is, as we observe them in greater detail. Edaphic conditions or the relief, when they present important variations, gives rise to borders which are anything but straight-edged or showing a smooth transition.

Limits and gradients

Environmental conditions can present more or less gradual transitions, depending on the characteristics of the relief. Thus in the case of an abrupt relief, there will be very marked transitions which will tend to give rise to very marked and defined limits in the landscape. On the other hand, in a smooth and homogenous relief, the transitions will also be smooth and give rise to gradual changes. In the first case, and especially in border zones between different formations or biomes, we will find a patchwork landscape, made up of clearly differentiated units. In the second case, we will find a landscape dominated by gradients with changes which lead almost imperceptibly from one type of landscape to another. These differences give rise to several different ways of understanding and describing landscapes. In the European continent, with a very marked relief, crossed by transverse mountain ranges and with considerable climatic differences, associated with a greater or lesser degree of continentality, and sitting between three large, different biomes, some landscape ecologists have described its structure as a mosaic. They use descriptive methods which define and distinguish between clearly differentiated and neighboring units, formed by groups of related species which are placed in a systematic category, in a scheme which is similar to the arrangement and classification of species. In the American continent, on the other hand, made up of large areas of relatively homogenous relief or broken by N-S mountain ranges, the landscapes change gradually. In this case the methods of description that have been developed are directed more towards the study of gradients than to that of borders, as they are faced with landscapes made up of different units of broad belts along which some species are gradually replaced by others, in a smooth transition between different types of landscapes or formations. It is perhaps for this reason that European and American phytosociology constitute two completely different schools as far as methodology goes. They provide, like the landscapes, responses to the different distribution of environmental conditions in space.

3.2 Humanized biomes

The mosaic formed by the biosphere's landscapes has become more and more complex. Humans have gradually transformed these landscapes and have introduced new factors that cause variation. The influence the human race has had on nature has changed considerably throughout its history, as has been pointed out in previous chapters. For a long time the heavy consumption of external energy has allowed humans to change the biosphere radically, to such an extent that today it is impossible to understand the distribution and structure of landscapes without taking human intervention into account.

The general trend for changes in the biosphere introduced by humans, as far as modification and transformation of landscapes is concerned, consists of a process of simplification and the introduction of new factors affecting spatial distribution. Structural simplification stems directly from exploitation which, in ecological terms, consists of a rejuvenation of ecosystems that places them in less mature though clearly much more productive states. The exploitation of forests to obtain wood, the conversion of forests to pasture or crop lands and the harvesting of fish resources are all different examples of the above-mentioned trend. In fact, whenever humans add a trophic level to an ecosystem, the lower levels experience a rejuvenation. Such rejuvenation, the reverse of the trend that leads ecosystems to their most mature states, encourages so-called opportunist species--ones that grow quickly and that are well-adapted to environmental fluctuations--because of their high reproductive capacity in particular. Humans, when transforming the landscape through exploitation, directly or indirectly benefit certain species whose role which was at first slight becomes pronounced and dominant. That causes a loss of diversity or, in other words, a reduction in the total number of species, accompanied by an increase in dominance of a few. If we compare the appearance of a forest, a pasture and a monoculture treated with insecticides, we get an extremely clear example of this series of transformations.

Exploited and exploitable ecosystems

Some ecosystems are clearly preadapted to exploitation. In other words, they experience structural and functional transformations of minor significance as a result of the extraction of resources. This is the case in systems previously adapted to fluctuating conditions, naturally dominated by opportunist species capable of rapidly rebuilding their populations. Formations dominated by herbaceous plants, like steppes and grasslands, and marine systems associated wiith areas of nutrient blooming are good examples of this. Humans effectively replace the action of grassland fires and large herbivores by transforming them into cereal farms whose structure is not that much simpler than that of the initial ecosystem; less drastic, but equally effective, is the straightforward changeover from the original herbivores to herds of domestic livestock. In the case of fishing grounds near nutrient blooms, humans exploit fish species that have an extremely high reproductive capacity and relatively short life span so that the populations can be rebuilt very rapidly.

In other cases, some characteristics of ecosystems make them better suited to certain types of exploitation. For example, the rich, deep soils in temperate deciduous forests can be turned into crop lands without any problem whatsoever, so long as the periodic application of fertilizers replaces the nutrients drained by the harvests. In this case, part of the system (the soil) copes well with exploitation, although the system as a whole suffers a far-reaching modification that turns it into something completely different.

A third case is systems that cannot cope with exploitation at all, and not necessarily because of difficult environmental conditons. This is the case of tropical rain forests and tropical coral formations. In both cases the ecosystems provide extremely favourable conditions and have a very substantial species and structural richness that cannot be matched by any other ecosystem on the planet. They are stable systems in stable conditions, though very sensitive to fluctuations and alterations. Most of the species that make up these ecosystems have a long life span and a relatively low reproductive capacity. The species are also very well adapted to particular environmental conditions. The system as a whole has a low level of resilience, in other words, it cannot readily recover from changes. In the case of tropical rain forests, the problem is worsened because of the structure of the soils: they are very poor in nutrients and not at all resistant to intense leaching by rain, an action that would be generated should the vegetation cover be removed. The model of agricultural exploitation from middle latitudes is obviously unsuitable for application in tropical rainy climates. Similarly, fishing exploitation in nutrient rich areas cannot be applied to coral reefs in warm seas.

Deliberate and involuntary alterations

Modifications made to the landscape by humans go far beyond simple transformations that stem from the extraction of resources or the replacement of vegetation by crops and pastures. In fact, unsuitable exploitation or overexploitation can lead to irreversible changes in a system as a result of degradation. This is the case, for example, of the overexploitation of wood in Mediterranean forests, excessive grazing or the cultivation of steep slopes within the same biome. Each one of these actions gives rise to intense erosion that destroys the already poor Mediterranean soils, which are often fossil soils or soils whose formation is extremely slow. Therefore, when these forms of exploitation are abandoned, because they are no longer productive, the original systems cannot be recovered; the degradation they have suffered prevents this from happening.

In very densely populated areas of the planet, territorial transformations go much further, as the effect of humanizing amounts to serious alteration of the landscape, changing it so as to adapt it to communication routes, housing, and other structures, all of which gives rise to new landscapes that only a few species are capable of sharing with man.

Furthermore, there are many alterations that are not caused by voluntary intervention, but are side-effects stemming from other actions. This is the case of the pollution and eutrophication of waters, the pollution of soils caused by the dumping of waste, the disappearance of species that cannot survive certain alterations in their environment, the side-effects caused by insecticides, atmospheric pollution, radioactive contamination or forest fires, to name but a few. Whether voluntary or not, the alterations are anthropic, induced by humans in the fabric of the biosphere.

Man's far-reaching tentacles

The human species represents but a small part of the biosphere's total biomass, yet its importance is by no means proportional to this minuscule fraction. Most of the planet's landscapes are humanized or, in other words, transformed directly or indirectly by man's activities. However, even in those landscapes where the human population is not permanent or is very sparse, the traces of humans are always present. The side-effects of pollution, of radioactive waste or of synthetic chemicals, for example, manage to reach all four corners of the earth.

Humans have changed, upset and complicated the mosaic of the biosphere that was itself already very complex. Humans have fragmented landscapes, causing a reduction in biological diversity but a multiplication of frontiers. They have directly or indirectly favoured some species, increasing their areas of distribution, whilst bringing about the disappearance of others, that were unable to tolerate the disturbances. They have humanized and disfigured the Earth's living mantle, either for their own benefit or as a consequence of side-effects which, although undesirable, have not been avoided. Therefore, as far as crops and livestock are concerned, with their associated weeds, fodder-producing plants, plagues and parasites, today's world displays a considerable level of homogeneity, due to what some authors have termed ecological imperialism, referring to the great success of the effects of colonization over the past 500 years by civilizations coming from temperate European biomes. Andean lands in Argentina and Chile, for example, are full of crops, trees, shrubs and animals that are typically European, which have displaced the indigenous flora and fauna to a lesser or greater extent.

Insular systems or those that are largely isolated have been hit particularly badly by the effects of these massive introductions of species connected with the colonization process. New Zealand, for example, which has been separate from the Australian continent for 100 million years and whose indigenous flora is 89% endemic, and which developed in the absence of large herbivores, saw its vegetation severely disturbed when European livestock were introduced in 1840. Some European plants, accidental human travelling companions, like furze (Ulex europaeus), bramble (Rubus fruticosus), broom (Sarothamnus scoparius) and the tree lupin (Lupinus arboreus), spread throughout New Zealand as if it were totally unoccupied, displacing vegetation that was incapable of competing with them. The rabbit, a permanent companion of European colonizers, has repeatedly plagued insular systems or those that are largely isolated. The 24 rabbits introduced into Australia in 1874 gave rise to a population of hundreds of millions in just a few years because of the absence of predators and diseases to control them. The introduction of foxes helped to keep numbers down but caused, as an indirect consequence, many marsupial populations to become rare, chiefly because they were not adapted to defend themselves against a predator of that type.

The same biogeographical revolution caused by man in some insular systems made them uninhabitable, as happened during the fifteenth century on the Atlantic island of Porto Santo (Madeira), when the colonizers were forced to leave by the uncontrollable population of rabbits. Donkeys introduced into Fuerteventura (Canary Islands) during the sixteenth century almost caused a similar exodus. In other cases, the biogeographical disturbance is more discreet and is limited to the disappearance of a few species without endangering the overall functioning of the systems. In the case of the island of Kerguelen, for example, sheep introduced in 1952 caused the virtual disappearance of two species of its flora (Pringlea antiscorbutica and Azorella selago) and the increase in numbers of a third (Acaena ascendens), which was rejected by the livestock and favoured by the lack of competition. The biogeographical revolution does not necessarily mean a reduction in the number of species: on the island of Reunion, for example, although the 5 indigenous species of mammals, 33 birds and five reptiles, were reduced to two, 14 and two respectively, they share their environment with 11 introduced species of mammals, 19 birds and 10 reptiles.

This general connectivity between biomes around the world is also taken advantage of by opportunist species and others with a high dispersion capacity. Ports and airports constitute recolonization centers from which new distribution areas open up for these new travellers. The Argentinean ant Iridomyrmex humilis, for example, appeared in New Orleans in 1891, in Madeira, Portugal, California, Cape Town and Chile between 1905 and 1910, on the Riviera in 1920, in Naples in 1936, in Melbourne, Hawaii and Australia between 1940 and 1950, and in Majorca in 1953, which just goes to show how it took advantage of transoceanic shipping lines. Outbreaks of malaria in the surroundings of European airports which have heavy traffic with tropical zones, caused by transporting mosquitos that transmit the disease, emphasizes the dangers that some of these small biogeographical revolutions can hold for man. Many sea organisms like molluscs and crustaceans also travel from one place to another by sticking to the hulls of ships or by simply being present in the waters used to fill ballast tanks. They then colonize areas far away from their initial ones. This spread can bring closely-related species into contact and, when crossed, give rise to new hybrids that can be viable or successful. The hybrid grass Spartina townsendi, for example, has successfully colonized large zones of the European Atlantic coastline.

Humans do not only link the continents by transport routes but also physically modify some of the biogegraphical barriers and even eliminates them. This is the case of the Suez canal, that has brought the waters of the Mediterranean into contact with those of the Red Sea, enabling 24 species of fish to pass from the latter to the former. In other cases, humans create peculiar environments within the biomes, that reproduce different climatic conditions, and involuntary transportation of species does the rest. Greenhouses in the temperate zone, for example, create a replica of tropical environments, and diverse species of seaweed, arthropods and Turbellaria typical of warm climates can be found in them.

The North American Indians called the plantain (Plantago lanceolata), very common along roadsides, "English foot," because it was associated with the presence of the colonizers. This close relationship emphasises the opportunist species' capability of rapidly colonizing altered habitats. In the last few centuries, these species have also become the privileged members of a biosphere that is ever more simplified and without frontiers. The old and absurd intercontinental bridges that paleontologists and biogeographers postulated in order to explain obvious relationships between species of different continents or separate specific areas, before the theory of continental drift provided a more realistic basis, have become much more of a reality, profuse than anyone would have dared to imagine. Humans have set in motion a biogeographical revolution whose consequences cannot easily be foreseen, full of notable events like the invasions by donkeys, rabbits and pigs, or like the tremendous thistle invasions of the Argentinian Pampa, or the orange groves that Charles Darwin found on the islands in the estuary of the Parana River. But there is also a far less spectacular side, consisting of changes that are more subtle yet no less radical or irreversible. Humans are homogenizing and simplifying the biosphere, changing the specific composition of ecosystems and modifying landscapes. The biosphere has and will

never be the same since humans learned to interpret the ocean winds and brought what had, in the distant past, been just one continent together again.

The end-product is a mosaic of virgin landscapes (the scarcest element), slightly altered landscapes (not very abundant), landscapes modified by forestry, livestock and agricultural exploitation (very abundant), highly modified or directly urbanized (found everywhere), and even completely eroded or destroyed landscapes (more and more frequent). The biosphere, complex by nature, is becoming even more diverse and difficult to interpret. In any event, however, this extremely humanized biosphere is the one we are left with. And it is, after all, the biosphere we now see.

3.3 Climatic change: the greenhouse effect

It is said that a new climatic change is taking place due to an increase in the Earth's average temperature. It is also believed that human action on the Earth is one of the main causes of this change: gases emitted by industries, cars, forest fires, cattle raising, paddy fields, etc., are making the greenhouse effect worse. In 1863 Svante Arrhenius, a Swedish chemist and the founder of modern chemistry, had already foreseen this. There is a great deal of truth and a great deal of speculation about this subject.

The hypothesis of a man-made climatic change

Although natural factors contribute, current climatic change is caused mainly by human action. Carbon dioxide and methane emissions increase the concentrations of these gases in the Earth's atmosphere, provoking the greenhouse effect. Short-wave radiation coming from the Sun warms up the Earth's surface, but the later emission of long wave radiation by the Earth cannot escape to outer space as these gases absorb it and reflect it back again to Earth.

It is very difficult, though, to separate this human-made climatic change from natural climatic change, that is, from normal fluctuations of the climate. The Earth has undergone climatic changes, some of them very considerable, throughout history. Separating the increase in temperatures observed this century from the planet's normal fluctuations is very difficult. Until the last few decades, meteorological data were scarce and collected by a variety of instruments, which makes interpretation very difficult. To this we must add that data from so few years do not allow us to be certain as to whether the increase in temperature which has been observed is due to the effect of humans, to natural causes, or an error in available data.

To evaluate the validity of the data, climatologists and physicists of the atmosphere use huge supercomputers to construct climatic models. The models are computer programmes which simulate a real phenomenon, in this case atmospheric circulation. These models are used to try and predict what would happen to the Earth's climate if different variables were changed, like the composition of the different gases in the atmosphere. However, the results vary: some models predict an increase in the average global temperature of 5-9[degrees]F (3-5[degrees]C), while others foresee a much smaller increase of only 1-2[degrees]F (0.5-1[degrees]C). The fact is that these models, although extremely complex (supercomputers have to be used to make the very complex and long mathematical calculations), are still too simple as they cannot take into account the myriad of complex processes which take place in the atmosphere and the oceans. For example, clouds, an extremely important atmospheric factor (their presence or absence can lead to marked changes in the terrestrial albedo), are not taken into consideration.

Even though the different models and different teams of researchers cannot come to an agreement over the degree of temperature change, they do agree on the qualitative aspect of this change. Most models agree in predicting an increase in the Earth's average temperature. This prediction also fits in with the meteorological data which we have available. These data show a generalized increase in temperatures. Thus, even though we are using data and models which are not ideal, we can assert, with a high degree of certainty, that the climate is tending to warm up.

The possible agents of the change

The possible natural causes are numerous and plausible. Thus, raising the enormous plains of Tibet and the western part of north America provoked important physical and chemical changes in the atmosphere which helped to shape modern climatic trends. In the past, important climatic changes could have been provoked by large changes in the relief of the land and the configuration of the continents. We can find another cause in the modification of the geometry of the Earth's orbit and the inclination of the Earth's axis, since orbital variations modify the climate as they alter the quantity of solar energy which the Earth receives at different latitudes and in different seasons. The existence of a connection between the history of the glacial periods and orbital variations has recently been demonstrated. Solar activity varies every eleven years. During these cycles the number of sun spots and eruptions changes. When the cycle is longer, solar activity decreases and that, in turn, makes the Earth's temperature fall. Although the formation of great plains and the geometrical changes in the Earth's orbit explain the climatic change only on a very large time scale, the change in solar activity could partly explain it on a much smaller time scale, of the order of 10 years.

However, part of the increase in temperature recorded this century is certainly due to the increase in the concentration of greenhouse gases, an increase produced by humans. Some of the gases which make up the atmosphere, mainly carbon dioxide (C[O.sub.2]), methane ([CH.sub.4]) and water vapour ([H.sub.2]O) are the so-called greenhouse [effect] gases. The short-wave radiation coming from the Sun can penetrate the atmosphere and warm up the Earth's surface, its oceans and continents. The Earth's surface, like any heated body, radiates heat--infrared radiation. Only infrared radiation of between 8 and 12 [micro]m can traverse the atmosphere and escape to outer space. The remaining, two thirds to three quarters of the radiation emitted by Earth, is trapped by those gases, which act like a heat trap, or like a screen that stops the Earth's radiation from escaping. Humans, through their activity, increases the concentration of these gases in the atmosphere and therefore increases the greenhouse effect, causing the Earth's temperature to rise.

One of the factors that most complicates the greenhouse effect is cloud cover. Clouds are water vapor and, as we have mentioned before, prevent long-wave radiation escaping into space. But clouds also increase the terrestrial albedo and, therefore, the reflection of solar rays back into outer space. Thus, with respect to climatic change, clouds present two opposite effects at the same time. On the one hand, they stop infrared radiation from escaping to outer space, and on the other hand they stop short-wave radiation from reaching the Earth's surface. The result of all this can be very variable, from a net gain to a net loss of heat, and depends on the type of cloud. Some clouds contribute more to the Earth's albedo, while others contribute more to the trapping of heat in the lower atmosphere. The high cirrus-type clouds which are flat on top and very bright, reflect more radiation coming from the Sun than they absorb from the Earth. On the other hand, cumulus-type clouds have the opposite effect. This added complexity due to the clouds is one of the most problematic factors when attempting to construct climatic models with computers that are capable of predicting future climatic changes with a degree of certainty.

Not only the clouds affect the global terrestrial albedo; human activity also directly modifies the nature of the Earth's surface, changing its albedo. The whole process of desertification due to human action is the most revealing example of this. When a rain forest is cut down and the soil is left exposed, the albedo suffers an important change and the reflection coefficient is much higher. These changes in the albedo of the Earth's surface change the proportion of energy retained versus energy reflected back into outer space in the form of short-wave radiation, and this has an effect on the thermal balance.

The possible consequences of climatic change

The phenomenon of the greenhouse effect and its accentuation through human action is extremely complicated. There are many factors which come into play. Which is a natural phenomenon with many variables, the links between them are still not well understood. For example, the increase in greenhouse effect gases increases the temperature on the Earth and, in turn, the amount of water vapour in the atmosphere. But no-one knows if this increase in water vapour contributes to an even greater increase in the temperature on Earth or on the contrary to a reduction of the same, thus counterbalancing the greenhouse effect. Evidently the consequences of one or the other are very different. The consequences associated with this climatic change induced by man can be catastrophic from the point of view of human societies. The extent of the catastrophe depends on whether the global climate heats up by between 0.9[degrees]F (0.5[degrees]C) or 1.8[degrees]F (1[degrees]C) or whether the rise in temperature is considerably greater, of between 7.2[degrees]F (4[degrees]C) and 9[degrees]F (5[degrees]C). Although predictions can have a high margin of error, the consequences if this climatic change were to occur, would be very great. It is a case where the uncertain predictions are coupled with a very real and serious risk. In other words, although we cannot affirm whether what has been forecast will happen or not, the consequences if it did happen are so great that they deserve the highest consideration.

A global increase in temperature does not mean that this increase would be uniform everywhere, at all latitudes and altitudes. In some places it will be more marked than in others. It has been predicted that temperatures will remain more or less the same at low latitudes while the biggest changes will take place at mid and high latitudes, winter temperatures at mid and high latitudes could rise to more than double the global average; summer temperatures would rise also, but less. We can understand the magnitude of this change if we consider that a temperature change of 1.8[degrees]F (1[degrees]C) is equivalent to a latitudinal change of 62-93 mi (100-150 km). Furthermore, it is believed these climatic changes would be irreversible on a human timescale.

A change of this type would make the forests migrate northwards. For example, it has been forecast that, because of the higher temperatures, the taiga would migrate to higher and consequently milder latitudes which are currently covered in tundra. It is not so simple, however as the vegetation's ability to adapt does not only depend on temperature but on edaphic conditions. It has also been forecast that in north America the limits of the prairies and the forests would move northwards at a speed of between 62 and 93 mi (100 and 150 km) per decade. We shall have to see if the edaphic factors help in this process or not, if the genomes of the species will be adaptable enough to perform such a rapid migration. If the climatic change does take place it is probable that the ecosystems will not have time to adapt to it, which could lead to important disturbances in them, with unpredictable consequences.

One of the most serious consequences is the rise in sea level: between 13 and 23 ft (4 and 7 m) as a consequence of the partial melting of the polar icecaps. We only have to remember that a large proportion of human settlements are situated in coastal regions throughout the world to get some idea of the magnitude of this phenomenon: cities like Barcelona or New York would be partially flooded. There is no doubt that this would lead to an important human displacement: millions and millions of people throughout the world would have to move to other areas. A considerable proportion of the surface area of the continents would end up under water, and very valuable areas would be lost. The combination of all these changes on a global scale would provoke an even greater human pressure on the environment than at present and could well be accompanied, in the worst cases, by social conflict and even war.

Change, Gaia and Humans

Supposing that global warming is a reality, we could face this situation in two radically different ways. We could let the Earth warm up and adapt to it, or we could take preventive measures. In either case, the costs associated with these measures are very high. It is therefore absolutely essential to lend all possible support to research in this area to clarify the situation, and so that the measures taken can be founded on a sound scientific basis. Adapting to an increase in temperature means taking corrective measures, like developing different types of crops suitable for the new climate, putting up flood barriers along coasts, feeding the oceans with carbon dioxide so that it turns into calcium carbonate and joins the marine sediments and reduces the quantity of carbon dioxide in the atmosphere, and so on. As for preventing climatic change, we could try to reduce the emissions of greenhouse gases, use more energy efficient technologies (cars that use less fuel, better insulated housing, etc.), change to alternative sources of energy (solar and tidal power, geothermic or nuclear power), and replant barren zones so that the vegetation absorbs more carbon dioxide, and so on. Whatever the case, one thing is certain: Gaia, if it really exists, will restore the dynamic balance which it never loses: humans, however may not.

212 The present appearance of the biosphere cannot be disassociated from human activity. Practically no landscape is free from changes directly caused or indirectly induced by human activities. Throughout the past few centuries, communication systems in particular have been weaving a dense network of roads and telegraph and telephone wires that connect, anatomically or physiologically, every corner with virtually every other. This view of a wood with a road running through it represents one of the most common landscapes: the humanized landscape, the biosphere we see.

[Photo: AGE Fotostock]

213 The Earth rotates during the year around the sun, describing an orbit whose intersection with the plane in which the sun occurs is called the ecliptic. During its rotation the Earth is at an angle and this inclination is what determines the seasons, since the sun's rays do not fall on the entire Earth's surface at the same angle.

[Diagram: Editronica, from various sources]

214 Distribution of the mean world temperature in the months of January and July. The isotherms, the lines that join up points with the same temperature, show a certain uniformity in an E-W direction due to the progressive decrease in insolation from the equator to the poles. This uniformity can be better seen in the southern hemisphere, below the 25th parallel, where there is more ocean than continent. In the northern hemisphere the iso-therms show considerable deviations towards the north or the south when they cross continents to the oceans; especially in the month of January, when the contrast between the temperatures of the continents and the oceans is most marked. In general terms, one can state that there is a temperature gradient from the equator to the poles and an increase in the fluctuations in temperature in the same direction. The mean temperature in the northern hemisphere is 59.4[degrees]F (15.2[degrees]C) while in the southern hemisphere it is 37.9[degrees]F (3.3[degrees]C), but the difference between the means in January and July is 57.7[degrees]F (14.3[degrees]C) and 45.3[degrees]F (7.4[degrees]C) respectively. The maximum temperatures of the Earth are around 158[degrees]F to 176[degrees]F (70-80[degrees]C), taken at the surface of dry dark soils, in a calm atmosphere; they are 136[degrees]F (57.8[degrees]C) in the shade, while the minimums recorded are--108[degrees]F (77.8[degrees]C) to -117.9[degrees]F (-83.3[degrees]C) in Antarctica. In more southern zones of the southern hemisphere there are no records of temperarure changes because of the lack of fixed observatories to record data continuously.

[Cartography: Editronica, from various sources]

215 Mean annual precipitation. In the equatorial zone, rainfall is very abundant, generally more than 79 in (2,000 mm) annually, since the high temperatures and the large expanses of ocean produce a great amount of water vapor. In the centers of high pressure in the subtropics, rainfall is scarce: this is the reason for the deserts in North Africa, Arabia and Iran, South Africa, and the west coast of South America. The effect of the trade winds on a coastal chain of mountains produces orographic rainfall as in Central America and Madagascar, where the rainfall can exceed 79 in (2,000 mm) annually. The dry coastal belt of the Peruvian littoral and the Kalahari desert are situated to the east of the subtropical oceanic anticyclones, where the air descends in a parcel from above and is warmed because of the vertical movement (adiabatically). The rains of the subtropical zone are zenithal, while those of the extratropical temperate zone depend on wind circulation (cyclonic).

[Cartography: Editronica]

216 The circulation of the atmosphere and of the oceans is due to differences in insolation between regions at different latitudes and tends to redistribute the heat on a global scale. The atmosphere is heated up from beneath (at high pressure) and cools down above (at low pressure), while the sea warms up and cools in the same area, at the same pressure. For this reason convection cells in the atmosphere are vertical, while in the sea they are horizontal. The air of the atmosphere circulates between the equator and the poles, but recirculates also in each of the convection cells.

[Cartography: Editronica, from various sources]

217 Surface circulation of oceanic water, in which the cold currents are indicated by blue arrows and the warm currents by red arrows. Note the circular type movements surrounding the subtropical anticyclones, at some 25[degrees] or 30o latitude N or S. An equatorial current indicates the trade wind belt (although the trade winds blow towards the NW and SW, the movement of the water follows the parallels). The N and S equatorial currents are separated by an equatorial countercurrent. At low altitudes and along the western edge of the continents, the equatorial current is directed towards the poles and creates a current parallel to the coast. As regards the poles in the northern hemisphere, where the Glacial Arctic Ocean is surrounded by continents, the cold water moves towards the equator along the western edge of the straits that connect it with the Atlantic basin. In the Antarctic region the there is an Antarctic polar current which moves clockwise.

[Cartography: Editronica, from various sources]

218 Pluviothermic diagrams bring together and relate the two principal climatic parameters, temperature and rainfall, based on their absolute annual values and annual distribution. A quick look at the diagram gives an intuitive idea of the climate of the place studied. The situation in a map of pluviothermic diagrams obtain-ed in different parts of the world allows an easy comparison to be made of their respective climates and to distinguish homoclimatic areas, that is, areas of the world with the same climate. These are bioclimatic zones or domains.

[Diagram: Editronica, from various sources]

219 Distribution of land and sea by latitude in an ideal continent, in which the asymmetry of the vegetation zones in the northern and southern hemispheres, which coincides with equally irregular distribution of the climate, can be observed. The symmetry is maintained only at tropical latitudes, where it coincides in both hemispheres, but as one moves away from the equator, the asymmetry of the climate and the vegetation, and of land distribution, becomes more evident.

[Diagram: Biopunt, from H. Walter, 1976]

220 The climatic zones of the Earth, according to the classification of Koppen. The map shows the areas of the Earth with a similar climate and defines five major categories, as well as the high mountain. It allows one to appreciate that there is a certain symmetry on both sides of the equator, which is lost as one moves away from the equatorial belt. But there are asymmetrical conditions to the N and S of the equator, caused by the predominance of oceanic surface in the S hemisphere that affects the climate in the sense of making it cooler and more in equilibrium and leads to the temperate zone being little developed (only South America passes slightly the line of 40o latitude S). In the Southern hemisphere the equivalent of the boreal zone is lacking.

[Cartography: Editronica, bas-ed on Strahler, 1984]

221 The contrasts in landscape that correspond to local variations of the general climate in a particular area, can be very important as this photograph shows (Montseny, Catalonia]. In the northern hemisphere, on north-facing slopes, with relatively little sunshine, there are beechwoods that are deciduous, soft-leaved trees that like damp and cool conditions. In contrast, the south-facing slopes, under the Mediterranean sun, are covered with holm oaks, which are woods that are evergreen and coriaceous and tolerant of dry and hot conditions (see an autumnal view of this same zone in fig. 257). This kind of microclimatic change complicates considerably the structure of the landscape in areas with marked relief since it increases the diversity of ecological conditions.

[Photo: Ernest Costa]

222 The presence of humus manifests itself in the darkening of the upper horizons of the soil by the phenomenon known as mel-anization. The speed of humification and the type of humus that ensues in a particular soil depends on the plant remains, the class of mineral substrate, and the climatic conditions.

[Photo: AGE Fotostock]

222 The presence of humus manifests itself in the darkening of the upper horizons of the soil by the phenomenon known as mel-anization. The speed of humification and the type of humus that ensues in a particular soil depends on the plant remains, the class of mineral substrate, and the climatic conditions.

[Photo: AGE Fotostock]

223 The weathering of soils and their components is shown in this diagram in which the environmental factors are indicated in a hypothetical transect going from the pole to the equator. Given that the temperature and degree of humidity decreases from the equator to the pole, the degree of weathering, like the decomposition of organic matter, is slower as the latitude increases in the Northern hemisphere.

[Diagram: Biopunt, from various sources]

224 In cold zones, in the Chukchi peninsula (in the extreme east of Russia, asiatic zone of the Bering Strait), the cold provokes the fragmentation of the minerals in the soil by the formation of ice and its subsequent thawing; this phenomenon is known as gelivation.

[Photo: Serguei A. Balandin]

225 Red earths are fossil soils characterized by having an A horizon formed by impurities in the underlying calcareous rock, which undergoes a slow dissolution. The reddish color is caused by the high content of iron oxides, which were usually formed under past climatic conditions.

[Photo: David Badia / Biopunt]

226 The horizons in the soil are formed by the joint action of physical, chemical, and biological processes. Weathering is the process of alteration of the rocks and minerals that are found on the surface of the Earth, since in these locations the materials are exposed to the characteristic climatic conditions of each zone.

[Photo: David Badia / Biopunt]

227 Within its great diversity, the characteristics of humus are very good indicators of global environmental conditions, since they combine biotic and abiotic factors over long periods of time. For this reason, a classification of them into large families that correspond to different environmental conditions has been made. A distinction is made between little-developed humus (peat, mor, moder, carbonated mull) and developed humus (mull, anmoor).

[Source: data supplied by the author]

228 Determination of the texture classes of the soil, according to the classification of the International Society of Soil Science and of the USDA (United States Department of Agriculture); different particle sizes have to be taken into account in determining the textures, as indicated in the diagram.

[Diagram: Biopunt, from various sources]

229 Both roots and the activity of soil fauna contribute considerably to the structure of soils; in particular conferring on them a spongy structure as shown in this profile. Slowly but relentlessly, the the movement of soil fauna, and even more their feeding and digestive activities, constantly turn over the soil particles. The roots, in a similar way, in opening up the way and absorbing nutrients and, and fauna enter into a profound interactive relationship and confirm that the soil is the interphase between the biosphere proper and the inanimate fraction (which is quantitatively dominant) of the planet.

[Photo: Anne and Jacques Six / Firo Foto]

230 Naming and characterization of the soil orders, according to Soil Taxonomy (1975).

[Source: data supplied by the authors]

231 Naming and characteristics of the soil units in the FAO system (1989).

[Source: data prepared by the authors]

232 Comparison of soil classifications (p.p.=pro par-te)

[Source: data prepared by the authors]

233 Edaphic humidity regimes, determined by the number of years in which certain requirements of numbers of days with dry soil are met. [s=summer; w=winter; st=soil temperature.]

[Source: data prepared by the authors]

234 Soil temperature is a factor that affects the agricultural use of soils, which the Soil Taxonomy quantifies according to the accompanying Table (temperatures taken at 20 in (50 cm) depth, or in lithic or paralithic contact, expressed in degrees Celsius). [p=in part; w/h=with horizon; s/h=without horizon; mast=mean annual soil temperature.]

[Source: data prepared by the authors]

235 Podzols are the climatic climax in boreal zones, while in zones at lower latitudes they may be a seasonal climax. The very humid climatic conditions and the acidifying vegetation, generally of conifers, are favorable factors, but it is the very permeable rocks--which are poor in minerals and that are easily modified--that accelerate the process of podzolization.

[Photo: David Badia / Biopunt]

236 Soils with a high concentration of salts provoke an increase in osmotic pressure, which has a negative effect on the growth of most plants.

[Photo: David Badia / Biopunt]

237 Laterites are soils of humid tropical climates and originate from siliceous rock through an intense process of leaching, as a result, the humus tends to break down rapidly; with the appearance of ferruginous crusts and the impoverishment in organic matter and nutrient elements, these soils become sterile.

[Photo: Geoscience Features Picture Library]

238 The lack of aeration in hydromorphic soils does not allow the roots of plants nor microorganisms to breathe, due to the lack of oxygen. This favors anaerobic microorganisms and leads to certain chemical elements (such as iron) with more than a single valency to change to their reduced states. This process is known as gleying (or gleyzation).

[Photo: David Badia / Biopunt]

239 The great edaphic domains of the world established from the data of the Soil Conservation Service of the United States Department of Agriculture, and based on the criteria of the Soil Taxonomy.

[Cartography: Editronica]

240 Epiphytism, a relationship in which the epiphyte does not harm the host but uses it only as a support, is an important phenomenon in tropical rainforests. In these, the humidity of the air allows mosses and lichens, and especially ferns, orchids, and bromeliads to grow on the trees without taking water from the soil. Evergreenness and richness are outstanding characteristics of the rain forests, in which there is a great exuberance of life.

[Photo: Waina Cheng-Oxford Scientific Films / Firo Foto]

241 The geographical interpretation of the various bioclimatic domains of the planet has given rise to many graphic interpretations. In fact, the differences between the various maps are more common and marked with regards the frontiers of the different climatic zones than with regards to the types of biomes considered; even so, it is still possible to find very different classifications, as a result of mixing or separating particular formations, thereby increasing or de-creasing the number of biomes considered.

[Cartography: prepared from various sources with the help of the World Conservation Monitoring Center, Cambridge / Simon Blyth]

242 The vegetation of the savannahs, herbaceous formations with scattered trees, shows the phenomenon of coevolution with the animals that live there. The acacias, in particular, share features in common with the majority of trees of these formations through morphological convergence: coiled trunks, corky bark, xeromorphic and coriaceous leaves, and spines on the trunks that prevent grazing by herbivores. The herbivores have shared their eating habits between the various strata of vegetation. In the photograph, a group of Waller's gazelle (Litocranius walleri) browse shoots and tender stems of Commiphora in a savannah-like wood in Kenya. These gazelles are able to stand on their hind legs to reach lower the branches. Other strata of the savannah-like vegetation serve as food for other animals.

[Photo: Mitch Reardone / Ph. Stone International]

243 Desert plants, like this aloe (Aloe dichotoma) from the Namibian desert, survive with very little water but have devised various strategies to do so; some manage to obtain an amount of water that is comparable to what is normal for plants of temperate or even humid climates. In the particular case of succulent plants, the system consists of keeping the stomata of the leaves closed during the day to avoid transpiration, without having to limit photosynthesis; at night the stomata open and take in carbon dioxide with which they make organic acids that they use to undertake photosynthesis during the day. In short, they photosynthesize with little transpiration, which is the strategy of so-called C4 plants (see also figure 121). The succulent habit of the stems in the case of the plants in the photograph, is shared by various families, such as the Cactaceae, re-stricted to the American continent, the Euphorbiaceae, especially in the African deserts, Crassulaceae, and others. In the coastal desert of Namibia, which can be considered a cold desert because of the humidity and the almost total lack of rain, the vegetation consists of succulent plants and acacias; in the granitic areas, spiny euphorbias, succulent members of the geranium family (Sarcocaulon), and aloes (Aloe dichotoma) are dominant; there are extensive stony areas without vegetation, and where there are sand dunes, the leaves of the very bizarre welwitschia (Welwitschia mirabilis).

[Photo: H.R. Bramaz / Firo Foto]

244 The dark, tough, persistent foliage of the bushes of Aulax cancellata is characteristic of the South African Mediterranean vegetation. Active all year round, the mediterranean vegetation can resist the winter cold and the long summer droughts due to its sclerophylly; a strategy which consists of adopting an active water regime, which consists of making the maximum use of the water that falls during the period when the vegetation is dormant, i.e. the winter, and at the same time saving as much of it as possible during the summer, through regulation transpiration, by closing its stomata.

[Phot: Colin Paterson-Jones]

245 The humid Japanese forests (in the Kyoto region in the photograph) corresponds to the most typical laurisilva, (i.e the moist woodlands of warm temperate climates), typical of the regions with a so-called Chinese climate (South China, Korea, and Japan) and the Macaronesian region (Canary Islands, A-zores, Madeira, and Cape Verde). It consists of a transitional form of woodland in which evergreen dorsiventral-leaved plants form the greater part of the multi-species canopy, while the undergrowth is made up of small bushes, lianas, and many epiphytes.

[Photo: Phototheque Stone International]

246 The strategy adopted by deciduous temperate woodlands is clear and well known: it is not so much a question of adapting to the cold as avoiding it, by losing its leaves during the unfavorable period. This is the main feature of deciduous woodlands in temperate environments. The photograph shows the physiognomic aspect of the wood, which adopts four different aspects each year. During the year, nutrients all go through a phase of movement, a phase of storage in the leaf litter and in the wood, and a phase of chemical transformation and mineralization. One can deduce that in this system, the decomposer microfauna play a qualitatively and quantitatively important role, while the above-ground fauna has a more limited role; they have less biomass and both their presence and their activity are also interrupted by the winter resting period.

[Photo: AGE Fotostock]

247 The Eurasiatic steppe, as in the photograph, at 328 ft (100 m) altitude, W of Hujirt in Arhangay (Mongolia), is steppe par excellence; the term steppe comes from there and it represents the richest, most diverse and typical in the world. The vegetation of these biomes is emblematic steppe herbaceous, dominated by grasses, or shrubby and xerophytic and continental in nature.

[Photo: Paolo Koch / Firo Foto]

248 When the cold is very intense during much of the year and the vegetative period is reduced, without reaching such extremes that trees cannot grow, needle-leaved perennials dominate the landscape. The small and very hard leaves are able to resist the rigors of the cold winter and begin to photosynthesize as soon as the temperatures rise at the beginning of the favorable period. The crowns, which are conical in shape, are well able to resist heavy snowfalls without suffering the significant damage that the snow causes in trees with a rounded crown and branches that stick out from the trunk at an acute angle.

[Photo: Tom Tracy / Photo-theque Stone International]

249 The young Arctic tundra formation, which has appeared since the ice retreated from the zone, occupies the coldest lands of the Northern Hemisphere. The reindeer (Rangifer tarandus), one of the most representative animals of the northern territories, is found throughout the Arctic region. The lifestyle of the reindeer is very much conditioned by the climatic conditions of the zones where it lives. In the northern winter, when the low temperatures linked with a drastic reduction in the hours of sunlight stops all plant growth and activity, the reindeer leave this biome and move south to occupy the taiga zone where more food is available. When the days lengthen, the reindeer move north again. In the short period of vegetative activity that the tundra experiences, the reindeer take advantage of the active growth of the vegetation to feed, as the rising temperatures make the snow disappear.

[Photo: Michael Friedel / Firo Foto]

250 The Antarctic biome, which is found only in the Southern Hemisphere, is separated from the rest of the continents by extensive circumpolar oceans. This land area occupies some 5.3 million mi2 (13.8 million km2) of which 98% is covered by ice. This means that the vegetation is scarce, but not the fauna. The richness of the waters that surround the Antarctic allows the growth of a large variety of animals. There are extensive populations, which make use of the continent only as a physical base for resting or for rearing young but not as a place to seek food.

[Photo: AGE Fotostock]

251 The landscapes of high mountains show some common features that clearly distinguish them. On the one hand, high mountains are subjected to the influence of the climate of the lowlands that surround them, but have a peculiar and unique climate that stems from a combination of latitude and altitude. On the other hand, due to their isolation, the flora and fauna are highly specific. This, combined with the spectacular adaptations to the harsh environmental conditions and the relative simplicity of the landscapes, makes the presence or dominance of particular species or biological forms stand out, and results in the landscapes of high mountains being so peculiar and distinct from each other.

[Photo: Christer Fredriksson / Bruce Coleman Limited]

252 The geographical situation of an island and its topographical relief determine the diversity of environmental conditions that are found in the interior and which, in effect, means that landscapes corresponding to one or more kinds of biome are dominant there. However, other features such as overall size of the island, its distance from the nearest continent, and the time of its separation, determine the number of species that grow there, as well as the degree of endemism or differentiation shown by its flora and fauna in relation to continental species. The biological systems of the island evolve when they separate from the continent, or they build up and structure themselves slowly with colonizing species as, in the case of recently formed islands such as volcanic ones.

[Photo: AISA]

253 Caves are systems in which the lack of light prevents the existence of primary products. Bats, which feed outside the caves and use them only as a place of rest or hibernation, constitute an important source of entry of organic material. Their feces are an importance source of food for many species of arthropod, which in turn serve as a food for other species of the cave system. The bats are not the only source of entry of organic material into the caves, but their abundance and their habit of concentrating in great numbers makes them a key element in the ecological functioning of many subterranean cavities.

[Photo: Josep Loaso]

254 The biological basis of marine systems is made up of microscopic organisms: plankton. Unlike terrestrial systems dominated by primary producers of considerable size and longevity, and having a very complex structure and a relatively slow dynamism, marine systems are much more dynamic. They have a base of primary producers that have a much more simple structure and high turnover rate. Marine landscapes, apart from not corresponding to the terrestrial notion of landscape because of their loose structure, are so dynamic that they can be considered landscapes

almost without memory; influenced only by recent history and not by a long cumulative process as occurs in terrestrial ecosystems.

[Photo: Claude Carre]

255 Phenomena are perceived differently according to the scale at which they are observed. On the scale of the accompanying photograph, one can see on this rock covered with lichens, a unique community where a number of elements repeat themselves in space in a more or less regular manner, while a more detailed view of a small fragment of this rock would make different species observable (in this case species of the genera Rhizocarpon, Lecidella, Per-tusaria, Aspicilli, Hafellia, etc.) and would show that there is a solution of continuity going from one species to the neighboring one. The same happens with biomes and the diversity within them: the biome which appears to be homogeneous at the scale at which it can be seen from a satellite analagous to (our view of the lichens that cover the rock), can appear very polymorphic when seen from one of its subunits (the view at the scale of a small arthropod inhabiting one of the lichen species).

[Photo: AISA]

256 Valley bottoms, be-cause of the greater availability of humidity coupled with the greater quality, or depth, of the soil, and the rainfall, have a different appearance from that of the slopes. In some cases, this microenvironmental difference causes a relatively slight difference in landscape, since the conditions remain similar and allow the presence of similar or identical biological forms. In particular situations, however, the differences lead to a major contrast as in the case of a zone that is near the boundary between two biomes, a zone in which a small difference represents the crossing of a threshold. In the photograph, the severe degradation of the slopes by erosion drastically reduces the water retention capacity of the soil. In this situation, in a matter of a few meters one passes from a degraded zone to a riparian wood.

[Photo: Ernest Costa]

257 Each biome corresponds to a major biological strategy of adaptation to the prevailing climatic conditions; which in the case of the habitat shown in the photograph, (Ripolles, Osona, Catalonia) involves the installation of sclerophyllous woods, that is, trees with small, tough, evergreen leaves, adapted to tolerating the small amount of water available and the marked summer drought characteristic of the Mediterranean climate. However, within a biome, there may be penetrations by other biomes, due to variations in, for example, altitude or slope: this will also be the case in this landscape where intrusions of the biome corresponding to temperate deciduous woodland are found in hollows or on cool, damp slopes (see also figure 221). Normally in the contact zones between two biomes, or in mountains subjected to several altitudinal climatic changes, this kind of interpenetration of biomes may be found.

[Photo: Ernest Costa]

258 Of the main remote sensing satellites, among the most sophisticated are the series of French SPOT (Satellite pour l'Observation de la Terre) satellites, which allow us to obtain data from the Earth's surface at a resolution of 33 ft (10 m). The significance of environmental problems today has led to the design of satellites that will allow us to follow the state of the biosphere to decide courses of action to improve the global environment. For example, the European Space Agency (ESA) has a satellite, the ERS-1 devoted exclusively to obtaining information about the environment; it has been designed in such a way that the operationally important data can be transmitted to users in less than three hours. The data obtained from the last generations of satellites, the ERS, for example, allow one to make weather and marine forecasts; monitor the development of frosts; detect the causes of pollution; assist agriculture, forestry, and fishing; and explore mineral resources. (a) MSS: Multi-Spectral Scanner; (b) TM: Thematic Mapper; (c) Sea Wide Field Sensor; (d) Thermal Infra Red; (e) Synthetic Aperture Radar; (f) Coastal Zone Color Scanner; (g) Heat Capacity Mapping Mission; (h) European Remote Sensing Satellite; (i) Active Microwave Instrument; (j) Wind Scatto-meter; (k) Radar Altimeter; (l) Along Track Scanning Radiometer; (m) Microwave Sounder; (n) Marine Obser-vation Satellite (Japanese); (o) Multispectral Electronic Self-Scanning Radiometer; (p) Microwave Scanning Radiometer; (q) Indian Remote Sensing Satellite; (r) Linear Imaging Self Scanning.

[Source: data supplied by the author]

259 Often the transformation of the land by humans is so profound that it is difficult to imagine what the landscape looked like before human intervention. In some places and situations, such a reconstruction is very difficult such as in the zones of transition between different biomes and in territories that have been subject to change for centuries. In many cases, human activity is an environmental factor that shapes the landscape as effectively as climatic or edaphic factors.

[Photo: A. Tovy / Index]

260 Some ecosystems, such as tropical forests, are very sensitive to the changes introduced by humans in the course of exploitation. The great fragility of tropical ecosystems--apparently so abundant and productive--combined with techniques for exploitation, and management methods imported from temperate biomes, which have been shown to be quite unsuitable for application outside the environmental conditions in which they were designed, have led to the irreversible destruction of large areas of tropical forest, such as those of Madagascar shown in the photograph. In many cases, however, this destruction is caused purely and simply by overexploitation, which takes only immediate benefits into account.

[Photo: Frans Lanting / Minden Pictures]

261 The case of rabbits introduced by the Euro-peans into the continent of Australia for game, is probably the best known of the biological invasions caused by the continuous traffic of species that humans have practiced either voluntarily or involuntarily for a variety of reasons. In Australia, a modest game animal became converted, in the absence of predators and without competitors that could stop it, into a veritable pest both for agriculture and for the natural ecosystems of the continent. The large population densities reached means that, even today, the rabbit destroys crops and makes the regeneration of natural vegetation difficult.

[Photo: Brendan Beirne / Auscape International]

262 The percentages of introduced species in various floras. Bearing in mind the intense and indiscriminate traffic in species that humans have undertaken as a result of the links between the different continents and the lands of the planet, the percentage of introduced species in the floras of different countries can be considered as giving an indication of the susceptibility to invasion of different environments. Thus, in contrast to percentages in the 15-20 percent range in continental zones, we find other areas that have much higher rates of introduction, such as 20% in the Hawaiian islands and the 47% in the territory of New Zealand area have been isolated for a long time. One has to take into account, however, that these percentages do not give a clear idea of the degree of environmental change caused by these invasions, since, sometimes a single species can cause drastic changes if it gives rise to very large and extensive populations.

[Source: Drake et al. (1989)]

263 The common starling (Sturnus vulgaris) was introduced in North America in 1891, when 100 specimens were released in Central Park, New York. Its great capacity to compete with native species for food meant that in only 50 years it spread throughout nearly the whole of North America. Humans were the vehicle, facilitating a transatlantic journey that they could never have managed by themselves, but once in North America their great competitive capacity allowed them to spread rapidly.

[Diagram: Biopunt, from At-tenborough, 1989]

264 The human-induced alteration of most forest landscapes of the world is seen clearly in this wood in the Azores (island of Sao Miguel); none of the dominant species there belongs to the native flora of the archipelago. In effect, the original laurisilva, which presumably covered nearly the entire area before humans arrived, was almost totally destroyed by repeated fires in the first years of settlement by the Portuguese and later the Dutch (in the fifteenth and sixteenth centuries). Native laurisilva has been replaced by crops, pasture, forest plantations, or buildings. Thus lush "pine" woodland is in fact a plantation of the Asian conifer Cryptomeria japonica, and the undergrowth is covered by the spontaneous Asian exotic Hedychium gardnerianum (Zingiberaceae), which was introduced to the archipelago accidentally, and by the hydrangea (Hydrangea opuloides), a third introduction from eastern Asia, which escaped from the hedges of the livestock enclosure. Today one can find similar kinds of landscape throughout the world, which are stable as this case, or might be in the need of constant human attention.

[Photo: Ramon Folch]

265 The large urban and industrial centers act as a focus for pollution and suffer especially from its effects, because of the accumulation of gases and particles under certain weather conditions. This high frequency of pollution--as well as creating poor environmental conditions from a health point of view--leads to certain changes such as the increase in the number of days with mist and rain and an increase in mean temperatures. This shows clearly how human activities can lead to important environmental changes.

[Photo: Bruno Barbey / Zardoya]

266 Sun spots are an indication of the amount of activity at different points and regions of the star that is found at the center of the solar system. The number and extent of the sun spots, which are a measure of the amount of radiation that reaches the surface of the planet, varies from year to year, but follow a cycle of about 11 years. The effects of these variations on processes such as photosynthesis and the growth of plants has been shown. There is no doubt that they contribute to the variation in the meteorological conditions of the whole planet from year to year.

[Photo: John Bova-Photo Re-searchers, Inc. / AGE Foto-stock]

267 The seriousness of the effects caused by a climatic change, though small, has led to the investment of considerable resources into the collection and processing of information about climatic variables. Observation satellites, which for some years have been gathering data on the planet, provide vast quantities of data that are very useful for weather forecasting and essential for studying trends in climate change. The meteorologists and students of climatic change are always faced with the uncertainty as to whether the flitting of the wings of a butterfly in Asia will provoke a cyclone in the Caribbean: more seriously, they have to deal with phenomena that they cannot forecast, at least for periods of more than 24 to 48 hours.

[Photo: Center for Medium Range Weather Forecast]
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Publication:Encyclopedia of the Biosphere
Date:Jan 1, 2000
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