Chapter 2: Plants and ecology.
After completing this chapter, you should be able to:
* Present the relationship of worldwide climate to plant growth
* Explain the term ecology
* Describe the limiting factors for the growth of plants
* Discuss the cycling of the ecosystem
* Illustrate the food web and food pyramid
* Describe ecological succession and recolonization
How Plants Affect Ecology
In recent years, conflicts between resource exploiters and conservationists have led to a public awareness of ecology, even though many of the vocal advocates of both points of view are poorly informed of what ecology really means. The term is derived from the Greek oikos, meaning "home." Thus, in a broad sense the home is simply the habitat for all living organisms, and ecology is a study of the factors that allow organisms to grow, compete, reproduce, and perpetuate the species. It is a study of the total environment and interrelates with many traditional fields of plant science.
A more specific question can be asked: Why do the producer organisms--green plants--live there? Here the answer is much more complicated. This question can be answered by analyzing the abiotic (nonliving) environment. One plant may live where it does because the light, water, temperature, nutrients, soils, relative humidity, wind, and other factors are satisfactory for growth and reproduction of the species. But mere survival is not enough; the plant must have sufficient quality of growth to allow reproduction and perpetuation of the species. Some time humans settle for less. For example, the cotton plant grows from year to year in the warm environment of the tropics, but agriculturists have seen fit to move the cotton plant to a more temperate climate where it cannot survive the winter. The farmer's only concern is with an economic yield (fibers and seeds from the fruit). It makes no difference that frost in late fall kills the plant, providing that the fruits (cotton bolls as in Figure 2-1) have matured. The farmer will simply plant cottonseed again next spring.
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Buying a tropical foliage plant for your home or office, on the other hand, is a completely different situation. With varying degrees of success, you will attempt to care properly for your plant year round. Those of us who have had our houseplants die anyway may feel that we are not blessed with the innate ability to grow plants; we lack a 'green thumb.' In fact, we may actually feel that we are afflicted with the brown thumb syndrome; somehow, mysteriously, the ultimate death is not incurable. In fact, the trick to successfully growing plants is no trick at all; rather, it is an understanding of the combined effects of water, nutrition, light, humidity, temperature, and pest control. Houseplants are not specifically bred for inside growth but are really outdoor plants that are well adapted to their native environment, usually a tropical one. When plants are introduced; into areas that are genetically adapted, their needs go unsatisfied. Therefore, we must compensate artificially for the requirements of houseplants--the better we can modify their immediate environment, the more successful we will be in ensuring their healthy growth. The more we know, the greener our thumbs will become.
WHAT IS AGROECOLOGY? Our language produces many new words. For example we understand agronomics, agronomy, and ecology but what about agroecology? K. H. W. Klages, an agronomist at the University of Idaho, is credited as one of the first to discuss ecology and agriculture. The term agroecology appeared in the late 1970s. It started from the recognition that green revolution-era agroecosystems were highly dependent on inputs such as pesticides, capital-intensive machinery, and specific seed varieties. According to various sources, agroecology is the application of ecological concepts and principles to the design and management of sustainable food systems. Agroecology infers the linking ecology, socioeconomics, and culture to sustain agricultural production, farming communities, and environmental health. It is holistic in its approach meaning it is comprehensive and integrated when considering all elements of the many systems associated with agriculture. The methods of agroecology have as their goal achieving sustainability of agricultural systems balanced in all spheres. This includes the socioeconomic and the ecological or environmental. While farming methods vary, traditional manipulated "agroecosystems" generally differ from natural ecosystems in six ways: maintenance at an early successional state; monoculture, crops generally planted in rows; simplification of biodiversity; intensive tillage, which exposes soil to erosion; use of genetically modified organisms; and artificially selected crops. Agroecology tends to minimize the human impact. An agroecosystem is a key idea in agroecology. Agroecosystems are defined as semidomesticated ecosystems that fall on a gradient between ecosystems that have experienced minimal human impact and those under maximum human control, for example cities. They are generally defined as novel ecosystems that produce food via farming under human guidance. Practitioners of agroecology are called agroecologists, and they take a critical view of modern industrial agricultural techniques, seeing the industrial model as fundamentally or radically (at its roots) unsustainable. The agroecologist views any farming system primarily with an ecologist's eye; that is, it is not firstly economic (created for a commodity and profit), nor industrial (modeled after a factory). In fact, agroecosystems are both understood and designed following ecological principles. For example, integrated pest management aims to control problematic pests through the introduction of other species, not application of pesticides to kill that pest. A common example of this would be intercropping to attract beneficial insects within rows of a given plagued crop. The insects would balance the disturbed ecology represented by the pest, thus, eliminating unsustainable practices such as increasingly intensified pesticide use.
The factors of precipitation, temperature, and light combine to provide most of the abiotic environment that controls worldwide plant distributions. Collectively these represent the climate to which a given green plant species must be adapted to survive. The study of climate, climatology, is essential to a basic understanding of where plants do and do not grow.
Of the climatological factors, water is undoubtedly the most important. Precipitation, which includes all forms of moisture deposited on the earth's surface, is the source of essentially all fresh water.
Water vapor, although always present to some extent in adequate amounts in the lower layer of the atmosphere, must exist in adequate amounts if condensation into water droplets or ice crystals is to occur. The droplets or crystals then must attain a sufficient weight to fall to the earth as precipitation. For condensation to occur, the air containing the water vapor must cool to a temperature below its condensation point, the temperature at which water vapor condenses into a liquid state. The condensation point varies with the amount of water vapor present and the air temperature at which the vapor is being held.
Air is generally warmed by radiant heat from the sun, particularly near the equator and at lower elevations. Warm air is capable of holding more water vapor than is cold air; thus, at and near the equator and at lower elevations, the air is warm and humid (full of water vapor). Warm air rises, cooling as it does so, and condensation occurs.
A complex set of events serves to produce high elevation air currents moving north and south, from the equator toward either pole. These circulation cells do not reach the poles, however, but descend at about 30[degrees] north and south latitude. This descending air has already lost most of its moisture as it nears the earth's surface. As it warms, its capacity to hold moisture increases, enabling it to absorb water vapor near the ground and produce a drying effect on the earth. This cell completes its circulation by moving back across the earth's surface to the equator and outward toward either pole. As the air travels, it continues to warm (especially air moving from 30[degrees] to the equator), and absorbs water vapor from the soil, ponds and streams (evaporation), and plants (transpiration).
As a result of the movement of these circulation cells and their effects on precipitation, most of the earth's natural deserts occur between 20[degrees] and 30[degrees] north and south latitudes (see Figure 2-2). Not all land between these two latitudinal belts is desert, and there are desert areas outside these belts. These anomalies occur because the precipitation affects the air circulation patterns in these cells, which are modified by topography, elevation, proximity to the coast, and ocean current temperatures.
[FIGURE 2-2 OMITTED]
A coastal mountain range has a significant effect on precipitation patterns. Warm, moist sea air moves inland and is forced upward in elevation by a mountain range; it cools as it rises, causing condensation of water vapor to turn into rainfall. The ocean (windward) side of the mountains therefore received considerable precipitation as a result of the same physical phenomenon described for air cell patterns. As this now cold, dry air moves over the range and descends the inland (leeward) side of the mountains, it begins warming as it descends, and its capacity to hold moisture increases. As it moves farther inland, the dry air takes moisture from the land and the plants, producing a rain-shadow desert--like the leeward side of the Andes Mountains in Argentina.
Generally, large continental landmasses are dry in their interior because cold air masses collide with the warm, moist air from the sea. This forces the warm air to rise, and the condensation causes precipitation to occur before the moisture can reach very far inland. Even though no mountains may block the flow, the effect is similar to a mountain-induced rain shadow. Also, cold temperatures greatly modify precipitation patterns, so higher elevations tend to receive less rainfall.
Although the effects of air cell circulation patterns do produce increased precipitation near 60[degrees] north and south latitudes, where air again rises away from the earth's surface (see Figure 2-2) as one moves farther north and south from 60[degrees], the effect of temperature, especially cold, becomes increasingly important. North and south of approximately 60[degrees] is in fact the most significant factor influencing precipitation.
Maximum and minimum annual ranges and diurnal (daylight) fluctuations in temperature produce another important set of climate conditions to which plants must be adapted if they are to survive in a given area. Because of the relative position of the earth to the sun, the angle of incidence for sunlight and the daylength results in the equator being warmer year-round than any other zone. As one moves north and south away from the equator, the annual temperature fluctuations become progressively more extreme, as shown in Figure 2-3a. The sun's rays that strike the earth's surface most perpendicularly do not necessarily result in higher temperatures. This is because of another unique property of water. Water (including water vapor) gains and loses heat very slowly. An area of high humidity, therefore, has less extreme temperature fluctuations than do areas with low humidity.
Two considerations are important when discussing light as a component of the climate affecting plant distribution and activity: both light intensity and daylength can have strong regulatory effects on plants.
Of the total radiation produced by the sun, light is only that portion known as the visible spectrum. The intensity of this light varies with latitude and season, and is greatest at the point the earth is most perpendicular to the rays of the sun at any given moment. Light intensity is progressively reduced from the point, as the rays become more oblique. In addition, not all the light emitted by the sun reaches the earth (see Figure 2-3b). Of the light energy that arrives at the outer limits of our atmosphere, only about half manages to get past the reflection and absorption by dust, clouds, water, and other gases, including the important ozone layer, which absorbs most of the ultraviolet radiation. Some of the light reaching the earth's surface is reflected from rocks.
[FIGURE 2-3a OMITTED]
[FIGURE 2-3b OMITTED]
Water, is absorbed as heat or, in the case of green plants, converted to the chemical energy of carbohydrates (the process of photosynthesis). More than half is in the infrared region of the spectrum and warms the earth. About 7% of the ultraviolet radiation manages to get past the ozone. The human eye sees light beginning at the short wavelength of violet and then in successively longer wavelengths as blue, green, yellow, orange, red, and finally fading at the beginning of the infrared region. The visible spectrum as shown in Figure 2-3b, represents only a tiny fraction of all radiation, which includes the very short waves (gamma rays and X-rays) and very long waves (radio waves).
Visible light contains the entire wavelength absorbed by photosynthetic pigments. In particular, the chlorophyll molecule absorbs light at wavelengths of about 4,330 nm (blue) and 660 nm (red). Wavelengths capable of exciting a chlorophyll molecule may have a great deal of energy (short, blue light) and easily cause excitation, or they may have less energy (long waves, red light) and still cause the chlorophyll molecule to be excited. The consequences of that excitation are included in the discussion of photosynthesis.
Our eyes perceive sunlight as white light, simply because it is a mixture of all colors of the rainbow. Yet specific wavelengths are absorbed and reflected differently; shorter wavelengths have more energy. Leaves are green because they have chlorophyll molecules that reflect green light while absorbing red and blue light; thus, any color appears to the human eye as that color because light of that wavelength is being reflected. If all wavelengths are being absorbed, the object appears black; if all wavelengths are being reflected, the object appears white (see Figure 2-4).
Daylength varies by season and according to latitude. As discussed in the preceding section on temperature, during the winter the sun's rays hit the earth at a more oblique angle. This produces both a shorter day and less light intensity (and less heat). There are actual effects of changing daylength in plants relative to hormonal control of plant activity. In general, however, plants become less active during the shorter days of winter. With the gradually longer days of early spring, increased light intensity and warmer temperatures combined to initiate new activities in most plants. Reproduction and other development phenomena are affected.
For a given plant species to successfully inhabit an area, it must have present in appropriate quantities all the necessary components of its environment. If any single requirement is insufficient, then the species in question will not be able to survive. Whichever environmental component causes the organism to fail is said to be the limiting factor.
Even though any of the requirements could be theoretically the limiting factor for a given species, only the one that actually prevents the species from surviving is so designated. Usually one of the abiotic factors, especially water or temperature, is the most likely candidate. For example, many plant species are limited from growing in a desert because there is not enough water. In fact, if irrigated, many of these plants could survive well in a desert. Others, however, would not be able to survive even with supplemental water because of the high summer temperatures. For them, excessive heat would be the limiting factor. Other plants are limited from growing in more northern latitudes because of freezing winter temperatures that they cannot endure.
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Many other abiotic and biotic (biological) components can be limiting factors. Nutrients, space, soil characteristics, and damaging interaction with other living organisms sharing the area are among the probabilities controlling plant success or failure. Other components of the total environment contribute to the success or failure of given organisms in various settings; they are considered in following sections.
Our planet has often been referred to as "Spaceship Earth," traveling through space with a critical atmosphere, critical energy input, and finite resources. Ecologists think of our entire earth as a giant system-the biosphere. Any portion of the earth that represented a relatively closed system in terms of nutrient cycling, energy input, and therefore a definable set of plants, animals, soils, and climate may be defined as an ecosystem. It may be very small, as in a backyard pond with a few organisms, or it may be very large, as in a deciduous forest or grassland. The size of the ecosystem is in the eye of the beholder, and its limits must always be a relatively homogeneous area made up of living and nonliving components that have something in common; then the system can be studied as a unit. The ecosystem concept is useful for all biological study.
Within an ecosystem, each species has a unique set of requirements, a spot in which it is better adapted than competing organisms. Niche is the term that denotes this position within the ecosystem. The niche of an organism should not be confused with the habitat in which the organism lives. The physical habitat is much more general and can include many different species, each with its own unique niche. In other words, the concept of niche includes every aspect of an organism successfully positioned within the habitat. The niche would include all the physical habitat characteristics of temperature, water, soil, and light plus all possible interactions with other organisms in the ecosystem. When two individuals of different species have some of the same specific requirements, they are sharing a part of the same niche. Such coexistence can theoretically occur for a while if there are abundant resources available, but ultimately they will be in competition with each other for this specific need. The more similar the niche of these two species, the sooner they will find themselves in competition with one another.
It is easier to visualize animals than plants competing for the same resource, but the term is still appropriate. Plants compete for water, light, nutrients, space, and other components in their habitat by being better designed to acquire those needs. The depth or total volume of the respective root system, the vertical growth rate and leaf production, and the abilities to withstand climatic and herbivore stress all enable one species to compete more successfully than another for common needs. The results of direct competition depend on the situation. When their respective niches are almost identical, one species could succeed while causing the other to become extinct. When their respective niches are less similar, one species may succeed in one area while the other species is superior in an environmentally different area. Such a situation might allow both species to survive, but with reduced total ranges. A third possibility is rapid evolution in divergent directions. Each of the two competing species would have individuals selected that were less directly competitive; their respective niches would become less similar. Whatever the end results, no two species can share exactly the same niche in the same place.
Other interactions between individual of different species are not competitive. In such situations, each organism functions in some way. If organism A benefits from its interaction with the organism but in the process affects B negatively, it is a parasitic relationship. For example, dodder (Cuscuta) is a parasitic vine that derives its nutrition from whatever host plant it lives on. The host plant ultimately dies in such a relationship.
Commensalism is an interrelationship in which one organism benefits and the other is unaffected. Some plants require partial shade to survive. Growing under a tall tree does not provide the necessary shade and the tree benefit nor does this association harm it. When both individuals benefit from an interrelationship, they are said to have a mutualistic or symbiotic relationship. A classic example of mutualism is the lichen, an organism composed of fungus and an alga (although some evidence indicates that the fungus benefits more than the alga).
In addition to plant-plant interaction, there are many plantfungus, plant-insect, and plant-animal interrelationships that exemplify these systems. Some plant-herbivore (plant-eating animals) interactions significantly affect the plant's total environment. Certainly the most obvious is the plant being a food source for the herbivore. This is not considered parasitic, however, because a parasite usually lives off its host for a long time before irreversible damage is done. There are also plant-animal interactions in which the plant causes a negative effect on the animal: there are toxic plants, plants with spines or thorns, and plants that provide shelter to one animal that in turn attacks herbivores that could damage its home. Some plants have more complex interrelationships and interdependencies with a wide variety of other organisms.
The likelihood of understanding all the possible niche interactions of any single species is improbable, so true comprehension of total ecological balance within even a single habitat therefore is impossible. Studying selected components shared by the many species within a given area, however, is worthwhile and provides ecologists with some basis for comparison of the general relationship. Therefore, even though the earth is extremely diverse in terms of numbers of species and numbers of individuals within a species, there are good reasons to consider the planet as a whole. Sometime broadscale conclusions about changes in gas concentrations in the atmosphere increase or decrease in temperature, changes in precipitation patterns, and other serious problems can be viewed only on a global basis. Ecologists have chosen to divide the world into a few major vegetation types based primarily on effective precipitation, temperature, and soil. These specific groups of ecosystems, representing recognizable types of vegetation that are remarkably stable with time (sometimes over hundreds or thousands of years) are called biomes.
Within a particular area in an ecosystem, all the living components are collectively referred to as a community. The term may encompass a bit more than what we refer to as a human community, since from an ecological standpoint the community represents all the plants, animals, and microorganisms living together in an area. Within that community, there may be groups of organisms, all of the same species, which constitute a population. A community might have a population of bluegrass, a population of rabbits, a population of foxes, and a population of grasshoppers. Each population could have only the number of individuals that its trophic, or feeding, level and food supply would support.
This limit on the number of organisms that a given area can support without causing degeneration of the area is termed the carrying capacity. For example, the carrying capacity of a specific area of prairie may be 1,000 field mice, 20 deer, and 1 coyote. In practical use, ranchers need to know how many sheep or cattle given sections can support without overgrazing the land, which could cause a smaller carrying capacity in subsequent years.
Many of the specific components of an ecosystem cycle go through set cycles within the normal functioning of the system. These cycles are all in balance at a worldwide level, but it is possible to throw them out of balance if the natural habitat is significantly altered. Overgrazing is only one of many such negative alterations possible.
Oxygen-Carbon Dioxide Balance
The earth's atmosphere is made up of a mixture of nitrogen ([N.sub.2]), oxygen ([O.sub.2]), carbon dioxide (C[O.sub.2]), water vapor ([H.sub.2]O), and a number of other gases of lesser importance. Practically all these substrates are critical for life processes, and the recycling of both oxygen and carbon dioxide is absolutely essential to almost all living organisms. The air around us contains only about 21% oxygen and 0.33% carbon dioxide. Essentially all the remainder is nitrogen (about 78%). The opposing process of photosynthesis, which produces sugars, and respiration, which allows those sugars to be used for energy, also involve oxygen and carbon dioxide exchange.
As the photosynthesis process starts with water, carbon dioxide and sunlight is producing the sugars, oxygen is given off into the atmosphere. At the same time, the respiration process consumes oxygen, "burns" sugars, and releases carbon dioxide and water. The relative rates of photosynthesis and respiration, then, determine the amount of oxygen and carbon dioxide in the atmosphere. All organisms carry on respiration and give off carbon dioxide, but only green plants carry on photosynthesis and produce oxygen. Therefore, green plants must reach an acceptable balance between the two processes. If the amount of carbon dioxide released is exactly equal to the amount of carbon dioxide consumed, the plant is said to be at the compensation point. Such plants cannot accumulate materials and thus do not grow. In agriculture, for example, it is important that photosynthesis far exceeds respiration if crops are to be productive. The balance of atmospheric oxygen and carbon dioxide is a critical interrelationship in which plants (which photosynthesize and respire) play the central role. The food chain, photosynthesis, respiration, and other organismal interactions are not isolated events. Their interrelationships and their responses to environmental factors are complex, so a broad understanding of the entire system is necessary.
By 1772, Joseph Priestly had discovered that if an animal, such as a mouse, were placed in a closed container, it would die after a period of time. If a green plant were placed in the same container and even if the container were glass so that sunlight could enter, the plant would also die. But if both the mouse and plant were placed in the container at the same time, they would coexist. If the gas balance ([O.sub.2] and C[O.sub.2]) were correct, they could theoretically live in this condition indefinitely. A closed terrarium represents a similar system. Green plants within it consume carbon dioxide and synthesize sugars, giving off oxygen: The microorganisms, worms, and other animals living in the soil (or perhaps on the plants) carry on respiration along with the green plants, consuming oxygen and releasing carbon dioxide back to the atmosphere of the terrarium. In effect, a balanced terrarium is an ecosystem.
Cycling in the Ecosystem
The ecological success of an ecosystem depends on its efficiency and stability. A great deal of that efficiency is related to nutrient and water cycling, essential components for all living organisms. If a single factor is missing, the entire system loses efficiency and slows down, and in a small system the balance may be altered permanently. The broadscale ecosystems the biomes comprise have evolved and increased in complexity over millions of years.
Organisms survive, grow, and produce because they have the energy, water, and nutrients to do so. The energy for our entire biosphere is derived from the sun (see Figure 2-5). This energy does not cycle, but it is converted from one to another, eventually ending up as heat, which has little value in the overall functioning of the system. Because the energy is lost as heat, there must be new input every day.
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On the other hand, our supply of water and nutrients is finite. At any given time, a portion of the water and nutrients is tied up in various parts of the system--in the air, the soil, the oceans, or the living or dead organic matter. As water and nutrients are transferred from one part of the system to another, a cycle is eventually completed and begins anew, depicted in Figure 2-6.
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We derive part of our understanding of ecosystem from knowing that water and nutrients cycle within certain physical boundaries. It makes no difference whether we are discussing a desert or a tropical rain forest; the manner in which nutrients cycle is exactly the same, although the rates may differ.
The Water Cycle
On the earth the total amount of water is enormous and essentially constant, but 99.4% of it is composed of salt water and ice found in oceans, inland seas, glaciers, and polar icecaps.
The salt water provides a saline habitat for organisms adapted to those conditions, and it provides a reservoir from which pure water molecules can evaporate. The energy for this process is derived from the sun.
As the surface waters of the oceans are warmed, evaporation occurs and water is moved into the atmosphere. When physical conditions are proper for condensation, clouds occur, and eventually precipitation is produced: rain, snow, hail, or sleet. As described earlier, air currents cause these cells of moist air to move around the earth, and precipitation may fall thousands of kilometers away from where the water was released by the ocean or land mass.
Some precipitation falls on land, a portion of it is evaporated back into the atmosphere, some goes to surface runoff (see Figure 2-7), some percolates through the soil to recharge underground water supplies, and a portion is stored in the soil as a water source for plants and ultimately animals.
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The plants absorb the water from the soil, transport it through their cells, and eventually release it back into the atmosphere through transpiration. About 99% of all water absorbed by the root system is given up this way. Only a small portion is stored in cells to be used for metabolism and maintaining water pressure, or turgidity. At any given time, a very small fraction of the total precipitation is tied up in the living tissues of plants and animals.
Both surface and underground runoff water eventually returns to the oceans. Thus, the problems of water management are not a matter of total supply but simply of having enough fresh water at the right place at the right time. Water is fast becoming our major nonrenewable resource.
The Carbon Cycle
All organic matter includes carbon and hydrogen. That carbon is also cycled through the living/nonliving systems in an orderly flow that allows for organic molecules to be constructed, as you can see in Figure 2-8. Carbon comes directly from the atmosphere as carbon dioxide. Earth's atmosphere contains about 0.033% or 330 parts per million (ppm) C[O.sub.2]. In the comparison with the amounts of nitrogen and oxygen, which dominates the air we breathe, this is a small percentage. It is remarkable that a gas so important should be present in such small concentrations. We shall see in a later discussion of photosynthesis that C[O.sub.2] concentrations in these ranges are absolutely critical to the photosynthesis process, and slight reductions in concentration can drastically reduce the fixation of carbon into sugars.
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In the process of photosynthesis, C[O.sub.2] is taken into the green plant and incorporated first into sugars and then later metabolized into all the organic molecules important to life. In the normal process or respiration, some C[O.sub.2] is cycled directly back into the atmosphere. Most of it is stored in living tissue, animals in normal food chains consume portions of it, and some occur in dead plant parts such as leaves, flowers, fruits, and seeds.
The level of C[O.sub.2] in the atmosphere is relatively constant, although there is considerable concern about its fluctuation. For example, there is good evidence that the level in the atmosphere has risen considerably during the past 100 years, primarily from the additional input from the burning of fossil fuels. In that period of time, the level has increased from 300 ppm to the present 330 ppm. At the current rate of increase, a recent National Academy of Sciences study predicts that the concentration of C[O.sub.2] in our atmosphere will double by the year 2020. Thus far the oceans have acted as an effective buffer by absorbing excess C[O.sub.2] as carbonates, but there is considerable controversy about when this buffering capacity might be overridden. If this were to happen, the CO2 in the atmosphere would rise dramatically, cause the sun's rays to be trapped at the earth's surface (the so-called greenhouse effect), and cause the temperature of the earth to rise. Studies indicate that only a 2[degrees] or 3[degrees]C increase in the earth's temperature would cause the polar icecaps to melt and result in serious ecological and economic damage. Such melting would raise the level of the oceans by as much as 5 m, according to some estimates, and significantly change the outline of our continents by submerging many millions of hectares of coastal cities.
Other sources of carbon released into the atmosphere include volcanic eruptions and the weathering of rocks, processes that are relatively stable over geologic time and about which we can do little. From the standpoint of the photosynthetic process, an increase of C[O.sub.2] concentration in the atmosphere should not be a problem. In fact, C[O.sub.2] enrichment of the environment in greenhouses and other confined spaces is now practiced commercially to improve yields. The ecological consequences of such an event worldwide, however, would far override any benefits derived from additional photosynthesis in nature.
The Nitrogen Cycle
The air we breathe is about 78% nitrogen gas ([N.sub.2]). As we shall see later, nitrogen is an extremely important element in the building of organic molecules, but most plants and animals have absolutely no way of incorporating atmospheric nitrogen gas. Instead, plants obtain nitrogen as nitrate (N[O.sub.3]) or ammonia (N[H.sub.3]) from the soil. Once nitrate or ammonia nitrogen has been taken up by the plant, it is converted onto organic matter and becomes a part of living cells. If the plant is eaten by an animal, the organic nitrogen is partially converted into new organic matter in the animal. Whenever the animal dies, the decomposers break down the organic matter into inorganic nitrogen in the soil. Most of this nitrogen will be recycled in the form of nitrate ammonia; some will be volatilized into the atmosphere in a form that produces the pungent odor of decay. Bacteria present in the soil are capable of converting nitrate to ammonia, and vice versa. Environmental conditions such as moisture, temperature, pH, and oxygen in the soil determine the balance of the two compounds. As Figure 2-9 shows, the cycle begins as plants take up nitrogen from the soil.
One way to provide these nutrients is to make them synthetically, usually involving methane, a fossil fuel. There was a time, not too many years ago, when fossil fuels were so inexpensive we felt that essentially all of our nitrogen fertilizer needs could be met this way. Now the energy crisis has forced us to look more seriously at natural nitrogen fixation. Many bacteria and cyanobacteria are capable of converting atmospheric nitrogen into inorganic nutrients in the soil that plants can use directly or that can be converted by other microorganisms into usable forms. They may do so as part of their independent, natural metabolism as free-living, nitrogen-fixing microorganisms, or in a mutually beneficial relationship in the roots of a higher plant as symbiotic, nitrogen-fixing bacteria. It has been known for many years that many legume (bean family) plants become infected with certain kinds of bacteria that cause a nodule (tumor) on the roots of those plants (see Figure 2-10).
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The nodules become infected with certain kinds of bacteria that have the ability to fix nitrogen from the air into NH3. In return for this "free lunch," the plant provides the bacteria with certain organic molecules necessary for the bacterial growth. These tumors do not harm the plant and, in fact, are considered highly desirable. Farmers routinely pull up alfalfa or soybean plants to see how many nodules occur on the root system and thereby judge the nitrogen nutrition of the plant.
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In recent years it has been discovered that different types of bacteria that do exactly the same thing infect many nonlegume plants. Such plants require essentially no nitrogen fertilization. They are usually found on soils very poor in nitrogen, and they play an important role in the fertility of such soils. Free-living bacteria and cyanobacteria appear to be far more important in nitrogen cycling than was originally believed. For example, the cyanobacteria that inhibit the water and soil of rice paddies in Asia are responsible for much of the nitrogen fertility of land farmed continuously for centuries. Most of these organisms have the ability to become dormant during periods of drought; then they grow vigorously in a matter of hours after receiving moisture, fixing nitrogen at a remarkable rate until water again becomes the limiting factor.
One other source of nitrogen from the atmosphere is important in certain regions. Electrical discharges during thunderstorms are capable of putting nitrite (N[O.sub.2]) into the air; rainfall carries it to the ground, and bacteria oxidize it to nitrate (see Figure 2-11). The burning of fossil fuels may put some ammonia and nitrogen oxides into the air. Ammonia is also soluble in water and may be brought to the ground with precipitation.
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The nitrogen cycles form the atmosphere by several fixation schemes to be captured in the soil or directly by the plant. It is taken up by the plant, and fixed into organic matter, and later broken down by decomposers to the inorganic from where the cycle begins again.
These cycling nutritional materials combine with the abiotic compound of the environment and the energy of the sun to control what kinds of plants grow where. Understanding these interrelationships increases our ability to grow them in other regions.
Organisms that make their own food directly from sunshine, carbon dioxide, and water are called autotrophic (auto, "self"; tropic, "feedings"). All green plants are autotrophic. Other organisms, including humans, are said to be heterotrophic (hetero, "others"). All heterotrophics lack the ability to make their own organic molecules from inorganic substances and light energy. Not only do plants produce their own food, but they also produce tissues that can be used as food by animals. Therefore, autotrophs provide food for the heterotrophs; plants are the producers.
A food chain is a hierarchy of organisms in which the producer organisms, the green plants, are the base. Figure 2-12 gives an example of a food chain. The green plants are consumed by plant-eating animals (herbivores), which are in turn consumed by meat-eating animals (carnivores). An example of such a simple food chain is a grass plant (producer) that is eaten by a cow (herbivore) that is then eaten by a person (omnivore). Consider that many grass plants are required to feed one cow, and the number of grams of grass required for conversion to 1 gm of beef is fairly large; in fact, the ratio is about 9:1 at best. Similarly, it takes at least 9 gm of beef to provide 1 gm of weight gain for an actively growing human child (the relationship becomes much more complicated for organisms that are fully grown).
[FIGURE 2-12 OMITTED]
What is apparent in this food chain is that it is indeed oversimplified. Cows may choose many different kinds of plants for forage, and humans eat many foods other than beef. A great deal depends on availability (many humans in the world have few choices).
We refer to the organic matter present in organisms as biomass, an ecological description of the weight of the matter itself. We measure our biomass by simply weighing ourselves, and the biomass of a plant can be determined by weighing both the above ground part and the below ground part. Such information is important in determining how efficiently the energy from the sun is being converted into the chemical energy of organic molecules.
For most organisms there is a choice of food, and therefore it makes more sense to think of a food web, which as shown in Figure 2-13 is a hierarchy of consumption in which alternate food sources are represented. The number of organisms available becomes very important because the total energy loss at each trophic level (actually a feeding level) is approximately 90%. Thus only about 10% of the total energy is incorporated (as organic matter) at each trophic level.
Since the green plants are at the first trophic level and are called producers, the herbivores that feed on them at the second level are called primary consumers. Animals that eat the primary consumers are the third trophic level and are called secondary consumers. The hierarchy continues until one reaches the top trophic level, occupied by the top carnivore. Nothing feeds on the top carnivore except scavengers, which consume the flesh after the top carnivore dies. Finally, decomposers (mostly bacteria and fungi) break down the organic matter into simple inorganic nutrients, which are cycled back to the soil, later to be absorbed by new plants, which start the cycle all over again.
A complex food web is one in which the organisms at each level eat many different species available to them. Such flexibility of food sources develops in nature out of necessity.
In unstable or unpredictable environments, where a given plant species may not be sufficiently available (or even at all) in a particular year, animals that would normally feed on that plant must feed on another species or fail to survive. These interrelationships are true at the secondary and tertiary consumer levels of the food chain. A desert, such as the one in Figure 2-14, is an example of such an environment because the precipitation is so unpredictable, and thus plant growth is not always ensured.
A simple food web, on the other hand, is one in which a high degree of specificity exists in food sources. In many cases, given animals feed on only a single plant species (for example, koalas feed only on Eucalyptus leaves). In turn, given carnivores may depend exclusively on a single species of animal as their sole food source. Such extreme specialization could have evolved only in a very stable environment, where each plant and animal species is always present in essentially the same abundance. Because these animals have never needed to search for and successfully compete for other food sources, they have lost the ability to do so. Their continued existence, therefore, is linked to the future stable supply of their food source.
[FIGURE 2-13 OMITTED]
[FIGURE 2-14 OMITTED]
Probably the most predictable and stable environments exist in the tropical rain forest. In these regions many highly specific ecological interrelationships exist--a simple food web. The elimination of any single species in such environments could well cause a chain reaction affecting several other species. That we are unaware of many of these specific interdependencies is one of the main reasons ecologists and conservationists concern themselves with protecting known endangered species.
Interestingly, another of the most predictable (stable) of the world's environments is the ocean, and yet a very complex food web exists there. The base of the aquatic food chain consists of single-celled algae generally referred to as phytoplankton (phyto, "plant"; plankton, "small, free-floating aquatic organisms"). These are in turn eaten by zooplankton (zoo, "animal"), single-celled animals that are then eaten by larger aquatic animals. Size and agility in avoiding capture govern the food source here almost entirely. Flavor, texture, and other factors that we find important in foods apparently have little to do with fish diet. The little fish must be large enough to be an enticement to the larger fish, but it must not be so large that it cannot be swallowed. Without benefit of knife or fork, size becomes very crucial in marine diets. Even on other food chains, size is important. Remember the difficulty in chasing a single pea around on a slick plate? It hardly seems worth the effort.
The best way to depict these energy relationships is three-dimensionally in the form of a pyramid. The broad base of the pyramid is made up of the first trophic level, the producer organisms with a very large biomass. Generally, that means a very large number of individual plants. The second trophic level is represented by the primary consumers, the herbivores. Fewer individuals and less biomass occur here because the energy conversion efficiency is only about 10%. In other words, 90% of the plant tissue eaten by a herbivore does not go to make animal tissue. Much of the energy lost is in the form of heat, some as indigestible waste that goes to the decomposers. At the third trophic level, decomposers take their toll as some organisms die and are recycled to the soil and air (see Figure 2-15). Even the green plants lose leaves, flowers, and fruit during growth, and the decomposers break them down. A compost pile is an example of this principle.
The pyramid concept is used to demonstrate that very little energy and biomass is left after only three or four trophic levels. Thus, food chains are never very long; the energy losses are simply too great. The lesson to be learned from this generalization is that there is more food for organisms that feed on more than just meat.
The food supply for omnivores such as humans is greatly enhanced if we eat plants rather than animals. If we are willing to shift our diet so that plants constitute a greater portion of our total intake, we have access to more food and the energy costs are much lower. High-priced beef is rapidly bringing this lesson home. This is not to say that all humans should become vegetarians, but modification in diet toward more plant foods would mean additional food for a hungry world.
[FIGURE 2-15 OMITTED]
In summary, the food pyramid clearly shows the impossibility of having a larger top than can be supported by the base. The plant material on earth can support only a finite amount of animal life, including humans. It would be foolish, therefore, to presume that human population can increase beyond a given size without proportionately increasing the available food base. This, of course, is what modern agriculture attempts to do. Continued pressure for ever-increasing productivity requires modifications in the habitats of the crop plants to allow production beyond what natural conditions would yield. Such modifications are not always in the best interests of the surrounding environment. In later chapters, we will look more closely at energy flow and changes in energy from plants as the all-important first step in the sequence.
Ecological succession is the sequential replacement of one community type by another through a series of development stages. These are known as seral stages until the final community structure is reached. This last stage in succession is called the climax community because it is the optimum assemblage of species that the environment can support in the area. The measure to determine whether a climax community has been reached is stabilization of the dominant species; when those species begin replacing themselves rather than being replaced by a new species, the climax community has been achieved.
The environment determines the community structure as the process of change occurs in spite of the fact that the climate patterns remain the same. The change from one community to the next is actually brought on by the modifications produced by each temporary community. As the dominant species alter the area in which they are growing, they actually produce conditions less favorable for themselves and more favorable for a new assemblage of species.
When succession occurs in a new or pristine habitat or in one that has not previously had a similar community occurring there, it is called primary succession. The classic example of primary succession is the normal transition of a pond or a bog and then to woodland as it slowly fills with silt and organic material. The south shores of Lake Michigan have been undergoing primary succession for many years as the lake slowly retreats to the north.
Secondary succession occurs when land that has been cleared for pasture or for farmland is no longer maintained. Such abandoned fields slowly revegetate with plants native to the area. The actual species vary with the region, but the first year finds annual weeds dominating the site. In the second year, perennial grasses and some herbaceous perennial broadleaf species join the annual weeds. For the next several years, the grasses dominate, but an ever-increasing number of shrubs begin to appear, as do some tree seedlings. While the trees are slowly reaching maturity, the shrubby species and grasses codominate. The fast-growing tree species shade out the shrubs and grasses as they become large enough to form a dense forest, but they might be replaced in time by slower-growing shade-tolerate deciduous tree species. If a deciduous forest is the dominant climax vegetation, a new group of shrubby plants take their place in the understory, maintained by the fact that deciduous trees do not form a shade canopy over them all year long.
Primary succession in ponds results from the slow filling of the body of water with silt and organic debris until plants can gradually invade from the banks toward the middle. As soon as submerged plants are able to root in the mud near the edge of the pond, the buildup of silt occurs much more rapidly, trapped and held by these plants. As the bottom continues to fill, floating surface plants like water lilies become common near the banks, and the submerged plants move farther toward the middle of the pond. Next, reeds, cattails, and similar rooted emergent plants become established in the deeper accumulation of sediment in the shallow water near the banks of the pond. The filling of the pond occurs much more rapidly now with plants occupying the entire area. Eventually true terrestrial plants establish along the original shallow zones of the pond, now dry land. The center of the rapidly filling pond becomes smaller and smaller, with an accompanying succession of seral vegetative zones, with it is a bog and finally dry land.
Generally, successional events share several common characteristics regardless of the wide variety of localities and plant species, and the rate of species replacement is higher in the initial stages, slowing and stabilizing in the older stages. In addition, the size of the plants and the total biomass increase through the seral stages until the climax community is reached. Finally, the food webs become more complex as the seral stages move toward climax, and thus a higher percentage of organic materials synthesized by the producer organisms are used in the older stages.
Occasionally, a natural phenomenon totally destroys life in an area. The eruption of a volcano in a vegetated area, a devastating forest fire, and the building of a shopping mall parking lot are all examples of such areas. Given time, all of these will again become revegetated through the process of recolonization and succession.
An extreme example of a recolonization occurred on the Pacific Island of Krakatau. In 1883, a volcanic eruption destroyed almost half the island and covered the remainder in a thick layer of lava pumice and ash. All forms of plant and animal life were eradicated, but as soon as the substrate cooled, the process of recolonization began. Single-celled algae quickly established in pockets of rainwater caught by the lava folds. Slowly, organic materials combined with ash in these pockets to provide a shallow layer of soil sufficient for hardy species of vascular plants. Gradually, more and more plant species reestablished, and they helped provide more humus for an ever-improving habitat, which contained a greater diversity of life forms. By 1934, only 51 years later, over 270 species of plants occupied the island with a commensurate number of animal species also in full residence.
The eruption of Mount St. Helens on May 18, 1980, in southern Washington, produced large areas denuded by lava flow and ash. The recolonization of this area is being monitored carefully by ecologists. Undoubtedly, succession to a climax forest community will take a long time.
The formation of a new volcanic island would undergo the same sequence of biological habitation, but technically this would be an example of colonization rather than recolonization. The island of Surtsey was formed in the 1960s off the coast of Iceland by volcanic eruption, and within only a few months living organisms had colonized the still warm lava.
1. The term ecology implies a thorough understanding of the biotic and abiotic components of the environment. Organisms exist where they do in nature because the combination of these factors is appropriate for their existence. Crop plants and houseplants need to be grown with the total needs of the plant in mind.
2. Precipitation is the single most important factor in determining plant distribution. The annual distribution of precipitation is dictated by a complex pattern of air circulation cells, which distribute moist air to certain parts of the world. These cells are modified by elevational changes and collisions with other air masses.
3. Temperatures are modified by the amount of water in the atmosphere. Moisture-laden air heats and cools more slowly than dry air. The equator is warm year-round because of the angle of incidence for the sun's rays striking the earth.
4. Only humans perceive a small portion of the entire radiation spectrum emanating from the sun as visible light. Fortunately, most of the harmful ultraviolet radiation is absorbed by the ozone layer in the outer layer of the earth's atmosphere and does not reach the surface. The ozone layers are being depleted by the abundance of pollutants, thus, causing a hole in the atmosphere ozone. Light in the red and blue portions of the spectrum excites the chlorophyll molecule to begin the energy capture process of photosynthesis.
5. Any of the abiotic factors critical to continued existence of an organism could also be the limiting factor for the same organism.
6. Within the earth's total ecological system, the biosphere, balanced ecosystems include many different physical habitats. Plants occupy a specific niche within a given habitat, and competition for resources occurs when different species have similar requirements. Plants and animals also develop specific interrelationships with each other: parasitism and commensalism are very common associations.
7. The earth's atmosphere is primarily nitrogen, oxygen, and a small amount of carbon dioxide. Water vapor also occurs in varying amounts. Photosynthesis puts oxygen into the atmosphere and uses carbon dioxide, whereas respiration uses oxygen and produces carbon dioxide. As long as worldwide photosynthesis and respiration rates remain equal, the relative concentration of oxygen and carbon dioxide are stable.
8. Water and nutrients are important to plant and animal growth cycles. At any given time they may occur in living or dead plants and animal tissue as organic matter, or they exist in the soil atmosphere as inorganic chemicals. Carbon and nitrogen both cycle through such stages. Carbon dioxide levels are critical to the carbon cycle; decomposition plus nitrogen fixation provide the necessary nitrogen for plant growth. The energy from the sun does not cycle but must be constantly replenished.
9. Organisms that obtain energy directly from the sun are called autotrophs or producers. These green plants are eaten by herbivores, which in turn are eaten by carnivores. A group of organisms that represent the producer-herbivore-carnivore sequence is called a food chain or more commonly a food web. The biomass relationships are best represented in the form of a pyramid.
10. The natural change in community structure is termed succession. This process results in the climax community. Primary succession and secondary succession have different points of origin but proceed in a parallel manner. Recolonization is the successional replacement of organisms in a habitat denuded by a natural disaster, such as a volcanic eruption.
Something to Think About
1. Explain how biotic and abiotic components of the environment relate to ecology.
2. What is the single most important factor in determining plant distribution?
3. Identify limiting environmental factors to the growth of an organism.
4. Explain a simple food web and food pyramid.
5. What is ecological succession?
6. Explain the oxygen-carbon dioxide balance.
7. What is recolonization?
Billings, W. B., Plants, man, and the ecosystem (2nd ed). Fundamental of Botany Series, Belmont, CA: Wadsworth Publishing.
Dale, V. H., L. A. Joyce, S. McNulty, and R. P. Neilson. 2000. The interplay between climate change, forests, and disturbances, Science of the Total Environment 2, 201-204.
Dale, V. H., P. Mulholland, L. M. Olsen, J. Feminella, K. Maloney, D. C. White, A. Peacock, and T. Foster. 2004. Selecting a suite of ecological indicators for resource management. West Conshohocken, PA: ASTM International.
Kapustka, L. A., H. Gilbraith, M. Luxon, and G. R. Biddinger, Eds. 2004. Landscape ecology and wildlife habitat evaluation: critical information for ecological risk assessment, land-use management activities and biodiversity enhancement practices. West Conshohocken, PA: ASTM International.
Kratsch, H. A., W. R. Graves, and R. J. Gladon. 2006. Aeroponic system for control of root-zone atmosphere. Environmental and Experimental Botany, 55, 70-76.
Internet sites represent a vast resource of information. The URLs for Web sites can change. Using one of the search engines on the Internet, such as Google, Yahoo!, Ask.com, or MSN Live Search, find more information by searching for these words or phrases: ecology, exobiology, abiotic, biotic, prevailing winds, rain shadow desert, diurnal, visible spectrum, daylength, biosphere, commensalism, and ecosystem.
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|Title Annotation:||PART 1: Plants and Nature|
|Publication:||Fundamentals of Plant Science|
|Date:||Jan 1, 2009|
|Previous Article:||Chapter 1: Why plant science?|
|Next Article:||Chapter 3: Biomes.|