3 Life on tideless coastlines.
1.1 The breaking of the waves
Tideless seas (or more accurately, seas with low tidal ranges, since there are always tides in any sufficiently large body of water) are small seas that are mainly located in the northern hemisphere, such as the Baltic, the Mediterranean, the Red Sea, and the Black Sea. In the Mediterranean, for example, the tidal range does not usually exceed half a meter, and large changes in the water level are usually caused by climatic factors (such as wind, barometric pressure, evaporation, and rainfall). Most are brackish due to the large volume of freshwater they receive, but some (such as the Mediterranean and the Red Sea) are more saline due to intense evaporation.
The transition zone
In tideless seas, the very special environmental characteristics of the transition zone between the terrestrial and the marine environment limit both the number of species and their biology. The upper limit of this transition zone corresponds to the disappearance of the upper halophilic plants. The lower limit is less clearly defined than the upper, and is located less than one meter below the surface of the water. In seas like the Mediterranean, the transition zone is divided into two parts: the supralittoral and the littoral. The difference between them is the amount of wetting they receive, which is occasional in the supralittoral and almost continuous in the littoral. So the most important limiting factor in this special habitat is the small length of time that the organisms are emerged and exposed to the air. The presence of terrestrial organisms, however, is limited by the amount of salt water to which they are subjected, almost always as splashes and waves, and the osmotic regulation problems this raises.
The intensity of light is another factor controlling the supralittoral and mediolittoral communities. High amounts of radiation lead to a large increase in temperature, and this affects the metabolic activity of organisms. Furthermore, increases in temperature imply a greater evaporation of surface water, which leads to a large increase in salinity. High temperatures and salinity, together with long periods of exposure, mean that these organisms' life is harsh and selection is severe. It must also be considered that environmental changes during the daily cycle can also be drastic. As an example, midday temperatures off Mediterranean coasts may be 10[degrees]C higher than at night. In the summer, wave intensity is much greater in the afternoon than during the rest of the day, because of the wind regime (gentle winds from sea to land).
Another important factor to consider is the nature of the substrate. Calcareous rocks encourage benthic organisms that drill into it to shelter from adverse environmental conditions, such as the impact of the waves. Granite substrates, however, develop fewer benthic life forms, while sandy substrates are hardly colonized because of their erosion by the sea.
Eutrophication, whether natural or caused by human action, is another very important factor in the littoral environment. Very high nutrient inputs occur in dense coastal concentrations of marine birds, for example, due to their excreta. Human action is increasing through the use of the sea as an uncontrolled dumping place, and the area that is most affected is the coast. Organic inputs alone are enough to increase nutrient concentrations, and this favors the growth of certain species--such as chlorophycaceous algae--that occupy almost all the available substrate, thus reducing the diversity of coastal communities.
Communities and sea level
The shaping of the coastal communities, in the sense of the number of species and their distribution pattern, varies with the orientation of the substrate. In zones receiving the direct pounding of the waves, the organisms show greater extension in space, as the splashes of water reach a few meters above the level of the water. In the calmest zones, the extent of the communities is reduced, and they are not very highly diversified, because the volume of water that periodically submerges the organisms has a lower rate of renewal. At the same time, the most sunlit surfaces show greater levels of desiccation and salinity, caused by the harsher temperature regime there. So, coastal communities show greatest development and the highest number of species on surfaces moderately exposed to the beating of the waves but sheltered from sunlight.
The period and degree of the exposure to the action of the waves is crucial in determining the distance that each community develops above the sea level. As wetting varies greatly over distances of just a few centimeters, coastal communities are shaped into narrowly-defined horizons.
The supralittoral horizon
The most environmentally limiting horizon is, obviously, the one furthest from the water level. Only a few resistant species have adapted to this horizon, such as the lichens of the genus Verrucaria and the prosobranchial mollusks of the genus Littorina, periwinkles. The very distinctive thin black patches formed by Verrucaria are common throughout the world's seas. Littorina form small groups in the fissures or clefts of rocks, and resist the impact of the waves by secreting a mucilaginous substance that sticks them securely to the rocks. Sticking tightly to the rock lets them retain their internal moisture, and is one strategy to avoid the desiccation caused by exposure to the air for long periods. A mobile fauna is associated with the Verrucaria horizon: an example is the isopod Ligia italica. This organism can migrate to the submerged zones when conditions are unfavorable, but it normally grazes on the cyanobacteria and chlorophytes growing around the Verrucaria colonies.
When the substrate forms a flat rock platform next to the Verrucaria horizon, and subject to conditions similar to those of that horizon, a series of small coastal pools occur. They contain a very special flora and fauna, consisting mainly of cyanobacteria, diatoms, benthic protoctists, and some harpacticoid crustaceans, as well as insect larvae. All these highly ephemeral species are adapted to the limiting conditions in this water, derived from both rainfall and the seawater left after strong wave action. These isolated systems lacking persistence have little effect on the rest of the coastal system.
The littoral horizon
Below the supralittoral horizon lies the littoral horizon, corresponding to the intertidal zone of the seas with tides. In seas like the Mediterranean, it is only one or two meters wide, and is found slightly above or below sea level. Unlike the supralittoral horizon, this horizon experiences almost constant wetting, thereby substantially increasing its biomass. Within this horizon there is a series of narrow levels (often clearly defined), each consisting of a species or community with a different degree of adaptation to desiccation. So, in communities that sometimes remain above water level, abiotic factors regulate species distribution and abundance. In permanently submerged communities, however, biotic factors such as competition and predation play the most important regulatory role. In this coastal environment, the success of a species depends on a minimum level of moisture; when a species is assured of this necessary supply, it can grow rapidly, become dominant and invade much of the habitat. This unusual, almost total occupation of the available space, is the reason for the pattern of zonation into horizons, or bands above the water level that consist almost entirely of a single species. The disappearance of some of these horizons occurs very slowly and is the result of pressure from another species, which becomes dominant through epiphytism or predation.
This zonation pattern is very clear on siliceous substrates that are regularly beaten by the waves. These zones coincide with the high latitudes, while at low latitudes zonation is less clear, mainly because the higher temperatures make the survival of species more difficult. The upper level of the littoral horizon is mainly occupied by acorn barnacles (Chthamalus), which are very abundant in the intertidal zone of the North Atlantic. The lichens and encrusting algae in this community often form part of the diet of the many gastropod mollusks that occasionally visit from lower horizons. Other species common in this horizon are periwinkles (Littorina) and limpets (Patella).
Below the Chthamalus community there are several communities of encrusting algae and fleshy, erect seaweeds that precede another community formed by encrusting seaweeds or polychaetes, located right at the water level. This encrusting belt grows in wave-beaten environments, and its degree of development depends on the intensity of the wave action. The belt thrives on steep slopes with little sunlight, forming a cornice or ledge (the "terrace" or "pavement") a meter or more in width, formed by the dead, cemented thalluses of coralline algae of the genus Lithophyllum. In more protected areas with greater water renewal, this is replaced by a belt of densely packed thalluses of upright seaweeds (such as Corallina, Ceramium, Gelidium), some of them with calcareous encrustations, and which look like small mattresses. The seaweed thalluses have a holdfast, a root-like system attaching them firmly to their substrate. Normally, communities are broader and have greater biomass on calcareous substrates. For organisms living in environments with intense and constant hydrodynamism, like the littoral, the development of structures to fix them to the substrate is as important as withstanding desiccation or high daytime temperatures.
The littoral environment is very inhospitable due to the desiccation and the high temperatures, but constant wetting can make it very favorable, leading to special development at the water level or just below it. Life proliferates in these conditions, generating complex and highly structured communities with great microhabitat diversity. Both environmental conditions and the activity of organisms contribute to this diversity.
Although the water mass moved by the waves has the same impact on all the members of the community, convoluted substrates or variations in relief break the flow down into differing intensities. As an example, water flowing over a ledge (or pavement) of Lithophyllum lichenoides moves twice as fast as the water only a few centimeters below, where it is slowed down by small cavities. This very heterogeneous distribution of water flow will lead to the creation of distinct microhabitats. Furthermore, the presence of different sedentary species that form dense populations, such as mussels, will favor the creation of new substrates for colonization. The spaces between shells are very suitable for interstitial organisms, which can shelter from the strong hydrodynamism among the shells of clump-forming mollusks, and during the dry periods they can use the water trapped in the interstices.
This leads to a great diversity of species, each adapted to very precise environmental conditions. These species may disappear from the community at some times of year due to the harsh environmental conditions. The summer conditions are very unfavorable in temperate seas, for example, as a result of the heat and the reduction of wave action. In the spring, however, primary production increases, and the space available increases when the seaweeds renew growth and new ones arrive. Many dormant organisms that have formed a stolon or are living at a low level of metabolic activity spring to life and occupy the habitats that are being created. The moderate spring hydrodynamism brings a constant supply of food for the suspensivores and nutrients for the seaweeds. The communities decline after the middle of summer, when there is little hydrodynamism and the many hours of sunshine cause the temperature to rise considerably. Furthermore, these periods of summer calm mean that the water of the coastal pools is hardly renewed, and these communities consequently decline and may even dry out. After surviving the months of desiccation, autumn storms help to increase the biomass of the community as a whole, although the lower availability of nutrients prevents the community developing as much as in the spring. Community production is maintained during the winter, although it is generally slowed by the strong storms and the reduced sunshine.
1.2 Biological strategies
Several organisms, especially animals, have adopted a series of strategies to exploit both the supralittoral horizon and the littoral horizon. The basic problem they must solve is how to remain in the right place in order to feed themselves and defend their territory.
Attachment to the substrate
Like seaweeds, many animals develop strategies to avoid damage by the action of the waves or being swept away. This is why very hard anatomical structures, such as shells or calcareous exoskeletons, are so common. Yet this rigidity imposes biomechanical limits and has a high metabolic cost, as it permanently increases the animal's weight. And so other organisms have adopted soft, very elastic forms that move in rhythm with the waves and currents without being detached. For example, the accordion-shaped series of rings that hydrozoan colonies have within their chitinous cuticle gives them some elasticity to resist the force of the currents.
In fact, high hydrodynamism favors sedentary species more than mobile ones, except those that develop some type of strategy to avoid being swept from the community; the most frequent strategy is to shelter in the interstices of the substrate or in the small cavities provided by the growth of encrusting organisms. In this way, in the Mediterranean, the ledges formed by the calcareous seaweed Lithophyllum lichenoides house a rich and varied fauna, with densities of more than a hundred individuals of different species per square centimeter. In the interior of calcareous formations, a circuit is formed consisting of small channels and microcavities full of polychaetes, mollusks, amphipods, isopods, etc. Most of these organisms pass all of their life in the interstices of the seaweed patches, only performing small migrations within it when high temperatures and dry conditions make the surface of the cornice very unfavorable. The water in these spaces is continuously renewed, as the water hitting the outside surface of the cornice rapidly penetrates by capillary action into the interior. The ebb and flow of the waves favors the renewal of the water the organisms extract their oxygen and food from.
Although the environmental conditions are very demanding for the species living in the coastal environment, it is the marine zone that receives the most reliable food supply. The wealth of nutrients and the greater amount of sunshine increase the biomass of the primary producers. These in turn serve as food for the herbivores and as a refuge for the rest of the animals, most of which are suspensivores.
Marine benthic environments, unlike terrestrial and planktonic environments, have few herbivorous or carnivorous species. Most of the species there feed on the organic material in suspension in the water. These suspensivorous organisms have different anatomical structures to capture prey or filter particles. In fact, this profitable strategy only requires them to settle on a substrate where currents ensure water renewal and absorb the incoming food particles, thereby saving them the energetic expenditure of actively seeking food. There are many suspensivores in an environment like the littoral, where wave action means there is a continuous flow of water. Many settle at water level or just below it, making best use of the flow, from which they also extract the gases they need for respiration. A clear example are the populations of the mussel Mytilus galloprovincialis located on the ledges formed by calcareous seaweeds or just above them. These bivalve mollusks use a system of syphons to filter the surrounding water and extract the small organic particles and the phytoplankton that they feed on. The dense mussel populations that often develop on the coastline show that the system is dominated by continuous hydrodynamism that transports water masses rich in suspended materials.
Most of the sedentary organisms of the littoral environment reproduce by means of planktonic larvae that are the result of sexual reproduction. After acquiring juvenile characters, these larvae have to establish themselves in the same habitat as the adult. In environments such as the littoral, this reproductive pattern is complicated by the fact that, after their release, the larvae are scattered very quickly by the currents. Many species in this habitat have developed alternative strategies to resolve this problem, such as incubating their larvae, producing enormous numbers of larvae to ensure some will return, and asexual reproduction by fragmentation or the production of stolons (runners or suckers).
The sea anemone Actinia equina exemplifies a reproductive strategy adapted to this environment. This species fertilizes its eggs by ingesting the spermatozoids released into the environment, but unlike most anemones, it does not release the eggs or the larvae, but instead incubates them until they are small adults, which immediately settle on the substrate on expulsion into their surroundings. The adults of this species have also adopted the reproductive strategy of dividing by transversal fission, avoiding the need to leave the substrate.
Many sessile (permanently attached) species can only reproduce sexually, which results in the formation of planktonic larvae (such as hydrozoan medusas) that remain suspended in the water until ideal conditions (new substrate) arise for return to the littoral habitat.
In spite of strong hydrodynamism, in an environment like the littoral where the food and nutrient supply is constant, the number of individuals may be very large. Yet space is one of the scarcest resources in benthic communities, especially in coastal benthic communities, because the space available for settlement is very limited. The heavy interspecific competition this creates is partly resolved by developing specialized colonization strategies.
Many sessile organisms colonize other organisms--for example the thalluses of seaweeds--as a secondary substrate. Many motile organisms shelter among the seaweeds clumps by holding on with a range of joints or adhesive substances. These epibiotic organisms' most serious problem is the ephemeral nature of their substrate; the solution they have adopted is to grow very fast (some can double their biomass in less than a week) so they can continuously occupy new substrate.
Many epibiotic organisms form stolons (hydrozoans, bryozoans, and ascidians) and orient them towards the base of the thallus. Things are easier for mobile animals because they can jump from thallus to thallus as soon as they are generated. When environmental conditions become unbearable, the substrate begins a process of degeneration and senescence.
2. Low-lying coasts: beaches, marshes, and deltas
2.1 Habitat distribution
As already mentioned, strictly speaking there are no seas without tides. Nevertheless, some seas have a very narrow tidal range, given that tide intensity depends on the mass of water involved and its geographical situation, as well as on local phenomena related to the form of the coast.
In enclosed seas, the boundary between the terrestrial and marine environments is very narrow. If the coastline is low, this limit takes the form of a beach and a more-or-less developed line of dunes, or in the case of a watercourse flowing into the sea, a zone of a marshes or deltas, varying in complexity with the river's size and the sediments it bears. The deltas of large rivers may have many active branches, inactive branches of various ages, or pools or lakes connected more-or-less directly to the sea or to rivers (e.g., the Nile delta and Rhone delta).
Life has diversified greatly in the frontier zone between dry land and the sea. As well as diversity in space, there is also diversity in time, because environmental factors change over the course of the year. The combination of climatic factors with salinity levels, soil types, and relief gives rise to numerous ecosystems and a mosaic of different habitats and vegetation types. Conditions are usually harsh due to exposure, lack or excess of water, the presence of salt, and other factors.
Beaches and coastal dune belts
Beaches form where the waves no longer have the energy to transport sedimentary material. The form of the beach depends on the geomorphological characteristics of the adjacent land, on the currents, on the quantity of sediment they bear, and on the strength of the waves. If there is a current eroding the coast, the beach is narrow. On the other hand, a wide beach will form if the current dissipates its energy in a bay or near the mouth of a large river.
A beach is a dynamic environment where the size of the particles forming the sand determines its porousness, as well as its levels of capillary action, humidity, and salinity in summer. The sedimentary particles' lack of cohesion, together with the force of the waves, are the cause of primary production being often very low, possibly even restricted to just a few algae (diatoms). Yet the decomposer chain can be very important owing to the organic material deposited by the sea. Even so, if the sand is not too saline and moisture permits it, some plants might develop, especially the sea rocket (Cakile), growing at higher levels.
The beach fauna consists of both marine organisms living within the sand and terrestrial organisms living on top of the sand. The marine, interstitial, or terricolous fauna, consisting of polychaete lugworms (Arenicola) and various bivalves such as clams (Cardium), take advantage of both the layer of subterranean marine moisture and the input of organic material to the surface. A number of birds, such as plovers (Charadrius), oystercatchers (Haematopus ostralegus), and turnstones (Arenaria interpres), feed on the fauna buried in the sand in the breaker zone. During favorable periods, a modest fauna develops in the area of contact with the sea, closely linked to this environment and consisting of crustaceans (Talochestia) and coleopterans (Cicindela trisignata). Landward beach areas and bare areas of dry sand play home to a fauna largely composed of generally nocturnal insects (coleopterans, curculionids, etc.) which feed on vegetable matter deposited by the sea.
Sand dunes are characterized by an extremely harsh environment, caused by dry conditions and the wind. Onshore winds exacerbate these effects (sea spray), while strong offshore winds can cause the structure of a coastal dune chain to break down. The loss of fine sand particles causes a relative increase in larger particles at the surface, a factor that is even more important when stabilization by vegetation is weak, leading to the formation of extensive low dune systems.
The dunes most exposed to the sea tend to be the highest, and their surfaces are always mobile. The dune retains rainwater above the water table and supplies the plants growing on the dune or at its base. The plants that are likely to colonize these dunes include typically cord grass (Spartina versicolor) and marram grass (Ammophila arenaria); not only do they help dune formation, but also ensure their conservation.
Some species of land snails inhabit dune chains, for example the monk snail (Eobania vermiculata) in the Mediterranean, and there are also various species of insects (orthopterans, coleopterans, and, above all, many tenebrionids and detritivorous scarabaeids). Conditions are unfavorable for amphibians, although some batrachians such as the spadefoot toad (Pelobates cultripes) survive, as do reptiles (such as lizards of the genus Psammodromus).
Inner dunes are higher and so they are less affected by the water table. They are normally beyond the influence of the waves and are poor in soluble salts. Thus, they are an ideal habitat for species with limited or no salt tolerance. Despite their enriched soil and the input of organic matter, the very dry conditions that occur on inner dunes still make life very harsh for plants.
Where the effects of the water table are absent, atypical species may occur that are adapted to marked seasonal drought and are present on a regional scale in other xeric environments. Dune systems throughout the Mediterranean, for example, provide suitable environments for shrubs such as Phillyrea, Pistacia, and junipers (Juniperus phoenicea, J. lycia). The spread of rainwater nearby allows the growth of other typical species, such as field wormwood (Artemisia campestris), crosswort (Crucianella maritima), and everlasting (Helichrysum stoechas).
Coastal lagoons and marshes are the result of coastal sedimentation, although very occasionally they are caused by the raising of marine terraces. They are usually connected to the sea, if only occasionally. This connection with the sea, or with rivers, governs features such as water quality (including its salinity), water circulation, and nutrient cycles.
Lagoon environments are sites of great biological activity, although species diversity is not great, and the species present are usually highly adapted to this environment. When contact with the sea is significant, the vegetation is mostly composed of eel grass (Zostera) and Ruppia, but several species of pondweed (Potamogeton) appear in less saline conditions. Excessively saline lagoons are unfavorable for fauna and only highly adapted species are present, such as a handful of crustaceans, mainly the brine shrimp (Artemia). When salinity levels rise above 120 g salt per liter, the only animals that appear are the larvae of some dipterans (salt flies of the genus Ephydra), microturbellarians, nematodes, and mobile protoctists, such as the ciliate Fabrea salina. When salinity levels oscillate between 80 and 120 g salt per liter, the absence of fish permits a thriving invertebrate fauna rich in dytiscids (diving beetles) and copepods, the basic food source for many marsh birds.
The presence or absence of fish depends on links with the sea. Variations in salinity, freshwater inputs, and changes in water levels, because they multiply the combinations of environmental factors, are responsible for increasing the number of species, which are mostly euryhaline (adapted to a wide range of salinity). The species that live mainly in salt water, such as the sand smelt (Atherina boyeri), three-spined stickleback (Gasterosteus aculeatus), common eel (Anguilla anguilla), and sea-perch (Dicentrarchus labrax), tend to come and go between the sea and the lagoons. Seaweeds such as Ulva and Enteromorpha often grow when there is little water flow or when the lagoon is cut off from the sea. This provokes an anoxic (oxygen depletion) crisis, worsened by excessive nutrient input, especially of nitrogen and phosphorus. These critical periods of anoxia are very harmful to flora and fauna alike.
Saltmarshes are found in low-lying areas that are not usually flooded directly by the sea (except in storms). The sea's influence is mainly in the form of seawater seeping inland underground. Subterranean water plays a very important role in deltas. Water circulation is conditioned by the alluvial belts deposited by abandoned river branches, which compartmentalize the deltas into small basins that constantly change from being connected to or separated from each other. Evaporation leads to capillary action and increases the levels of salts in the subsurface soil horizons and on the surface. Precipitation of salts during dry conditions and their return to the soil by rainfall mean there are great variations in the salinity levels, and these in turn have a decisive effect on the vegetation. Winds can modify the sea levels and those of coastal lagoons, and may speed underground water flow up or slow it down, causing changes in the water table.
Far from the beaches, rainwater is the main, and often the only, source of water. The broad alluvial plains extending behind the dune belts are flooded in the winter rains. This means that in summertime, when salinity in the subsurface soil horizons may be high, the actual topsoil suffers a shortage of salt and from drought. The actual degree of salinity depends basically on the physical characteristics of the area (altitude, particle size, etc.).
The flora consists of only a few species, mainly chenopods, such as saltwort (Salsola), seablite and purslane (Suaeda, Halimione), and glasswort (Salicornia [=Arthrocnemum]). Greater distance from the shore and increasing altitude decrease salinity levels and modify their distribution. Reduction of the intensity of flooding leads to the appearance of other halophytes; the composite, golden samphire (Inula crithmoides) is potentially dominant when a large area of sand is affected. Plants that are less tolerant of flooding and salinity are found on the microrelief. These include common sea-lavender (Limonium vulgare), sea purslane (Halimione portulacoides), and the Poaceae Agropyron elongatum and Puccinellia festuciformis.
Slight flooding by not very salty water favors the development of spring flowering annuals that exploit the annual periods of lowered salinity, such as the crucifer Hutchinsia procumbens and a ranunculaceous plant, the mousetail (Myosurus minimus). As far as Poaceae are concerned, Aeluropus littoralis often dominates the edges of flooded saltmarshes and temporary marshes, either alone or with tamarisk (Tamarix gallica). Where salinity is low, tall rushes such as sea rush (Juncus maritimus) and sharp-flowered rush (J. acutus) grow. A whole series of grasses appear on soils at greater elevations and with less risk of flooding, such as wall barley (Hordeum murinum), Polypogon maritimus, and the saltmarsh rush (Juncus gerardi), all typical plants of salt meadows that are subject to occasional flooding. Although in these habitats internal drainage is still relatively inefficient, salinity levels in their deeper water table do not usually reach 10 g per liter. Evaporation still causes important quantities of salt to rise to the surface, but these environments typically show increased species diversity, with an abundance of annual plants. However, the commonest plants found in these areas include sea-lavenders (Limonium), Buck's-horn plantain (Plantago coronopus), orache (Atriplex hastata), sedges such as mud rush (Juncus gerardi), or Poaceae such as wall barley (Hordeum murinum), brome (Bromus hordeaceus), and Glyceria festuciformis.
A diverse but essentially invertebrate fauna makes use of temporarily flooded saltmarshes: crustaceans, arachnids, coleopterans, dipterans, and mollusks such as the small gastropods Hydrobia acuta and Lauria cylindracea. The increased number of species is linked to the reduction in salt concentration. Reduction of salinity leads to the appearance of a nonhalophilous fauna which, in turn, favors the presence of residual moisture under the plant cover.
Dry salt meadows
On higher ground, salinity is low and flooding is uncommon or non-existent. Nevertheless, salt continues to be an important factor owing to the influence of the water table, as shown by the presence of saltmarsh rush (Juncus gerardi) and Limonium. Together with inland dunes, these meadows are the most species-rich environments in coastal areas. The summer drought means annual species are important, especially wall barley (Hordeum murinum), brome (Bromus hordeaceus), and the daisy (Bellis annua).
The heterogeneity of the vegetation permits a rich fauna to thrive which, because the conditions are less rigorous, is not typically coastal. The animals present tend to be locally common in the region's xeric environments and generally show no particular affinity for saline environments. Likewise, these grassy areas and other meadows not subject to flooding are the habitat of several species of batrachians, reptiles, and small rodents. Seasonal marshes
Seasonal coastal marshes are mainly found in wet coastal areas subject to alternating wet and dry seasons. They cover large areas in the Mediterranean region, an area where the hot season is very marked and where rainfall is most intense in spring and autumn. However, flooding can be caused by storms at sea, by a rise in the flow of rivers supplying the deltas, or by heavy rain.
The salinity of seasonal marshlands varies greatly with their distance from, and the salinity of, the water table; the porosity of the substratum which governs capillary action; the origin of floodwater (sea, river, or rain); the salinity of the river basins; and other factors. Salinity may change during the flood cycle (or the yearly cycle) according to the balance between the freshwater input in the form of rain and evaporation during drier periods.
Primary production varies from one marsh to another, as well as from one year to another. The primary producers vary greatly with the water regime, depth, salinity, and the physico-chemical characteristics of the surface water and those of the substrate. Seaweeds are the leading photosynthesizers in highly saline marshes. Submerged angisoperms tolerant of large fluctuations in salinity, especially those of the genera Ruppia and Althenia, are dominant in saline marshes.
In freshwater or slightly saline marshes there is a greater variety of primary producers. Marsh plants (helophytes) dominate: mainly reeds (Phragmites), bulrushes (Typha) and rushes, as well as club rushes (Scirpus), while rushes such as Juncus maritimus are restricted to the lagoon edges. Bulrushes are characteristic of freshwater, whereas reedbeds with rushes are found in more saline areas. Water depth also affects the distribution of species. The common club-rush (Scirpus lacustris), for example, prefers deeper water than the sea club-rush (S. maritimus). Submerged plants have to compete with marsh plants for sunlight and often can only thrive if their competitors are grazed, cut, or burned.
Submerged macrophyte pioneers are mainly charophytes (especially Chara vulgaris and C. aspera), and are generally dominant during the period immediately after flooding. During the actual flood season, the dominant species that replace them are slower growing and are generally represented by species of pondweed Potamoge-ton.
The distribution of submerged macrophytes also depends on salinity: Chara vulgaris and C. aspera, or other species such as the charophyte Tolypella glomerata and Potamogeton pusillus, are characteristic of freshwater. Increased salinity favors the development of communities dominated by plants of the genera Callitriche and Zannichellia. Water crowfoot (Ranunculus baudotii) is able to grow in water with salinity of up to 15 g per liter, on the edge of the transition to the saline waters dominated by Ruppia communities.
Freely floating species such as the duckweeds (Lemma minor, L. triscula, and Spirodela polyrrhiza), and small aquatic ferns of the genus Azolla also grow in great abundance in temporary marshes.
The vegetative parts and the seeds of submerged macrophytes constitute a very important part of the diet of plant-eating birds. For many animals, the duration, timing, depth, size, and isolation provided by flooded pools are the crucial factors. Fish only occur when flooded areas are in contact with larger bodies of water.
Connections between temporary lagoons and brackish waters permit the coexistence of halophilous and freshwater fish such as the carp (Cyprinus carpio), tench (Tinca tinca), sunfish (Lepomis gibbosus), pike (Esox lucius), and mosquito fish (Gambusia affinis). They spawn in the spring to avoid the summer drought months. If fish are absent, a group of organisms develops that these predators would otherwise eliminate. Their planktonic fish belong to many genera of branchiopods (of the orders Anostraca, Conchostraca, and Notostraca) and copepods. Insects thrive: culicids and chironomids attract predatory fauna dominated by dytiscids and odonates. The absence of fish also encourages amphibians, and in slightly saline or fresh marshes, the marsh frogs (Rana ridibunda and R. esculenta) are abundant, as are toads such as the natterjack toad (Bufo calamita) and predatory reptiles, mainly water snakes of the genus Natrix.
Most birds found in marshy areas are migratory. These large bodies of water provide ideal breeding spaces for cormorants (Phalacrocorax), squacco herons (Ardeola ralloides), and little egrets (Egretta garzetta). Habitats rich in invertebrates and amphibians are extensively exploited. In winter, large numbers of aquatic birds arrive on the shores of the Mediterranean's freshwater marshes from all over the western palearctic. The most common are mallard (Anas platyrhynchos), shoveler (A. clypeata), gadwall (A. strepera), greylag goose (Anser anser), pochard (Aythya ferina), and coot (Fulica atra). These temporary marshes provide food, above all, at night. Many migratory waders such as the black-tailed godwit (Limosa limosa) and green sandpiper (Tringa ochropus) take advantage of wet or temporarily flooded areas, as do other non-migratory birds like the ruddy shelduck (Tadorna ferruginea) and black-winged stilt (Himantopus himantopus). The mammals present tend not to be confined to these environments or to marshes in general. Their abundance is more due to the abundance of natural or protected environments in many wetlands. Most mammal species only feed incidentally in flooded areas, although some, such as wild boars (Sus scrofa) and foxes (Vulpes vulpes), make full use of them. The wild boars root out the tubers of Juncus and Scirpus, their main wild food source in these areas, and foxes principally search for ill or injured sea birds.
Forest trees that can colonize areas on or near dunes have to overcome this environment's many specific problems. In particular, they have to live with salinity, fog, and drought, as well as coping with the occasional mobility of this habitat. Stone or umbrella pines (Pinus pinea), maritime pines (P. pinaster), and Aleppo pines (P. halepensis) temper the force of the winds and thus help to fix dunes.
It is easier for woody species to colonize inland dunes and grow on them. The higher areas in deltas do not suffer anoxic conditions or excessive salinity, and they are thus optimal areas for the development of forest ecosystems. However, these areas also tend to be the most suitable as agricultural land for intensive cultivation. As a result, most forest ecosystems now only consist of a narrow band of deciduous trees running along river banks and some alluvial torrents.The lack of tides means that marine influences are almost imperceptible and so riverine forest tends to be little different from more inland woods, although right next to the sea there is saline stress (fluctuating salinity). Species distribution depends on their tolerance to flooding and the depth of the water table. The commonest species are poplar (Populus), ash (Fraxinus), alder (Alnus), and elm (Ulmus).
2.2 Adaptation to fluctuating environments
It is adaptation that ensures species survival and continuity. And the harsher the conditions, the greater the specialization necessary. Where there is no tide, the main environmental difficulties that typically arise in coastal areas are salinity, flooding, and drought.
Salinity has two elements, toxicity (toxic ions, especially Na+ and Cl-) and osmotic pressure (the greater the salt concentration, the greater the plant's difficulties in nutrient uptake), both of which become worse in dry conditions. Water excess during floods is often a crucial factor for plants, regardless of whether they are submerged or not, whether the soil is waterlogged, or whether the water table is near the surface. Flooding, for example, leads to a decrease in soil oxygen, which results in a reducing environment (with a weak redox potential) and the accumulation of toxic decomposition products that limit nutrient availability.
Nevertheless, the three factors mentioned above (salinity, excess water, and a reducing environment) may accumulate or follow one another, depending on the individual case. Salinity combined with flooding causes zonation between species with different responses to these two factors. However, in coastal areas in the Mediterranean, a further difficulty must be taken into account--the great variability in the time each factor lasts. This variability takes the form of winter floods when the quantity of salt is low, alternating with summer droughts when salinity is high. Likewise, there can be great variations in the actual dates of flooding and drying out of the marshes from one year to another. All in all, this tends to reduce the importance of competition, and population survival tends to be largely a function of their tolerance of the most severe physical conditions.
In order to survive, the plant species that have had no difficulty in moving have either had to adapt and acquire tolerance of high salinity, or have adapted to avoid these problems by completing their life cycle (germination, growth, flowering, and seed ripening) during their favorable periods. The first strategy, resistance, is found in perennial species and seeks to ensure the long-term survival of the individual plant. Avoidance strategies are found in annual species with life cycles measured in weeks.
Plants possess a wide variety of mechanisms for limiting the effects of salt. Thus, apart from some dune plants, most salt-tolerant plants are sensitive to salt spray, which they can only tolerate to some degree. Some plants, the true halophytes, are stimulated by the presence of salt. Many of the monocotyledons that grow in saline environments can excrete salt through their roots, whereas the dicotyledons can generally accumulate salt without suffering any damage. Chenopods, for example, possess storage vesicles, and Salicornia, Suaeda, and Halimione can dilute the salt concentration. Some Plumbaginaceae, such as the genus Limonium, have saltexcreting glands.
Often problems of salinity go hand in hand with those of drought, and plant adaptations are frequently a response to both. For example, chenopods such as Salsola and the grasses of the genus Ammophila possess osmotic regulation mechanisms in their cells to prevent the destruction of their enzyme systems.
Some physiological adaptations may respond directly or indirectly to problems associated with anoxic conditions. Some plants have anaerobic breakdown mechanisms so they can avoid self-intoxication caused by lack of oxygen. The oxidation or reoxidation capacity at the root level facilitates the immobilization of toxic factors, such as iron and copper.
Anatomic adaptations are mainly designed to increase drought resistance. Thus, many species have a xeromorphic structure that limits transpiration. The sea-lavender (Limonium) and the sea club-rush (Juncus maritimus) are sclerophyllous, and most dune species have sclerenchyma fibers. To deal with the effects of dry conditions and salt, dune-dwelling monocotyledons as a whole and many of those living in saline marshes have protective films, hairy leaves, or waxy cuticles. These mechanical adaptations also help combat the indirect effects of flooding and, especially, the consequences of the anaerobic way of life. To some extent, root aerenchyma alleviates oxygen deficiency and limits the toxicity of the compounds that accumulate.
Reducing the role played by roots (as assimilation of nutrients is by aerial structures) and adopting aquatic pollination permit plants that are submerged for long periods of time to survive and grow.
Life cycle adaptations
High salinity, and to an even greater extent drought, can be lethal to plants and so many of those found in coastal environments are annuals. Irregular climatic conditions and variability in the dates of drying out and flooding forces plants to adopt particular forms of reproduction. Vegetative reproduction is often very active and permits rapid occupation of space, although a seed bank must also be created. Dormancy, delayed germination, seed dimorphism, and germination in the absence of light exemplify the range of plant mechanisms to ensure that there are offspring in spite of the differences in conditions between one year and another. The glasswort (Salicornia ramosissima) occupies transition zones between temporarily flooded areas and those which are permanently flooded; those growing in the center of the population produce seeds that do not need light to germinate and are little affected by saline conditions, while those produced by plants on the periphery need light, require vernalization, and are sensitive to salinity.
The submerged macrophytes that are so abundant in temporary marshes survive the summer droughts by shortening their vegetative cycle. The first seeds of Zannichellia pedunculata to germinate do so when the first floods arrive, often in the autumn, and have often finished growth before the onset of winter. Species of this type must also maintain a permanent seed bank, often accompanied by a mechanism to stagger germination over several years. Generally, the propagules can survive long periods of time and disperse well.
The problems caused by drought, flooding and salinity have led the fauna of these areas to develop a series of mechanisms based on their ability to protect themselves and on physiological adaptations to withstand environmental variations. However, in the case of animals, we must add their possibility of movement, which is absent or only passive in plants. Each group usually has its own strategy.
In the most disturbed environments such as coastal sands, river banks, seasonal marshes, and saltmarshes liable to flooding, the fauna's reproductive strategies are similar to those shown by the plants. When conditions are favorable, many invertebrates undergo a population explosion, followed by an equally sharp mortality when conditions are no longer so favorable. These patterns also affect some animals such as crustaceans, mollusks and chironomids which inhabit more stable environments, such as shallow lagoons. Other environmental factors, especially temperature, also oblige species to behave as if they were living in a seasonal environment.
The most common drought-resistance mechanisms used by invertebrates include encystment, temporarily burying themselves, making use of microvariations in humidity, migration to permanent water, the acquisition of winged adult forms, and the adaptation of reproductive periods to the most favorable times of year.
The movements of arthropods tend to be seasonal and mainly vertical. Oligochaetes (earthworms) are isolated in the non-flooded areas and their life cycle, directly correlated with availability of rain, is interrupted during the hottest months of the year. Other groups, however, such as the isopods found in a great variety of habitats, generally move horizontally. These horizontal movements are important in temporarily flooded areas. When increases in temperature make the humidity drop drastically, these crustaceans have to move to the leaf litter, the edges of lagoons, or empty shells. They remain there until the autumn rains arrive, when they can migrate in the reverse direction. Sometimes, movements depend on the presence of plant cover, and the residual moisture means the population can limit its migrations even during the most critical periods. Arachnids generally cope very well with high temperatures and dry conditions, but because these factors limit food availability, they are also forced to migrate to environments where there is still some water.
The arrival of dry conditions is not such a determining factor for winged invertebrates. For example, most species of coleopterans that inhabit both more continental and xeric environments can easily tolerate these summer conditions. They are only occasionally found in saline flood-prone environments, and their adult forms can fly long distances in search of more favorable habitats.
During droughts, relatively immobile aquatic species adopt very effective resistance mechanisms. The often harsh changes in environmental conditions, together with their irregular nature, lead to important changes in reproductive cycles and population levels from one year to another. Especially illustrative is the case of the gastropod Anisus rotundatus, common in seasonal marshes. During the summer it hibernates in the first few centimeters of sediment, below leaf litter, at the base of plant stems, or in dead portions of trees. It secretes an epiphragm to seal its shell and prevent desiccation. Its periods of growth and reproduction are closely linked to high temperatures. Low temperatures block not only reproduction, but also the development and growth of eggs. When the rains return at the beginning of autumn, it reproduces quickly before winter comes and, for a period of time, two generations coexist. The offspring gradually replace the adults still present after the egg-laying period is over. On the other hand, when the rains do not return until the beginning of winter, the fact that reproduction does not take place in the autumn means there is only one generation until the following spring.
Another resistance mechanism is for many individuals to group together. A small snail, Theba [=Euparypha] pisana, responds to abiotic factors (humidity, temperature, wind) and biotic (density of individuals) by climbing plants to form colonies, which create moister air conditions. Depending on atmospheric conditions, these groups may last only a few weeks or as long as several months.
Differing uses of environmental resources by adults and juveniles of some fish species ensure the continuity of generations. Temporary marshes that do not form closed systems are often frequented by the numerous fish that find them ideal places to spawn. Carps (Cyprinus carpio), if possible, move in spring into temporary marshes with suitable vegetation. For most adult carps and for many younger fish, temporary marshes represent a potential trap that many predators (gulls [Larus], wild boar [Sus scrofa], and foxes [Vulpes]) take advantage of. On the other hand, newly hatched fry can easily reach channels where food is abundant and grow to maturity, thereby guaranteeing that there is a new generation.
Many arthropods possess well-developed water-saving mechanisms. Excreting solids permits them to live in very dry environments. Amphibians possess similar mechanisms. They are able to store urinary excreta for long periods of time without endangering themselves, and thereby reabsorb the large quantities of water that would otherwise be used for this purpose. Retention of excreta, which facilitates osmotic regulation, also permits them to absorb environmental humidity. These physiological adaptations are largely controlled by the kidneys (in turn regulated by neuro-hypophysial and medullo-suprarenal secretions), but amphibians may also have other abilities, such as skins that can reduce their water loss by evaporation or increase their permeability, depending on the humidity of the air.
The capacity to regulate osmosis by means of urinary excreta also allows amphibians to resist changes in salinity. Most land animals do not, however, show any mechanisms that adapt them to salinity. Migrations at the appropriate time and refuges from dry conditions also help protect them from the negative effects of salt. Saline environments are only used for feeding at very particular periods of time, even in the most favorable epochs. The strict dependence of fish on an aquatic environment has forced those species that live in coastal areas to develop very effective adaptive mechanisms to cope with salinity. One of the most important characteristics of estuaries, lagoons, and channels is the great variation in salinity caused by seasonal climatic cycles. This variation means most of these fish have the ability to modify their own internal environment, although this ability is limited and not immediate. Environmental changes are normally drastic, whereas the osmotic regulation processes involved require the changes to be relatively gradual, in order for the fish to adapt to the external environment. Most of the fish that live in coastal waters are not only euryhaline, in other words able to tolerate a wide range of salinity, but also undertake seasonal migrations. Marine species and those able to withstand higher salinity can always go back to the sea when abiotic conditions in lagoons become unfavorable. Nevertheless, physiological adaptations in fish are very precise. For example, evidence has been found of a genetic preadaptation mechanism in atherinid fish that allows them to select this plasticity or not. This flexibility mechanism is more necessary in isolated environments where variability is greater and is probably widespread in the coastal environment.
Adaptations in birds to artificial environments
In coastal wetlands and, in particular, in river deltas, most habitats are now highly artificial. Regular flooding provides fauna, especially aquatic birds, access to complementary food sources. Because of their richness, freshwater marshes are excellent feeding grounds for colonial ardeids (herons), which adapt the size and exact species composition of their colonies to the area of marsh available.
The birds that spend most time in saline lagoons and lakes on their migration do so in order to feed on fish and invertebrates, although some also build their nests there. Small islands are occupied as long as the surrounding water level is high enough to ensure it does not dry out before the young birds have fledged. Premature drought causes abandonment of the nest and migration to a more favorable area. In the Rhone delta, these patterns of behavior have led flamingoes and numerous larids (gulls and terns) to use the sites created by human activities. Thus, flamingoes breed exclusively in salt marshes whose water levels are constant from the beginning of spring to the middle of summer, and where they enjoy a regular food supply and safety from predatory mammals.
In most of the world, marshes and wetlands have been undergoing great changes for centuries. Many have been dried out and turned into pasture, while others are now cultivated. Rice grows better than any other cereal in tropical, subtropical, and Mediterranean wetlands. Some land animals have discovered this abundant source of food and have learned to exploit it. Among birds, the ploceids are highly specialized grain-eaters that find the ears of rice an abundant and easily obtained food source. In the Mediterranean this family is only represented by the house sparrow (Passer domesticus), but in the tropics the many species of the genera Ploceus, Lonchura, and Munia are serious pests of this crop. Another family of land birds that has discovered rice is the psittacids, the parrots and cockatoos, and in parts of southeast Asia members of this family, such as the ringnecked parakeet (Psittacula krameri), can wreak havoc in paddy fields. In the same part of the world, several species of mammal are also attracted by pastures and cereals. The most serious problem, because of their size, are the few still-wild elephants, who often invade paddy fields to graze. The damage to the rice crop is easy to imagine.
2.3 The biological wealth of coastal wetlands
Marshes are very productive ecosystems and so many organisms exploit their resources at some times of year, although they do not live there permanently. The almost constant presence of water and the abundance and variety of animal life in wetlands ensure that food is always plentiful and easy to find. Many animals travel long distances every day in search of water or to hunt the wildlife concentrated around it.
The abundance of pasture and food
Wetlands are vitally important in Mediterranean and semiarid climates, or in any area that suffers a prolonged dry season. This is mainly because the water on which most animals depend is almost always present. To a lesser extent, it is because there is plentiful, good quality grazing in the most low-lying areas, even at the height of a drought, especially when the water is fresh.
In large marshy areas, reeds and wet meadows form the only patches of vegetation with high nutritional value. These wetlands are extremely important for the survival of populations of wild herbivores. Today, few of the world's freshwater coastal marshes remain unaltered, Nearly all the world's coastal zones were among the first areas to be settled and transformed by humans, and large wild herbivores were generally the first animals to suffer the effects of this intensive use, and they were rapidly replaced by domesticated animals, such as horses, cows, and water buffaloes.
By studying some of the lakes and marshes in the interior of Africa, it is possible to observe something similar to how freshwater coastal wetlands might have functioned naturally. During the dry season herbivores seeking pastures perform large movements and migrations from mountains and other dry areas to wetlands on the plains and on the coast. It is almost impossible to witness this type of phenomenon in the Mediterranean or in similar areas in the Americas, where the vegetation and animal populations have been drastically altered.
The presence of large wild ungulates affects the distribution and zonation of vegetation around flooded areas. The outermost margins (those which are only flooded temporarily) are the most heavily grazed and so the large helophytes cannot grow and must seek refuge where the water is too deep for animals but not too deep to prevent the plant itself from growing. Grazing lands are characterized by their diversity of plants and animals, and this is where most aquatic invertebrates, as well as fish, amphibians, reptiles, and birds, are concentrated.
In some coastal marshes, there is a variety of large wild animals, principally wild boar, fallow deer, and red deer, that coexist with herds of semi-wild domesticated animals, mainly horses and cows. A very primitive race of horse that is perfectly adapted to spend most of the year in aquatic environments still exists in the Camargue at the mouth of the river Rhone. Such situations closely simulate what would occur in an hypothetical natural population of herbivores in a freshwater marsh on the Mediterranean coast.
In almost all types of marshes, large populations of flying insects proliferate with the commonest being chironomids, culicids, and ephemerids, all of which form large swarms over lakes and reedbeds. This abundant form of nutrition is exploited by many insectivores that come from far afield. Large groups of chiropterans (bats) fly over marshes in the evening. In the tropics, there are even some highly specialized bats that can capture fish and certain amphibians. At dusk and at dawn many Caprimulgiformes, an order of birds well-represented in almost all continents and in Europe by the red-necked nightjar (Caprimulgus ruficollis) and nightjar (C. europaeus), gather in wetlands to hunt medium-sized flying insects. However, by day the airspace over the marshes is taken over by the swallows and martins (Hirundo) and swifts (Apus) that come from different and often distant habitats to take advantage of the insect swarms found there.
Concentrations of large predators
The large quantities of resources present in marshes encourage many predators to use them as occasional hunting grounds, even normally terrestrial species.
Birds of prey are a good example of this. Apart from those that live permanently in wetlands, many large eagles occasionally hunt in marshes. The imperial eagle (Aquila heliaca) lives in Mediterranean habitats but in southern Europe often flies to coastal marshes to hunt. It makes sporadic incursions into the Ebro delta to catch ducks, coots, moorhens, and other water birds. In eastern Europe, for example in the marshes around the Baltic Sea and more rarely in the Danube Delta and other Black Sea marshes, the spotted eagle (A. clanga) sometimes appears. In western and southern Asia (as well as in the Balkans and on the shores of the Black Sea) the lesser spotted eagle (A. pomarina), and in north Africa and southwest and southern Asia the tawny eagle (A. rapax), all of which are excellent hunters, occasionally make use of the abundant resources of the wetland.
Another more cosmopolitan terrestrial bird of prey that often hunts in marshes is the peregrine falcon (Falco peregrinus). This powerful hunter is capable of bringing down almost any wader and most species of duck.
As well as birds of prey, some large predatory terrestrial mammals are also present in marshes. The jaguar (Felis onca) is a highly skilled predator that used to be common in many wetlands in the Americas, such as the Venezuelan Llanos. Other smaller predators include the puma (F. concolor), which specializes in capturing deer but will also feed on prey taken in wetlands.
Another important group of terrestrial predators only visit the wetlands at certain periods of the year to exploit the abundance of eggs in bird and reptile nests. In Europe they are mainly foxes and wild boar, while in the Americas they are skunks and foxes.
Refuges for fauna
In many wetlands, there are two kinds of situations and formations that provide protection from predators: islands and reedbeds. In the former, a broad expanse of water over a meter deep is an impassable barrier for many predators. Consequently, in wetlands many islands are chosen by birds as a safe place for nesting and resting. The dense vegetation and the water that covers them entirely except the stems and leaves makes reedbeds and communities of bulrush, cattails and rushes, and other helophytes impassable to many predators, and they rarely enter such areas.
These areas, far from the shores and the water surface itself, are like islands that are almost impossible to reach. This is why many terrestrial birds roost on islands and in reedbeds at certain times of year, especially when they form flocks outside the breeding season. In Europe species such as starlings (Sturnus), reed buntings (Emberiza schoeniclus), and swallows (Hirundo rustica) roost in reedbeds. Wooded islands, on the other hand, are used by wood pigeons (Columba palumbus), stock doves (C. oenas), and magpies (Pica pica). In North America, some Icteridae such as the redwinged blackbird (Agelaius phoeniceus) use marshes to breed, although they live in neighboring agricultural areas, and in winter they occasionally form "dormitories" with as many as 15 million birds.
In Africa, various ploceids spend a large part of the year in trees, agricultural lands, or in savannas feeding on grain and seeds, but congregate in wetlands to breed. Some species build their nests over the water, hanging from trees, while others such as the red bishop (Euplectes orix) weave nests into reeds and bulrushes. Bulrush and reed leaves are almost always used to construct their nests.
Apart from birds, some mammals also seek refuge in reedbeds and on islands from other animal predators and human hunters. The classic example in Europe is the wild boar (Sus scrofa), which in winter often takes refuge from hunters and their dogs in the impenetrable large reedbeds. In North America, the white-tailed deer (Odoi-coileus virginianus) acts similarly and has been able to survive very high levels of hunting by seeking refuge in wetlands that humans cannot enter.
2.4 Marsh birds
Of all the vertebrates, birds are the group that has adapted best to wetlands. Large numbers of species and individuals are found everywhere from arctic tundra to mangrove swamps and other tropical wetlands. Birds make very efficient use of marshes' high productivity and have managed to occupy all the ecological niches available. In cooler temperate areas, birds use wetlands basically as breeding areas and there are few non-migratory species that can survive all year round, but in lower latitudes there are nonmigratory species, as well as wintering and migratory birds that stop to feed and rest.
Many bird migration routes are not merely international but intercontinental. Many migratory routes are well-known, such as those between Europe and Africa across or around the Mediterranean, those between north and south Asia (with some species even reaching Australia), and those between North America and Central and South America. Tropical bird populations are less mobile, although large numbers of wintering birds arrive annually from colder climates and there are more local movements due to the seasons and the presence or absence of water.
Marsh birds are generally large, typically show a great variety of forms and colors, and tend to form large flocks for the purposes of breeding, feeding, refuge, or rest. Thus, these birds often provide some of the world's most dramatic natural spectacles. The study and observation of aquatic birds, relatively easy and very rewarding, is carried out by millions of amateur and professional ornithologists around the world. As a result, many nature conservation groups concentrate their efforts on saving wetlands and the birds dwelling there. Another consequence is that in many partially-managed areas or other places where habitats are being altered, conservation efforts are basically intended to provide breeding, feeding, and resting areas for aquatic birds, and most projects to recover or regenerate wetlands take mainly these birds into account. However, this may have disadvantages given that some less visually pleasing but more threatened groups of plants or animals may not be included in the protective activities.
In these habitats birds can find food freely and easily. Only swimming and diving species have access to the deep water. Swimmers obtain food directly from the water's surface or by submerging their heads and stretching their necks down to reach the bottom. Diving species, on the other hand, feed on the bottom or chase prey under the water.
One of the orders of birds residing in open water that is most characteristic and well-represented throughout the world is the grebes (Podicipediformes). Almost all build floating nests that they fix to the emergent vegetation at the water's edge or on submerged macrophytes that reach the surface. Most feed on fish and large aquatic invertebrates.
Another important group is the duck family, the anatids. Although there are ducks in almost every wetland environment, most feed, rest, and congregate on large bodies of water (lakes, lagoons, or marshes). While surfacefeeding ducks only go to areas of deep water to rest and for safety, almost all ducks can feed easily in water less than 3 ft (1 m) deep. Diving ducks can dive to considerable depth in search of plant or animal food.
A typical habit of ducks is to congregate in large numbers, especially to overwinter. For many years census data for wintering ducks and coots were the only criteria used to assess wetland importance. In winter 1973, almost 400,000 ducks and coots were counted on Lake Ichkeul in Tunisia. In the United States, Texas and Louisiana marshes are well-known for large gatherings of wintering ducks. The coots (Fulica), which are a genus often associated with ducks, are widespread throughout Europe, Africa, and America and are common in most wetlands. Curiously, in Europe and Africa there are only two species of coot, but seven species are found in the Americas. Most require calm waters with abundant helophytic vegetation, which they use as a refuge as well as for nest-building. They also need submerged macrophytes and other vegetation to feed upon.
The pelicans (Pelecanus) and cormorants (Phalacrocorax) are both important groups in open waters. Both fish, but with very different techniques. Pelicans nosedive from the air or form large groups on the water's surface and then encircle the fish, catching them in their large pouched beaks, while cormorants swim, dive, and chase fish underwater. Cormorants are normally associated with marine environments, although some species breed and feed in coastal marshes and even inland. However, among the pelicans, the only species that is almost exclusively marine is the brown pelican (Pelecanus occidentalis), which normally nests in mangrove swamps.
Skimmers (Rynchops) are a highly specialized group of birds that catch small fish near the surface by skimming the surface of the water with their lower mandible in flight. Clearly, this fishing technique can only work in calm waters. They can only fish at sea when it is totally calm, early in the morning or in late evening. However, lagoons behind coral reefs and the lower courses of rivers are easier fishing grounds. This genus is represented by three species in America, Africa, and Asia, normally in tropical waters.
In shallow water at the edges of calm waters unaffected by strong winds, the dominant vegetation consists of submerged macrophytes. The leaves of some species reach the water's surface and may almost cover the whole sheet of water. Water lilies, lotus and water hyacinths are the most common species, along with plants from the genera Potamogeton, Ceratophyllum, Trapa, and Elodea. These floating mats of vegetation create a very unusual environment that has been colonized by specialist birds: moorhens and gallinules (Gallinula, Porphyrio) and jacanas (Jacana, Irediparra and other genera of jacanids). Their large toes, reduced weight, and agility enable them to walk on floating leaves without sinking, and so they can exploit an environment no other bird can reach (except ducks and coots, and even then only with great difficulty). Some colonial nesting birds, such as terns, use these areas to build their floating nests and form large breeding colonies. Whiskered terns (Chlidonias hybrida) are found in Europe, Africa, Asia, and Australia. The black tern (C. niger) is only missing from Australia and the white-winged black tern (C. leucopterus) is only absent from the Americas.
In tideless coastal lagoons, tidal fluctuations are replaced by fluctuations caused by alternating dry and wet periods. During dry months water levels drop and muddy margins appear that are used by many species. The most important group, the waders, are very numerous throughout the world and typically show a series of adaptations to this environment. Different species have beaks and legs of different lengths, which means they can feed at different depths. They employ a variety of different techniques. Some locate food by sight and then chase and catch it, while others rely on touch as they stir up the sediments and seek the hard particles corresponding to mollusks, worms, and grubs. Most mud-dwellers nest at high latitudes on the tundra or in similar environments.
River banks lacking vegetation are ideal places to fish from and therefore often witness large concentrations of ardeids. In the dry season when temporary pools evaporate, fish, amphibians, and aquatic invertebrates are concentrated in an area of a few square meters, where they can be captured extremely easily. Ardeids and other birds generally time the moment when their young leave the nest to coincide with these periods of abundance when feeding is so easy. In some freshwater marshes bare banks appear where the vegetation has been totally eliminated by domesticated or wild herbivores. The combination of trampling, grazing, and the accumulation of herbivore excrement makes these river banks very productive. Under these conditions, water temperatures are high and the microhabitats created by trampling are ideal for many aquatic invertebrates and even amphibians. Many of these microhabitats are not connected to open waters and these animals are therefore safe from predatory fish. This explains why many birds frequent these muddy shores in search of food.
Large herbaceous communties
Where the depth of water is less than 3 ft (1 m) in fresh or slightly saline coastal marshes, large stands of aquatic herbaceous plants may develop: helophytic vegetation. Reedbeds, stands of bulrushes, and papyrus swamps are dense formations, almost invariably consisting of just a few plant species in which many types of birds shelter.
Some birds breed and feed on the mud that appears at the base of the vegetation when water levels drop, and some on floating and decomposing vegetable matter. The most important group of birds of this type is the rallids (moorhen, water rails, and crakes). Most are fairly small and tend to be poor fliers but good runners, able to move around with ease in difficult conditions. They are found in all but the coldest climates. The most cosmopolitan is the moorhen (Gallinula chloropus), with approximately 12 subspecies scattered throughout Europe, Asia, Africa, the Caribbean, America, and part of Oceania.
Higher up, in the reeds, bulrushes, and papyrus live a large number of passerines specialized in catching the insects and other invertebrates living among the vegetation. These birds include many sylviids such as the reed warblers (Acrocephalus), timaliids such as the bearded tit (Panurus biarmicus), and representatives from many other families that have members adapted to life in the great banks of vegetation found in coastal marshes. All are characterized by being able to hang from vertical reed and rush stalks. Nests are skillfully constructed by weaving leaves around three or four stalks for support, or may sometimes be built on accumulations of plant remains.
This dense, inaccessible environment is also used by other species as a safe nesting area. Some of the most important are the pelicans (Pelecanus); the Threskiornithidae, such as the glossy ibis (Plegadis falcinellus) and the ibises (Threskiornis, Eudocimus); many ardeids; and a few birds of prey, such as the marsh harrier (Circus aeruginosus) and snail kite (Rostrhamus sociabilis).
260 The gentle Mediterranean tides, as in other seas little affected by daily oscillations in level, logically lead to a narrow intertidal band, as can be seen in this coast in the south of the Iberian peninsula with the supralittoral horizon exposed to the air.
[Photo: Juan Carlos Calvin]
261 Black patches of the lichen Verrucaria amphibia are typical of the supralittoral horizon in the Mediterranean. They live next to barnacles (Chthamalus), which characterize the upper part of the mediolittoral horizon. The photograph shows also some snails of the species Melaraphe neritoides, as well as tiny black cyanobacterian colonies (Rivularia, Brachytrichia), found on rocky coasts throughout the world.
[Photo: Enric Ballesteros]
262 A typical mediolittoral zonation pattern in the northwestern Mediterranean consists of an upper layer consisting of barnacles (Chthamalus), an intermediate level of the laminar rhodophytes (Rissoella verruculosa), and a lower level with mussels (Mytilus galloprovincialis) and some coralline algae (Corallina elongata). Below this are the permanently submerged regions, inhabited by organisms that cannot withstand exposure to air.
[Photo: Juan Carlos Calvin]
263 The tubes of vermetids and tubicolous polychaetes may be an important element forming the Mediterranean pavement or ledge.
[Photo: Josep Maria Gili]
264 An idealized Lithophyllum ledge or pavement. On many wave-beaten rocky shores in seas like the Mediterranean, this type of ledge-like structure forms due to the activity of different organisms, mainly coralline algae. As well as receiving the main impact of the waves, they slow them down and prevent their force from affecting the levels immediately above them. The drawing shows a large mass of L. lichenoides with crevices where other seaweeds live. These include the chlorophyte (Ulva rigida and Bryopsis muscosa), the rhodophyte (Scottera nicaeensis, Plocamium cartilagineum [=P. coccineum] and Lomentaria articulata [at bottom]) or Corallina elongata, Laurencia pinnatifida or Gelidium pusillum (immediately above). Sessile organisms living here include the red anemone (Actinia equina), the Mediterranean mussel (Mytilus galloprovincialis), the barnacle Balanus perforatus, and the colonial hydrozoan Aglaophenia kirchenpaueri (above the fish). Several ambulatory animals, such as the sea urchin Paracentrotus lividus, blennies (Blennius canevae), the polyplacophoran Acanthochiton fascicularis (in the lower right corner), one of several species of limpet (Patella coerulea), top shell (Monodonta turbinata), and the hairy crab Eriphia verrucosa, can also be found here. Running over them is a rock crab (Pachygrapsus marmoratus) and on the ledge itself are dark patches of the encrusting phaeophytes Ralfsia verrucosa, a different species of limpet (Patella rustica) and the star barnacle (Chthamalus stellatus). Higher up, in the zone that is merely splashed, there are black patches of the lichen Verrucaria amphibia, a third species of barnacle (C. [=Euraphia] depressus), the small crustacean Ligia italica, and the black periwinkle (Melaraphe neritoides).
[Drawing: J. Corbera, based on data from the author]
265 Predatory fish, such as this blenny (Blennius [= Aidablennius] sphynx), exploit the food available in the benthos of rocky shores. This species, shown here on a patch of the seaweed Jania rubens, eats polychaetes, mollusks, and cirripedes.
[Photo: Adolf de Sostoa]
266 Diagram of beach and dune vegetation on the shores of the western Mediterranean. A section drawn perpendicular to the coastline, from the sea inland, first cuts through a zone with no vegetation. This area is followed by discontinuous populations of two grasses (sand couch grass [Agropyron junceum] and Sporobolus pungens), both with well-developed rhizomes that firm up and fix this very unstable habitat. These grasses often grow in company with sea knotgrass (Polygonum maritimum) and the sedge Cyperus capitatus. The dunes are stabilized and encouraged to develop by marram (Ammophila arenaria), along with sea medick (Medicago marina) and Echinophora spinosa. A whole series of sand-loving plants can be found on the beach as well as halfway up the dunes, and even alongside the marram right on top of the dune chain: sea rocket (Cakile maritima), sea daffodil (Pan-cratium maritimum), sea bindweed (Convolvulus soldanella), and sea holly (Eryngium maritimum). Behind the dunes where the substrate is more stable and sheltered from the sea winds, plant populations are denser and the commonest plant is Crucianella maritima. Nearby grows Thymelaea hirsuta and a great variety of more continental species such as large yellow restharrow (Ononis natrix) and felty germander (Teucrium polium subsp. dunense var. maritimum), which have adapted to life next to the sea in sandy habitats. If a hollow behind the dune chain floods regularly and remains moist and saline for any length of time, a rush community with club-rush (Scirpus holoschoenus) and Saccharum [=Erianthus] ra-vennae will develop. If human or animal action has enriched the nitrogen content of the soil, the depression will develop yellow-horned poppy (Glaucium flavum) colonies, accompanied by sea rocket.
[Diagram: Editronica, from data provided by authors]
267 Dunes and areas behind the dunes are fascinating habitats where species that can exploit sandy as well as salty habitats live. The photograph, which shows Cape Sao Vicente in the Algarve (Portugal), illustrates well the populations of marram (Ammophila arenaria) that cover the summits and flanks of the dunes and also the shrubs and bushes that grow in the area behind the dunes on more structured soils.
[Photo: Ernest Costa]
268 One of the most interesting fish in saline coastal lagoons and also in freshwater rivers is the three-spined stickleback (Gasterosteus aculeatus). This minute fish 2-3 in (5-8 cm), widely distributed throughout the temperate waters of the northern hemisphere, lives up to its name, for it possesses a series of sharp spines on its back. The fish in the photograph is a male outside of the breeding season. During the breeding season, the males turn red and begin to mark and jealously defend territories in which they construct a spherical nest out of vegetable matter stuck together by a secretion they themselves produce. The female deposits her eggs (100-400) in the nest and they are then carefully looked after by the male.
[Photo: C. Blaney / Natural Science Photos]
269 A saltmarsh in winter in the Ebro Delta dominated by a species of glasswort (Salicornia [= Arthrocnemum] fruticosum). Cold winter weather typically turns these chenopods the reddish shade shown in the photograph.
[Photo: Lourdes Sogas]
270 Halophytic rush community bordering a seasonal coastal lagoon in the Sant Pere Pescador Marshes in the Emporda region, coast of a Catalonia, Spain. These lagoons are normally formed behind the dunes by accumulations of seawater blown across by strong northeasterly winds. In the foreground sharp rush (Juncus acutus) is growing.
[Photo: Lisette Pons]
271 The fragile beauty of coastal marshes is an ever-changing panorama; the time of day, the light playing on the surface of the water, and the presence of migratory birds all play their part. Here, twilight is falling on the Emporda Marshes (on the Costa Brava, northeastern Iberian Peninsula) and snipe, ducks, and lapwings, framed by bulrushes and tamarisks, are silhouetted on the mirror-like surface of the becalmed water.
[Photo: Antoni Agelet]
272 Succulent halophytes occur commonly in coastal areas and especially in saline marshes. Their tissues take up large amounts of water in order to dilute the salt that enters from the soil, and thereby decrease the osmotic pressure within the cells. This phenomenon gives the plants a swollen, inflated appearance, seen here in Zygophyllum fontanesii from the island of Fuerteventura in the Canary Islands.
[Photo: Josep Maria Barres]
273 A group of greater flamingoes (Phoenicopterus ruber subsp. ruber) flying over the Caribbean near the islands Las Aves (Venezuela). Flamingoes are halophilic birds and are specialists in filtering food from the bottoms of saline coastal lagoons.
[Photo: Pere Navalles]
274 Coastal marshes are food-rich areas and are used by domesticated animals, especially horses and cows, and wild animals alike. These bulls (Bos taurus), up to their ankles in water, share their pastures with coots (Fulica atra) and greater flamingoes (Phoenicopterus ruber) in a flooded area of rushes (Scirpus) in the Guadalquivir Marshes on the Atlantic coast of Andalusia, Spain.
[Photo: Xavier Ferrer]
275 The white-tailed deer (Odocoileus virginianus) is essentially a woodland species but often takes refuge from increasing human pressure in the Atlantic coastal marshes of North America.
[Photo: Erwin & Peggy Bauer / Bruce Coleman Limited]
276 Coots (Fulica atra) and tufted ducks (Aythya fuligula) are common on open coastal waters in Europe, above all, in the case of the tufted duck, in winter (it prefers the interior of the continent in summer). The photograph shows a group of these two birds on a coastal lake in northern Germany. The coot is a member of the rail family and is distinguished by its white beak and facial shield. The tufted duck has a characteristic crest and marked sexual dimorphism: the male has white flanks, while the female's flanks are brownish.
[Photo: Uwe Walz / Bruce Coleman Limited]
277 The moorhen (Gallinula chloropus) is another rail, similar to the coot but with a yellow and red bill that makes identification easy. This bird was photographed in coastal marshes in Florida.
[Photo: John Cancalosi / Auscape International]
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|Publication:||Encyclopedia of the Biosphere|
|Date:||Oct 1, 2000|
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