Final Paper Outline
CHAPTER 25
Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson.
25.1 The Intertidal Zone Is the Transition between Terrestrial and Marine Environments
Rocky, sandy, muddy, and either protected from or pounded by incoming swells, all intertidal shores have one feature in common: they are alternately exposed and submerged by the tides. Roughly, the region of the seashore is bounded on one side by the height of extreme high tide and on the other by the height of extreme low tide. Within these confines, conditions change hourly with the ebb and flow of the tides (see Section 3.9). At high tide, the seashore is a water world; at low tide, it belongs to the terrestrial environment, with its extremes in temperature, moisture, and solar radiation. Despite these changes, seashore inhabitants are essentially marine organisms adapted to withstand some degree of exposure to the air for varying periods of time.
At low tide, the uppermost layers of intertidal life are exposed to air, wide temperature fluctuations, intense solar radiation, and desiccation for a considerable period, whereas the lowest fringes on the intertidal shore may be exposed only briefly before the rising tide submerges them again. These varying conditions result in one of the most striking features of the coastal shoreline: the zonation of life.
25.2 Rocky Shorelines Have a Distinct Pattern of Zonation
All rocky shores have three basic zones (Figure 25.1), and each is characterized by dominant organisms (Figure 25.2). The approach to a rocky shore from the landward side is marked by a gradual transition from lichens and land plants to marine life dependent at least partly on the tidal waters. Moving from the terrestrial or supralittoral or supratidal zone, the first major change from the adjacent terrestrial environment appears at the supralittoral fringe, where saltwater comes only once every two weeks on the spring tides. It is marked by the black zone, named for the thin black layer of cyanobacteria (Calothrix) growing on the rock together with lichens (Verrucaria) and green alga (Entophysalis) above the high-tide waterline. Living under conditions that few plants could survive, these organisms represent an essentially nonmarine community. Common to this black zone are periwinkles, which are small snails of the genus Littorina (from which the term littoral zone is derived) that graze on wet algae covering the rocks.
Below the black zone lies the littoral or intertidal zone, which is covered and uncovered daily by the tides. In the upper reaches, barnacles are most abundant. Oysters, blue mussels, and limpets appear in the middle and lower portions of the littoral, as does the common periwinkle. Occupying the lower half of the littoral zone (midlittoral) of colder climates, and in places overlying the barnacles, is an ancient group of plants—brown algae, which are commonly known as rockweeds (Fucus spp.) and wrack (Ascophyllum nodosum). On the hard-surfaced shores that have been covered partly by sand and mud, blue mussels may replace the brown algae.
The lowest part of the littoral zone, uncovered only at the spring tides and not even then if wave action is strong, is the infralittoral fringe. This zone, exposed for short periods of time, consists of forests of large brown alga—Laminaria (one of the kelps)—with a rich undergrowth of smaller plants and animals among the holdfasts. Below the infralittoral fringe is the infralittoral or subtidal zone.
Grazing, predation, competition, larval settlement, and action of waves heavily influence the pattern of life on rocky shores. Waves bring in a steady supply of nutrients and carry away organic material. They keep the fronds of seaweeds in constant motion, moving them in and out of shadow and sunlight, allowing more even distribution of incident light and thus more efficient photosynthesis. By dislodging both plants and invertebrates from the rocky substrate, waves open up space for colonization by algae and invertebrates and reduce strong interspecific competition. Heavy wave action can reduce the activity of such predators as starfish and sea urchins that feed on sessile intertidal invertebrates. In effect, disturbance influences community structure.
The ebbing tide leaves behind pools of water in rock crevices, rocky basins, and depressions (Figure 25.3). These pools represent distinct habitats, which differ considerably from exposed rock and the open sea and even differ among themselves. At low tide, all pools are subject to wide and sudden fluctuations in temperature and salinity. Changes are most marked in shallow pools. Under the summer sun, temperatures may rise above the maximum that many organisms can tolerate. As water evaporates, especially in the shallower pools, salt crystals may appear around the edges. When rain or drainage from the surrounding land brings freshwater to the pools, salinity may decrease. In deep pools, this freshwater tends to form a layer on top, developing a strong salinity stratification in which the bottom layer and its inhabitants are little affected. If algal growth is considerable, oxygen will be high during the daylight hours but low at night, a situation that rarely occurs at sea. The rise of carbon dioxide at night lowers the pH (see Section 3.7). Most pools suddenly return to sea conditions on the rising tide and experience drastic and sudden changes in temperature, salinity, and pH. Life in the tidal pools must be able to withstand these extreme fluctuations.
25.3 Sandy and Muddy Shores Are Harsh Environments
Sandy and muddy shores often appear devoid of marine life at low tide in sharp contrast to the life-filled rocky shore (Figure 25.4), but sand and black mud are not as barren as they seem. Beneath them life lurks, waiting for the next high tide.
The sandy shore is a product of the harsh and relentless weathering of rock—both inland and along the shore. Rivers and waves carry the products of rock weathering and deposit them as sand along the edge of the sea. The size of the sand particles deposited influences the nature of the sandy beach, water retention during low tide, and the ability of animals to burrow through it. In sheltered areas of the coast, the slope of the beach may be so gradual that the surface appears to be flat. Because of the flatness, the outgoing tidal currents are slow, leaving behind a residue of organic material settled from the water. In these situations, mudflats develop.
Life on sand is almost impossible. Sand provides no surface for attachment of seaweeds and their associated fauna, and the crabs, worms, and snails characteristic of rocky crevices find no protection there. Life, then, is forced to live beneath the sand.
Life on sandy and muddy beaches consists of epifauna, which are organisms living on the sediment surface, and infauna, which are organisms living in the sediments (see Figure 16.17 for an illustration of patterns of zonation on a sandy beach). Most infauna occupy either permanent or semipermanent tubes within the sand or mud and are able to burrow rapidly into the substrate. Other infauna live between particles of sand and mud. These tiny organisms, referred to as meiofauna, range in size from 0.05 to 0.5 millimeters (mm) and include copepods, ostracods, nematodes, and gastrotrichs.
Sandy beaches also exhibit zonation related to the tides, but you must discover it by digging (see Figure 16.17). Pale, sand-colored ghost crabs (Ocypodinae) and beach hoppers (Talitridae) occupy the upper beach—the supralittoral. The intertidal beach—the littoral—is a zone where true marine life appears. Although sandy shores lack the variety found on rocky shores, the populations of individual species of largely burrowing animals often are enormous. An array of animals—among them starfish and the related sand dollar—can be found above the low-tide line in the littoral zone.
Organisms living within the sand and mud do not experience the same extreme fluctuations in temperature as do those on rocky shores. Although the surface temperature of the sand at midday may be 10°C (or more) higher than the returning seawater, the temperature a few centimeters below the sand remains almost constant throughout the year. Nor is there a great fluctuation in salinity, even when freshwater runs over the surface of the sand. Below 25 centimeters (cm), salinity is little affected.
Near and below the low-tide line live predatory gastropods, which prey on bivalves beneath the sand. In the same area lurk predatory portunid crabs (Portunidae) such as the blue crab (Callinectes sapidus) and green crab (Carcinus maenas) that feed on mole crabs (Emerita), clams, and other organisms. These species move back and forth with the tides. The incoming tides also bring small predatory fish, such as killifish and silversides. As the tide recedes, gulls and shorebirds scurry across the sand and mudflats to hunt for food.
The energy base for life on the sandy shore is an accumulation of organic matter. Most sandy beaches contain a certain amount of detritus from seaweeds, dead animals, and feces brought in by the tides. This organic matter accumulates within the sand in sheltered areas. It is subject to bacterial decomposition, which is most rapid at low tide. Some detrital-feeding organisms ingest organic matter largely as a means of obtaining bacteria. Prominent among them are many nematodes and copepods (Harpacticoida), polychaete worms (Nereis), gastropod mollusks, and lugworms (Arenicola), which are responsible for the conspicuous coiled and cone-shaped casts on the beach. Other sandy beach animals are filter feeders that obtain their food by sorting particles of organic matter from tidal water. Two of these—alternately advancing and retreating with the tide—are the mole crab and the coquina clam (Donax).
25.4 Tides and Salinity Dictate the Structure of Salt Marshes
Salt or tidal marshes occur in temperate latitudes where coastlines are protected from the action of waves within estuaries, deltas, and by barrier islands and dunes (Figure 25.5). The structure of a salt marsh is dictated by tides and salinity, which create a complex of distinctive and clearly demarked plant communities.
From the edge of the sea to the high land, zones of vegetation distinctive in form and color develop, reflecting a microtopography that lifts the plants to various heights within and above high tide (see Figure 16.16 for an illustration of vegetation zonation in a coastal salt marsh). Commonly found on the seaward edge of marshes and along tidal creeks of the Eastern coastline in North America are tall, deep green growths of salt-marsh cordgrass, Spartina alterniflora. Cordgrass forms a marginal strip between the open mud to the front and the high marsh behind. It has a high tolerance for salinity and is able to live in a semi-submerged state. To get air to its roots, which are buried in anaerobic mud, cordgrass has hollow tubes leading from the leaf to the root through which oxygen diffuses.
Above and behind the low marsh is the high marsh, standing at the level of mean high water. At this level, tall salt-marsh cordgrass gives way rather abruptly to a short form. This shorter form of Spartina is yellowish, in contrast to the tall, dark green form. This short form is an example of phenotypic plasticity in response to environmental conditions of the high marsh (see Chapter 5). The high marsh has a higher salinity and a decreased input of nutrients that result from a lower tidal exchange rate than in the low marsh. Here also grow the fleshy, translucent glassworts (Salicornia spp.; Figure 25.6) that turn bright red in fall, sea lavender (Limonium carolinianum), spearscale (Atriplex patula), and sea blite (Suaeda maritima).
Where the microelevation is about 5 cm above mean high water, short Spartina alterniflora and its associates are replaced by salt meadow cordgrass (Spartina patens) and an associate, spikegrass or saltgrass (Distichlis spicata). As the microelevation rises several more centimeters above mean high tide, and if there is some intrusion of freshwater, Spartina and Distichlis may be replaced by two species of black needlerush or black grass (Juncus roemerianus and Juncus gerardi)—so called because their dark green color becomes almost black in the fall. Beyond the black grass and often replacing it is a shrubby growth of marsh elder (Iva frutescens) and groundsel (Baccharis halimifolia). On the upland fringe grow bayberry (Myrica pensylvanica) and the pink-flowering sea holly (Hibiscus palustris).
Two conspicuous features of a salt marsh are the salt pans interspersed among meandering creeks. The creeks form an intricate system of drainage channels that carry tidal waters back out to sea (Figure 25.7). Their exposed banks support a dense population of mud algae, diatoms, and dinoflagellates that are photosynthetically active all year. Salt pans are circular to elliptical depressions flooded at high tide. At low tide, they remain filled with saltwater. If the pans are shallow enough, the water may evaporate completely, leaving an accumulating concentration of salt on the mud. The edges of these salt flats may be invaded by glasswort and spikegrass (Figure 25.8).
Although the salt marsh is not noted for its diversity, it is home to several interesting organisms. Some of the inhabitants are permanent residents in the sand and mud, others are seasonal visitors, and most are transients coming to feed at high and low tide.
Three dominant animals of the low marsh are ribbed mussels (Modiolus demissus), buried halfway in the mud, fiddler crabs (Uca spp.), running across the marsh at low tide, and marsh periwinkles (Littorina spp.) that move up and down the stems of Spartina and onto the mud to feed on alga. Three conspicuous vertebrate residents of the low marsh of eastern North America are the diamond-backed terrapin (Malaclemys terrapin), clapper rail (Rallus longirostris), and seaside sparrow (Ammospiza maritima).
In the high marsh, animal life changes as abruptly as the vegetation. The small, coffee-colored pulmonate snail (Melampus)—found by the thousands under the low grass—replaces the marsh periwinkle. The willet (Catoptrophorus semipalmatus) and seaside sharp-tailed sparrow (Ammospiza caudacuta) replace the clapper rail and seaside sparrow.
Low tide brings a host of predators into the marsh to feed. Herons, egrets, gulls, terns, willets, ibis, raccoons, and others spread over the exposed marsh floor and muddy banks of tidal creeks. At high tide, the food web changes as the tide waters flood the marsh. Small predatory fish such as the silversides (Menidia menidia), killifish (Fundulus heteroclitus), and four-spined stickleback (Apeltes quadracus), which are restricted to channel waters at low tide, spread over the marsh at high tide, as does the blue crab.
25.5 Mangroves Replace Salt Marshes in Tropical Regions
Replacing salt marshes on tidal flats in tropical regions are mangrove forests or mangals (Figure 25.9), which cover 60 to 75 percent of the coastline of the tropical regions. Mangrove forests develop where wave action is absent, sediments accumulate, and the muds are anoxic (without oxygen). They extend landward to the highest vertical tidal range, where they may be only periodically flooded. The dominant plants are mangroves, which include 8 families and 12 genera dominated by Rhizophora, Avicennia, Bruguiera, and Sonneratia. Growing with them are other salt-tolerant plants, mostly shrubs.
In growth form, mangroves range from short, prostrate forms to timber-size trees 30 meters (m) high. All mangroves have shallow, widely spreading roots, and many have prop roots coming from trunk and limbs (Figure 25.10). Many species have root extensions called pneumatophores that take in oxygen for the roots. The tangle of prop roots and pneumatophores slows the movement of tidal waters, allowing sediments to settle out. Land begins to move seaward, followed by colonizing mangroves.
Mangrove forests support a rich fauna, with a unique mix of terrestrial and marine life. Living and nesting in the upper branches are many species of birds, particularly herons and egrets. Littorina snails live on the prop roots and trunks of mangrove trees. Also attached to the stems and prop roots are oysters and barnacles, and on the mud at the base of the roots are detritus-feeding snails. Fiddler crabs and tropical land crabs burrow into the mud during low tide and live on prop roots and high ground during high tide. In the Indo-Malaysian mangrove forests live mudskippers, fish of the genus Periophthalmus, with modified eyes set high on the head. They live in burrows in the mud and crawl about on the top of it. In many ways they act more like amphibians than fish. The sheltered waters about the roots provide a nursery and haven for the larvae and young of crabs, shrimp, and fish.
25.6 Freshwater Wetlands Are a Diverse Group of Ecosystems
The transitional zones between freshwater and land are characterized by terrestrial wetlands. These unique environments form ecotones between terrestrial and adjacent aquatic ecosystems, sharing characteristics of both. Wetlands cover 6 percent of Earth’s surface. They are found in every climatic zone but are local in occurrence. Only a few—such as the Everglades in Florida, the Pantanal in Brazil, the Okavango in southern Africa (Figure 25.11), and the Fens of England—cover extensive areas of the landscape (see this chapter, Ecological Issues & Applications).
Wetlands range along a gradient from permanently flooded to periodically saturated soil (Figure 25.12) and support specialized plants that occur where the soil conditions remain saturated for most or all of the year. These hydrophytic (water-adapted) plants are adapted to grow in water or on soil that is periodically anaerobic (lacking oxygen) because of excess water (see Chapter 6). Hydrophytic plants are typically classified into one of three groups: (1) obligate wetland plants that require saturated soils, which include the submerged pondweeds, floating pond lily, emergent cattails and bulrushes, and trees such as bald cypress (Taxodium distichum); (2) facultative wetland plants that can grow in either saturated or upland soil and rarely grow elsewhere, such as certain sedges and alders, and trees such as red maple (Acer rubrum) and cottonwoods (Populus spp.); and (3) occasional wetland plants that are usually found out of wetland environments but can tolerate wetlands. The third group of plants is critical in determining the upper limit of a wetland along a gradient of soil moisture.
Wetlands most commonly occur in three topographic situations (Figure 25.13). Basin wetlands develop in shallow basins, ranging from upland depressions to filled-in lakes and ponds. Riverine wetlands develop along shallow and periodically flooded banks of rivers and streams. Fringe wetlands occur along the coasts of large lakes. The three types are partially separated because of the direction of water flow (see Figure 25.13). Water flow in basin wetlands is vertical as a result of precipitation and the downward infiltration of water into the soil. In riverine wetlands, water flow is unidirectional. In fringe wetlands, flow is bidirectional because it involves rising lake levels or tidal action. These flows transport nutrients and sediments into and out of the wetland.
Wetlands dominated by emergent herbaceous vegetation are marshes (Figure 25.14). With their reeds, sedges, grasses, and cattails, marshes are essentially wet grasslands. Forested wetlands are commonly called swamps (Figure 25.15). They may be deep-water swamps dominated by cypress (Taxodium spp.), tupelo (Nyssa spp.), and swamp oaks (Quercus spp.); or they may be shrub swamps dominated by alder (Alnus spp.) and willows (Salix spp.). Along many large river systems are extensive tracts of bottomland or riparian woodlands (Figure 25.16), which are occasionally or seasonally flooded by river waters but are dry for most of the growing season.
Wetlands that are characterized by an accumulation of partially decayed organic matter with time are called peatlands or mires (Figure 25.17). Organic matter accumulates because it is produced faster than it can decompose. The water table is at or near the soil surface, which creates anaerobic conditions that slow microbial activity. Mires that are fed by groundwater moving through mineral soil, from which they obtain most of their nutrients, and are dominated by sedges are known as fens. Mires dependent largely on precipitation for their water supply and nutrients and that are dominated by Sphagnum are bogs. Mires that develop in upland situations—where decomposed, compressed peat forms a barrier to the downward movement of water, resulting in a perched water table (zone of saturation above an impermeable horizon) above mineral soil—are blanket mires and raised bogs (Figure 25.18). Raised bogs are popularly known as moors. Because bogs depend on precipitation for nutrient inputs, they are highly deficient in mineral salts and low in pH. Bogs also develop when a lake basin fills with sediments and organic matter carried by inflowing water. These sediments divert water around the lake basin and raise the surface of the mire above the influence of groundwater. Other bogs form when a lake basin fills in from above rather than from below, creating a floating mat of peat over open water. Such bogs are often termed quaking bogs (Figure 25.19).
25.7 Hydrology Defines the Structure of Freshwater Wetlands
Wetland structure is influenced by the phenomenon that creates it: its hydrology. Hydrology has two components. One involves the physical aspects of water and its movement: precipitation, surface and subsurface flow, direction and kinetic energy of water, and chemistry of the water. The other component is the hydroperiod, which involves duration, frequency, depth, and season of flooding. The length of the hydroperiod varies among types of wetlands. Basin wetlands have a longer hydroperiod. They usually flood during periods of high rainfall and draw down during dry periods. Both phenomena appear to be essential to the long-term existence of wetlands. Riverine wetlands have a short period of flooding associated with peak stream flow. The hydroperiod of fringe wetlands, influenced by wind and lake waves, may be short and regular and does not undergo the seasonal fluctuation characteristic of many basin marshes.
Hydroperiod influences plant composition because it affects germination, survival, and mortality at various stages of the plants’ life cycles. The effect of hydroperiod is most pronounced in basin wetlands, especially those in the prairie regions of North America. In basins (called potholes in the prairie region) deep enough to have standing water throughout periods of drought, the dominant plants are submergents (Figure 25.20). If the wetland goes dry annually or during a period of drought, tall or midheight emergent species such as cattails dominate the marsh. If the pothole is shallow and flooded only briefly in the spring, then grasses, sedges, and forbs will make up a wet-meadow community.
If the basin is deep enough toward its center as well as large enough, then zones of vegetation may develop, ranging from submerged plants to deep-water emergents such as cattails and bulrushes, shallow-water emergents, and wet-ground species such as spike rush. Zonation reflects the response of plants to the hydroperiod. Those areas of wetland subject to a long hydroperiod support submerged and deep-water emergents; those with a short hydroperiod and shallow water are occupied by shallow-water emergents and wet-ground plants.
Periods of drought and wetness can induce vegetation cycles associated with changes in water levels. Periods of above-normal precipitation can raise the water level and drown the emergents to create a lake marsh dominated by submerged plants. During a drought, the marsh bottom is exposed by receding water, stimulating seed germination in the emergents and annuals characteristic of mudflats. When water levels rise again, the mudflat species drown, and the emergents survive and spread vegetatively.
Peatlands differ from other freshwater wetlands in the accumulation of peat that results because organic matter is produced faster than it can be decomposed. In northern regions, acid-forming, water-holding Sphagnum add new growth on top of the accumulating remains of past moss generations, and their spongelike ability to hold water increases water retention on the site. As the peat blanket thickens, the water-saturated mat of moss and associated vegetation is raised above and insulated from mineral soil. The peat mat then becomes its own reservoir of water, creating a perched water table.
Peat bogs and mires generally form under oligotrophic and dystrophic conditions (see Section 24.4). Although usually associated with and most abundant in boreal regions of the Northern Hemisphere, peatlands also exist in tropical and subtropical regions. They develop in mountainous and coastal regions where hydrological conditions encourage an accumulation of partly decayed organic matter. Examples in coastal regions are the Everglades in Florida and the pocosins on the coastal plains of the southeastern United States.
25.8 Freshwater Wetlands Support a Rich Diversity of Life
Biologically, freshwater wetlands are among the richest and most interesting ecosystems. They support a diverse community of benthic, limnetic, and littoral invertebrates, especially crustaceans and insects. These invertebrates, along with small fishes, provide a food base for waterfowl, herons, gulls, and other birds, and supply the fat-rich nutrients ducks need for egg production and the growth of young. Amphibians and reptiles, notably frogs, toads, and turtles, inhabit the emergent growth, soft mud, and open water of marshes and swamps.
Herbivores are a conspicuous component of animal life. The dominant herbivore in prairie marshes is the muskrat (Ondatra zibethicus). Muskrats are the major prey for mink (Mustela vison), the dominant carnivore on the marshes. Other predators, including the raccoon, fox, weasel, and skunk, can seriously reduce the reproductive success of waterfowl on small marshes.
Ecological Issues & Applications Wetland Ecosystems Continue to Decline as a Result of Land Use
For centuries, we have looked at wetlands as forbidding, mysterious places: sources of pestilence, home to dangerous and pestiferous insects, and the abode of slimy, sinister creatures that rise out of swamp waters. They have been looked upon as places that should be drained for more productive uses by human standards: agricultural land, solid waste dumps, housing, industrial developments, and roads. The Romans drained the great marshes around the Tiber to make room for the city of Rome. William Byrd described the Great Dismal Swamp on the Virginia–North Carolina border as a “horrible desert, the foul damps ascend without ceasing.” Despite the enormous amount of vacant dry land available in 1763, a corporation called the Dismal Swamp Land Company, owned in part by George Washington, failed in an attempt to drain the western end of the swamp for farmland. Although severely affected over the past 200 years, much of the swamp remains as a wildlife refuge.
Rationales for draining wetlands are many. The most persuasive relates to agriculture. Drainage of wetlands opens many hectares of rich organic soil for crop production. In prairie country, agriculturalists viewed the innumerable potholes (see Figure 25.20) as an impediment to efficient farming. Draining them tidies up fields and allows unhindered use of large agricultural machinery. There are other reasons, too. Landowners and local governments view wetlands as an economic liability that produces no economic return and provides little tax revenue. Many regard the wildlife that wetlands support as threats to grain crops. Elsewhere, wetlands are considered valueless lands, at best filled in and used for development. Some major wetlands have been in the way of dam development projects. For example, the large Pymatuning Lake in the states of Pennsylvania and Ohio covers what was once a 4200-hectare Sphagnum–tamarack bog (see Figure 25.19). Peat bogs in the northern United States, Canada, Ireland, and northern Europe are excavated for fuel, horticultural peat, and organic soil (Figure 25.21 ). In some areas, such exploitation threatens to wipe out peatland ecosystems.
Many remaining wetlands, especially in the north-central and southwestern United States, are contaminated and degraded by the pesticides and heavy metals carried into them by surface and subsurface drainage and sediments from surrounding croplands. Although inputs of nitrogen and phosphorus increase the productivity of wetlands, a concentration of herbicides, pesticides, and heavy metals poisons the water, destroys invertebrate life, and has debilitating effects on wildlife (including deformities, lowered reproduction, and death). Waterfowl in wetlands scattered throughout agricultural lands are also more exposed to predation, and without access to natural upland vegetation they breed less successfully.
Fifty-one percent of the human population of the contiguous United States, and globally 70 percent of all people, live within 80 kilometers (km) of the coastlines. With so much humanity clustered near the coasts, it is obvious why coastal wetlands are threatened and disappearing rapidly. During colonial times, the area now embraced by the 50 United States contained some 160 million hectares of wetlands. Over the past 200 years, that area has decreased to 110 million hectares (Figure 25.22), and many of these remnants are degraded. Coastal Europe has lost 65 percent of its original tidal marshes, and 75 percent of the remaining are heavily managed. Since the 1980s, 35 percent of tropical mangrove forests have been diked for aquaculture—pond rearing of shrimp and fish—and cut for wood chips and charcoal. In a satellite-based assessment of the extent and changes in the global distribution of wetlands, Catherine Prigent and colleagues at Observatoire de Paris found that wetlands declined by 6 percent between 1993 and 2007 as a result of conversion for agriculture, drainage, and water diversion. The study found wetland loss was the greatest in the tropics and subtropics, where population growth and agricultural expansion is the greatest. Other research has shown that wetland loss, especially in Malaysia and Indonesia, has continued since 2007. Plantation development for palm oil and pulp and paper production are key drivers of wetland loss in Southeast Asia.
Commonly regarded as economic wastelands, salt marshes have been and are still being ditched, drained, and filled for real estate development (everyone likes to live at the water’s edge), industrial development, and agriculture. Reclamation of marshes for agriculture is most extensive in Europe, where the high marsh is enclosed within a sea wall and drained. Most of the marshland and tideland in Holland has been reclaimed in this fashion. Many coastal cities such as Boston, Amsterdam, and much of London have been built on filled-in marshes. Salt marshes close to urban and industrial developments occasionally become polluted with spillages of oil, which becomes easily trapped within the vegetation.
Losses of coastal wetlands have a pronounced effect on the salt marsh and associated estuarine ecosystems. They are the nursery grounds for commercial and recreational fisheries. There is, for example, a positive correlation between the expanse of coastal marsh and shrimp production in the coastal waters of the Gulf of Mexico. Oysters and blue crabs are marsh dependent, and the decline of these important species relates to loss of salt marshes. Coastal marshes are major wintering grounds for waterfowl. One-half of the migratory waterfowl of the Mississippi Flyway depend on Gulf Coast wetlands, and the bulk of the snow goose population winters on coastal marshes from the Chesapeake Bay to North Carolina. Through grazing or uprooting, these geese may remove nearly 60 percent of the belowground production of marsh vegetation. Forced concentration of these wintering migratory birds into shrinking salt-marsh habitats could jeopardize marsh vegetation and the future of these salt-marsh ecosystems.
The loss of wetlands has reached a point where both environmental and socioeconomic values—including waterfowl habitat, groundwater supply and quality, floodwater storage, and sediment trapping—are in jeopardy. In the United States, however, some progress has been made in recent decades to reduce the trend of continued loss. In a report developed for Congress, Thomas Dahl of the United States Department of Interior examined the status and trends in wetland ecosystems in the conterminous United States over the period from 1998 to 2004. The study indicated that there were an estimated 110.1 million acres (44.6 million ha) of wetlands in the conterminous United States in 2009, of which an estimated 95 percent of all wetlands were freshwater and 5 percent were in the marine or estuarine (saltwater) systems. Salt marsh made up an estimated 66.7 percent of all estuarine and marine wetland area, and forested wetlands made up the single largest category (49.5 percent) of wetland in the freshwater system. Dahl found that between 2004 and 2009, wetland area declined by an estimated 62,300 acres (25,200 ha). The reasons for this are complex and potentially reflect economic conditions, land-use trends, and changing wetland regulation and enforcement measures. Certain types of wetland exhibited decline whereas others increased in area. Collectively, marine and estuarine intertidal wetlands declined by an estimated 84,100 acres (34,050 ha) or an estimated 1.4 percent. In contrast, freshwater wetlands realized a slight increase in area between 2004 and 2009.
Although the United States has made some progress toward preserving the remaining wetlands through legislative action and land purchase, the future of freshwater wetlands is not secure. Apathy, hostility toward wetland preservation, political maneuvering, court decisions, and arguments over what constitutes a wetland allow the continued destruction of wetlands.
The Everglades National Park in South Florida is one of the largest natural wetlands in the world. Over the past century, the draining of lands and the diversion of water to meet growing residential and agricultural needs in this region have threatened this wetland ecosystem. Efforts are currently underway to restore the flow of water that is critical to preserving this unique ecosystem. Information on the history of the Everglades ecosystem and the Comprehensive Everglades Restoration Plan are available online at www.evergladesplan.org/.
Summary
Intertidal Zone 25.1
Sandy shores and rocky coasts occur where the sea meets the land. The drift line marks the farthest advance of the tide on sandy shores. On rocky shores, a zone of black algal growth marks the tide line.
Rocky Coasts 25.2
The most striking feature of the rocky shore—the zonation of life—results from alternate exposure and submergence by the tides. The black zone marks the supralittoral fringe, the upper part of which is flooded only once every two weeks by spring tides. Submerged daily by the tides is the littoral zone, characterized by barnacles, periwinkles, mussels, and fucoid seaweeds. Uncovered only at spring tides is the infralittoral, which is dominated by large brown laminarian seaweeds, Irish moss, and starfish. Distribution and diversity of life across rocky shores are also influenced by wave action, competition herbivory, and predation. Left behind by outgoing tides are tidal pools, distinct habitats subject to wide fluctuations in temperature and salinity over a 24-hour period and inhabited by varying numbers of organisms, depending on the amount of emergence and exposure.
Sandy Beaches 25.3
Sandy beaches are a product of weathering of rock. Exposed to wave action, the beaches are subject to deposition and wearing away of the sandy substrate. Sandy and muddy shores appear barren of life at low tide; but beneath the sand and mud, conditions are more amenable to life than on the rocky shore. Zonation of life is hidden beneath the surface. The energy base for sandy and muddy shores is organic matter carried in by tides and made available by bacterial decomposition. Basic consumers are bacteria, which in turn are a major source of food for both deposit-feeding and filter-feeding organisms.
Salt Marshes 25.4
The interaction of salinity, tidal flow, and height produces a distinctive zonation of vegetation in salt marshes. Salt-marsh cordgrass dominates marshes flooded by daily tides. Higher microelevations that are shallow, flooded only by spring tides, and subject to higher salinity support salt meadow cordgrass and spikegrass. Salt-marsh animals are adapted to tidal rhythms. Detrital feeders such as fiddler crabs and their predators are active at low tide; filter-feeding ribbed mussels are active at high tide.
Mangrove Forests 25.5
In tropical regions, mangrove forests or mangals replace salt marshes and cover up to 70 percent of coastlines. Uniquely adapted to a tidal environment, many mangrove tree species have supporting prop roots that carry oxygen to the roots, and their seeds grow into seedlings on the tree and drop into the water to take root in the mud. Mangroves support a unique mix of terrestrial and marine life. The sheltered water about the prop roots provides a nursery for the larvae and young of crabs, shrimp, and fish.
Freshwater Wetlands 25.6
Wetlands can be defined as a community of hydrophytic plants occupying a gradient of soil wetness from permanently flooded to periodically saturated during the growing season. Hydrophytic plants are adapted to grow in water or on soil periodically deficient in oxygen. Wetlands dominated by grasses and herbaceous hydrophytes are marshes. Those dominated by wooded vegetation are forested wetlands (riparian forests) or shrub swamps. Wetlands characterized by an accumulation of peat are mires. Mires fed by water moving through the mineral soil and dominated by sedges are fens; those dominated by Sphagnum and dependent largely on precipitation for moisture and nutrients are bogs. Bogs are characterized by blocked drainage, an accumulation of peat, and low productivity.
Hydrology Structures Wetlands 25.7
The structure and function of wetlands are strongly influenced by their hydrology—both the physical movement of water and hydroperiod. Hydroperiod is the depth, frequency, and duration of flooding. Hydroperiod influence on vegetation is most evident in basin wetlands that exhibit zonation from deep-water submerged vegetation to wet-ground emergents.
Diversity of Wetland Life 25.8
Wetlands support a diversity of wildlife. Freshwater wetlands provide essential habitats for frogs, toads, turtles, and a diversity of invertebrate life. Nesting, migrant, and wintering waterfowl depend on these critical habitats.
Wetlands Decline Ecological Issues & Applications
Wetland ecosystems continue to be drained and converted to other land uses such as agriculture. The extent of both freshwater and coastal wetlands continues to decline the greatest losses occurring in the tropics and subtropics, where population growth and agricultural expansion is the greatest.
