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Wetland Ecosystems of North America

Wetlands are important ecosystems. They are nature's flood-control systems. They take up excess water unleashed by storms and release it slowly, thereby reducing or eliminating flooding. Without wetlands, flood waters run quickly over paved areas and into storm sewers or conduits, concentrating in volume and speed as they go; when this water reaches streams and rivers, erosion and flooding occur. Wetlands also filter sediments and break down pollutants. Still another important function of wetlands is groundwater recharge. Wetlands absorb water that eventually returns to the water table. Without wetlands, aquifers would not be replenished.

What is more, wetlands are very productive. The lush wetland vegetation captures the sun's energy and converts it into living plant matter. Much of the vegetation produced in a growing season remains in the wetland, releasing nutrients as it decays. This nutrient-rich water supports a vast array of animals that feed on decaying vegetation. These animals form the foundation of an important food web that includes mink and otter, frogs, turtles, butterflies, dragonflies, and many other creatures. Prairie pothole wetlands are waterfowl factories; Arctic wetlands are essential to shorebirds as well as waterfowl; coastal saltmarshes produce seafood and fish. Without healthy wetlands, we would not have a healthy environment filled with a diversity of beautiful and useful species.

Marshes, bogs, swamps, and saltmarshes are the major wetland types.

Meat-Eating Plants

The greatest concentration of insectivorous plants in North America is found in the southern Coastal Plain, where pitcher plants (Sarracenia species), Venus's-flytrap (Dionaea muscipula), sundews (Drosera species), and bladderworts (Utricularia species) all catch and consume insects. The sandy and peaty soils of coastal savannahs and bogs are low in nutrients, especially nitrogen. Insect-eating plants have evolved to catch the food that they need in order to survive. Pitcher plants use a pit-trap method to catch their prey: the insects are lured to the beautiful leaves by sweet excretions. They slip on the edge and fall into a tiny pool of water inside the leaf, where they drown and are digested. The other plant carnivores use active traps to catch their prey. The Venus's-flytrap is the most dramatic, with a snare that snaps shut when an insect walks across the trap. Carnivorous plants are fascinating to grow, but require special care. They need sterile, acidic soil and must be watered with distilled or rain water. Alkaline and mineral-rich tap or well water will slowly kill them. Many insect-eating plants are collected from the wild for sale, and populations of Venus's-flytrap, for example, are imperiled as a result. Purchase them only from reputable dealers.

Marshes

Marshes are open, shallow wetlands with standing water for all or part of the year. In general, marshes are tree-less, dominated by herbaceous plants. Emergent plants grow in standing water, and sedges, grasses, and water-tolerant forbs grow in the seasonally inundated soils around the margin of the wetland. Emergent plants reproduce by seed during natural drawdown cycles caused by drought. Marshes are classified as shallow or deep, based on the water depth and the persistence of water through the season. Deep marshes have water up to three feet deep. Shallow marshes have 6 inches or less of standing water.

Marshes are among the most productive wetlands for birds and mammals. Ducks, geese, wading birds such as herons and egrets, rails, terns, and songbirds make their home in marshes. Raptors, including ospreys, bald eagles, northern harriers, and short-eared owls, hunt and nest in or adjacent to marshes. Muskrats, mink, voles, and mice are common. Marshes serve as nurseries for young fish. They also stabilize soil and improve water quality by filtering sediments and absorbing nutrients.

Peatlands/Bogs

Peatlands form in glacial lake beds in regions where temperatures are too cold for rapid decay of organic matter. The organic matter, called peat, accumulates faster than it decomposes, and open wetlands fill with peat over time. Bogs are specialized, acidic peatlands derived mostly from species of sphagnum moss. Bogs are nutrient poor and generally form in basins with no inlet or outlet for surface water. The lack of fresh water causes the acidity to increase to a level that is toxic to all but specialized plants.

Bogs start as floating mats of vegetation over the open water of a basin. As the mats grow and thicken, the basin begins to fill in with accumulated peat. The older the bog, the drier it becomes, until trees are able to grow. Open bogs are dominated by shrubs in the heath family (Ericaceae) and herbaceous plants. Older bogs are dominated by spruce and larch trees in the dry areas where the mat is thickest, by shrubs in the intermediate zones, and by herbs in the saturated soils.

Bogs are not totally restricted to northern latitudes. The are found in the Appalachian and Rocky Mountains, as well as in the southeastern states. These southern bogs often lie over shallow bedrock or hardpan soils that trap water at the surface. They are similar in ecology to northern bogs, but the peaty soils are much shallower.

Bogs are home to an astonishing array of plant and animal life. Bears, lemmings, amphibians, and many songbirds live amongst shrubs, orchids, ferns, and pitcher plants.

Swamps

Swamps are wetlands dominated by woody vegetation, with soil that is inundated for all or part of the year. There are three main categories of swamps: shrub swamps, or carrs, deciduous swamps, and coniferous swamps. Shrub swamps grow on the edges of bogs, wet meadows, streams, floodplains, and other wetlands. They are often dominated by a mixture of dogwoods (Cornus species) and willows (Salix species). A variety of sedges (Carex species), grasses, and forbs grows in the dappled shade of the shrubs. Shrub wetlands are important nesting sites for songbirds such as sparrows and warblers, and provide winter shelter for mammals. Shrub carrs dominated by alders (Alnus species) are known as alder thickets.

Deciduous swamps are found in ancient lake beds and other low-lying areas with poor drainage, seepages, and the oxbows and floodplains of rivers. These seasonally wet areas are critical for floodwater retention and home to a wide variety of plants and animals. Lowland trees such as black ash (Fraxinus nigra), red maple (Acer rubrum), tupelos (Nyssa species), and bald cypress (Taxodium distichum) tower above a carpet of marsh marigold (Caltha palustris), skunk cabbage (Symplocarpus foetidus or Lysichiton americanum), orchids, sedges (Carex species), and ferns. In southern swamps, red bay (Persea borbonia), loblolly bay (Gordonia lasianthus), titi (Cyrilla racemiflora), and hollies (Ilex species) form thickets amongst the cypress trees.

Coniferous swamps are found in northern areas where winter cold is hard on deciduous trees. In acidic soils, larches (Larix species) dominate. Where soils are sweet and nutrient rich, white cedar (Chamaecyparis thyoides) is common. The dense year-round shade of coniferous swamps limits the herbaceous vegetation to plants with evergreen leaves such as goldthreads (Coptis species), as well as orchids, sedges, and ferns.

Saltmarshes

Saltmarshes are coastal wetlands found in estuaries and backwaters, behind barrier beaches, and along bays and inlets. Saltmarshes are subjected to dramatic, twice-daily water-level fluctuations from high and low tides. Grasses dominate saltmarshes, but a variety of sedges (Carex species), glassworts (Salicornia species), and other forbs are also found. Spartina grasses are a common component of healthy saltmarshes, but pollution and sedimentation have allowed common reed (Phragmites communis) to dominate in many situations. This plant forms dense monocultures where it gets a foothold, and it soon crowds out other species. The muck soils are nutrient rich, making saltmarshes very productive. Gulls, terns, and other birds nest and winter in the dense cover of the grasses. Wrens, sparrows, and red-wing blackbirds are common in the drier areas. Rails, ducks, shorebirds, and wading birds frequent the mudflats at low tide. Fish and crustaceans breed in the shallow waters.

Seafood Nurseries

Saltmarshes are virtual seafood factories. The twice-daily influx of tides bathes them with oxygen and nutrient-rich water. Fish, crustaceans, and other marine life spawn in the shallow waters. Crabs, mussels, scallops, and many prized food fish such as striped bass depend on the marshes to shelter their young. Yet these wetlands are some of the most adversely affected by human activity. Not only are they filled or dredged to make room for resorts and marinas, but they also are repositories for upstream pollution. Many saltmarshes are part of estuary systems at the bottom of the watersheds of great rivers. Upstream from the marshes, development and agriculture are common. Erosion dumps soil, fertilizers, and pesticides into streams and rivers. They concentrate as they move downstream toward the saltmarshes, where they are dumped by the slowing currents. The sediment smothers eelgrass and other submergent vegetation, eliminating spawning and hiding places. The pesticides kill plants and animals. The nutrients accelerate the growth of pest plants at the expense of others. The health of saltmarshes depends on careful land management, both upstream and along the coast.