Environmental Factors Controlling Diversity

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Water level is a critical factor in all wetlands. Since each kind of plant and animal can withstand only certain amounts of flooding, wet-

Figure 2

Zonation of Birds, Mammals, and Amphibians in Relation to Water Level and Vegetation

A. Birds and Mammals

B. Amphibians

Source: Keddy, Paul A. 2000. Wetland Ecology. Cambridge: Cambridge University Press. fig. 2.3, pp. 88-89. (Reprinted with permission) Note: Different water levels create different plant communities, which in turn generates diversity in birds, mammals, and amphibians.

lands with different water depths will support the most kinds of species (see Figure 2). A typical shoreline marsh on lakes will often show distinct bands of vegetation (so-called zonation), with each kind of plant occupying a narrow range of water levels. Most animals, including frogs and birds also have their own preferred set of water depths.

Seasonal changes in water levels are normal; spring floods alternate with midsummer droughts. Spring floods enlarge marshes and swamps by killing terrestrial plants at higher elevations. Droughts allow some species of plants to regenerate from buried seeds. Humans often reduce these seasonal changes with dams. Spring floods are retained by the dams (see Figure 3), then released later to augment periods of low water. This almost invariably reduces biological diversity.

Nutrients (principally nitrogen and phosphorus) enhance productivity, but, paradoxically, added nutrients often reduce diversity. The high productivity is channeled into a few dominant species; rarer species disappear. Humans often increase nutrient levels in watersheds and wetlands. This process (called eutrophication) can be the result of either point sources (such as sewage from cities, runoff from feedlots) or diffuse sources (for example, erosion from farmland or logging operations).

Natural disturbances may remove biomass and increase diversity. In interior wetlands, disturbances include waves on lakes, fire, grazing, or, in the north, scouring by winter ice. Disturbances create gaps in the vegetation, allowing new kinds of plants to establish themselves from buried seeds. Most interior wetlands have buried seeds; in fact, densities may exceed 1,000 seeds per square meter. Gaps resulting from disturbance can also provide different kinds of food or nesting conditions for wildlife.

See also: Coastal Wetlands; Dams; Freshwater; Lakes

Figure 3

Effects of Water-level Variation and Stabilization on Wetland Zones

Source: Keddy, Paul A. 2000. Wetland Ecology. Cambridge: Cambridge University Press. fig 4.9, p. 195. (Reprinted with permission)

Note: Variable water levels (top) generate diversity in plant communities, particularly extensive areas of marsh and wet meadow. Changes in water level over decades, perhaps associated with changes in rainfall, are particularly important on large lakes such as the Great Lakes in North America. If dams stabilize water levels, the two types of wetland are lost, and the shoreline has only aquatic and shrub zones.


Goulding, Michael. 1980. The Fishes and the Forest: Explorations in Amazonian Natural History. Berkeley: University of California Press; Keddy, Paul A. 2000. Wetland Ecology. Cambridge: Cambridge University Press; Mitsch, William J., and James G. Gosselink. 1986. Wetlands. New York: Van Nostrand Reinhold; Patten, Bernard C., ed. 1990. Wetlands and Shallow Continental Water Bodies. Vol.1.: Natural and Human Relationships. The Hague: SPB; Whigham, Dennis F., Dagmar Dykyjova, and Slavomil Hejnyt, eds. 1992. Wetlands of the World 1. The Netherlands: Kluwer.

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