sist. However, hydroperiod variability can result in major fluctuations in the numbers of species from year to year (Snodgrass et al. 2000), with the consequence that alternative wetlands must be reached for breeding and feeding opportunities in some years. Many species take advantage of, and actually require, alternative wetlands during periods of drought. To avoid extirpation from natural causes, a variety of isolated wetlands must be accessible by overland routes (see Box 3–2). Species need alternative wetlands in the landscape when a particular wetland experiences a period of environmental duress.
The aquatic and semiaquatic fauna that use wetlands are key components of wetland structure, productivity, and overall functioning. How-
Ecological Functions of Small, Isolated Wetlands
Of 371 isolated depression wetlands known as “Carolina bays” in South Carolina, most (87%) are smaller than 4 hectares (ha), and 46% are 1.2 ha or smaller. Because they are small, the Carolina bays are more variable than larger wetlands and more likely to dry temporarily during most years. The smaller their size, the lower the probability that species predatory on amphibians, such as fish and dragonfly larvae, will be present during winter and spring when many amphibians are developing. Most fish are restricted to permanent water systems, whereas dragonfly eggs are laid in the warmer months with larvae that persist until the following spring. Thus, if the wetland dries in autumn, neither fish nor dragonfly larvae are present when autumn- and winter-breeding amphibians enter the wetland or while larvae are developing.
Field research in Carolina bays shows that these small isolated wetlands are critical for amphibians. When the bays are of a suitable water depth, they are used for breeding by a wide array of salamanders and frogs. When the smaller wetlands are too dry, the larger bays act as refugia, so that collectively the “metapopulations” of amphibians persist. Additional studies indicate that the maximum dispersal distance for many amphibian species may be less than 1 kilometer (km). The ability of a population to persist is thus limited by the proximity and juxtaposition of small isolated wetlands. As the distance between wetlands increases, the potential for migration and recolonization by amphibians decreases.
Using a GIS with maps of the locations of wetlands of different sizes, Semlitsch and Bodie (1998) showed that if all wetlands smaller than 4 ha were removed, the nearest-wetland average distance would increase from 471 meters (m) to 1,633 m—beyond the critical dispersal distance for most amphibians. The coupling of data on amphibian life histories, dispersal distances, and wetland size and distribution provides convincing evidence that a network of small isolated wetlands is essential for ecosystem function in many regions.