little else” (Hammer 1997). Thus, the use of compensatory wetlands for storm-water treatment may achieve watershed water-quality goals but at the expense of other ecological functions lost or degraded at the impact site. It is important that all lost or impacted functions be mitigated. Stormwater wetlands might best serve watershed water-quality goals, in which case other functions might best be compensated for at another location within the watershed. But such decisions can be effectively addressed only by applying appropriate functional assessment tools on a watershed scale.

In the southeastern United States, there is a large acreage of inland nonriverine wetlands, such as pocosins and pine savannahs, that occur on broad flats and often exist at higher relative elevations in the watershed. The areas are relatively flat, so water moves slowly across the soil surface. They are often located miles from a naturally occurring stream, and excess rainfall can take several weeks to dissipate. The hydrology and degree of wetness are driven by rainfall and evapotranspiration. Although the quality of water discharged from these wetlands is high (Richardson et al. 1978; Richardson et al. 1981; among others), these wetlands do not provide a cleansing water-quality function in the watershed, because they rarely receive natural inputs of poor-quality water (Evans et al. 1993). By releasing storm water slowly, they moderate peak storm flow and provide extended baseflow through the watershed.

Wetlands as Animal Dispersal Corridors in Watersheds

Dispersal of plants and animals is influenced by the proximity and number of wetlands in a geographic area. Connectivity between (Harris 1988) and functional interdependence of wetlands with other landscape units (Bedford and Preston 1988) can also affect animal use because many species (e.g., some amphibians) require an upland-wetland matrix.

Most wetland species of reptiles, amphibians, small mammals, and possibly nonflying invertebrates do not have capabilities for overland migration if terrestrial corridors are obstructed. Birds and flying insects are exceptional in that a disrupted terrestrial landscape can be negotiated without complication, permitting movement to another wetland when necessary.

The functioning of many wetland animal populations on a long-term basis is inherent to the source-sink dynamics of metapopulations that require connectivity in the terrestrial landscape (Gibbs 1993; Burke et al. 1995; Semlitsch 2000). Although populations of many or most wetland animals can fluctuate dramatically in numbers seasonally and annually (Pechmann et al. 1991), most wetland species will remain associated with a particular wetland as long as environmentally suitable conditions per-



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement