and New Hampshire, have lost more than 20 percent of their original wetland acreage. The highest percentage of loss, 91 percent, has taken place in California, where only 454,000 acres (ca. 183,700 ha) remain, while the highest acreage loss, approximately 9,286,713 acres (ca. 3,759,800 ha) has occurred in Florida (a 46 percent loss; Figure 6.1).
Alterations to wetlands fall in three sometimes overlapping categories—biological, chemical, and physical (Table 6.3). The biological alterations result from management to maximize one or a few specific wetland values, from harvesting or removal of natural vegetation or animals, and from introduction of nonnative plants and animals. Like lakes and streams, wetlands are also subject to chemical alterations through point and nonpoint nutrient runoff from surrounding uplands; discharge of toxic, hazardous, or other chemical waste; and increased nutrient loadings.
For wetlands, the most destructive alteration has been physical, often eliminating the topography and hydrology that supports the wetland ecosystem. The most significant historical loss of wetlands has resulted from agricultural practices (87 percent; Tiner, 1984), with most due to wetland drainage (Figure 6.2). The construction of dams and the dredging of river, stream, and coastal waterways eliminate wetlands at the project site and also affect downstream systems. The stabilization of water levels for rivers or lakes eliminates the vital pulsing function that flooding provides, thus interrupting nutrient and sediment delivery (Loucks, 1989). The natural maintenance and expansion of wetlands often depend on sedimentation events, which are interrupted by dams (long-term reduction in sediment load) and dredging projects (short-term increase in sediment release). In urban areas, filling has accounted for significant wetland loss. This continues to be the case where restoration of one wetland is proposed as mitigation for additional development of other wetlands (Fonseca et al., 1988; Zedler, 1988a). In most physical alterations of the habitat, the wetland ecosystem is obliterated, and there is little opportunity for restoration once the habitat has been flattened or replaced by urban development, reservoirs, ports, or marinas.
Biological, chemical, and physical alterations often occur together, and the result is a cumulative impact that may well exceed the "sum" of the individual disturbances. A wetland that has been bulldozed to remove vegetation would recover more rapidly and more completely if it were not also contaminated with toxic materials or affected by hydrologic alterations. In addition, the alteration of one wetland