It's an assessment supported by recent studies, including two in Pennsylvania. A 1996 Pennsylvania Department of Environmental Protection evaluation of 99 replacement wetlands found 59 had failed, with most of the failures due to lack of water. In King County, Wash., a 1998 review of 29 wetland and stream mitigation sites found that 23, or 79 percent, were unsuccessful. Only one of the 29 replaced the functions of the wetlands destroyed by development. The study also found that nine promised mitigation sites hadn't been built at all.
Wetlands can fail for many reasons.
The hydrology problems cause wetland plants to fail to take root. Wetland plants also can be decimated by animals, including deer, which browse on tender emergent shoots. Invasive, non-native plant species, such as purple loosestrife, also can crowd out wetland plantings. Following a century in which Pennsylvania's 1 million acres of wetlands were reduced by more than half, the state Department of Environmental Protection says more than 3,100 acres of wetlands have been added to the state's half-million acre total over the past several years. Most of the state's new muddy areas are the result of work by the U.S. Fish and Wildlife Service, the Natural Resources Conservation Service, individual landowners and private clubs or environmental groups that have built wetlands to clean up acid mine drainage.