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Use of Macroinvertebrates to Identify Cultivated
Wetlands in the Prairie Pothole Region


Wetlands in the United States have been the focus of considerable controversy since European settlement. Initially perceived as wastelands that produced mosquitoes and other insects associated with human diseases and as impediments to agricultural production, federal policies targeted wetlands for conversion to cropland and other land uses. By the 1950s, growing concern over the loss of prairie wetlands and their importance to waterfowl and migratory birds led to the enactment in 1958 of the small wetlands acquisition program (Public Law 87-383). Increasing interest and knowledge of wetland functions, coupled with concern over continuing wetland losses, stimulated additional interest in protecting wetlands. Wetland protection increased in 1972 with the passage of the Federal Water Pollution Control Act (Public Law 92-500), in 1977 under amendments to Section 404 of the Federal Water Pollution Control Act (Clean Water Act; Public Law 95-217), and in 1985 under the Food Security Act (Public Law 99-198). Wetlands originally comprised about 9% of the land surface and about 90 million ha in the United States; about half of that area had been drained or converted to other land uses by the mid-1980s (National Research Council 1995). Today, wetlands are the only ecosystems that are regulated on both public and private lands in the United States.

To support national needs, several manuals have been developed to help federal regulators identify and delineate wetlands. The first such manual was developed by the U.S. Army Corps of Engineers (USACE) in 1987 (U.S. Army Corps of Engineers 1987). In 1989, an interagency manual (Federal Interagency Committee for Wetlands Delineation 1989) was developed by the USACE, the U.S. Fish and Wildlife Service, the Environmental Protection Agency (EPA), and the U.S. Department of Agriculture (USDA). The 1989 interagency manual was strongly criticized because it was perceived as designating areas as wetland that should be classified as upland (National Research Council 1995). Hence, a revised manual was prepared in 1991, but it was criticized for excluding many important wetlands and was not adopted. To further complicate the development of a uniform scientific definition of wetlands, the USDA developed a separate manual for agricultural wetlands relying on definitions established under the 1985 Food Security Act (U.S. Department of Agriculture 1994). With neither of the wetland identification manual versions proving satisfactory and a separate manual being used for agricultural wetlands, confusion existed over how to identify and delineate wetlands. As a result, Congress requested in 1993 that the EPA ask the National Research Council (NRC) to form a committee to evaluate the scientific basis for identifying and delineating wetlands and to assess the adequacy of existing manuals. The NRC report made a number of recommendations, including the need to develop a new manual that would serve the broad needs of all federal regulators and be based on scientifically credible procedures that would accurately identify wetlands and avoid misclassifications (National Research Council 1995).

Three features (wetland hydrology, hydric soils, and hydrophytic vegetation) are used to define wetlands according to each of the three wetland definitions currently used in the United States — those of the U.S. Army Corps of Engineers (1987), the 1985 Food Security Act (Public Law 99-198), and the U.S. Fish and Wildlife Service (Cowardin et al. 1979). Current indicators of wetland hydrology (watermarks, drift lines, sediment deposits, visual observation of saturation, etc.) are much more variable on a short time scale than are the indicators of hydric soils (U.S. Department of Agriculture 1996) and hydrophytic vegetation (Reed 1988), so that hydric soil and vegetation indicators often are used to infer wetland hydrology (National Research Council 1995). Identification of wetlands within agricultural landscapes using current indicators of wetland hydrology, hydric soils, and hydrophytic vegetation is often difficult in the Prairie Pothole Region (PPR) of North America because indicators of wetland hydrology are often destroyed, soils are often disturbed by tillage, and/or plant communities may have been artificially altered or removed by farming practices (U.S. Army Corps of Engineers 1987). The problem is exacerbated during drought years by increased agricultural activity within wetland basins. A need exists for additional indicators that can be used to identify wetlands during periods of drought or when basins have been disturbed by intensive agricultural activity.

The recalcitrant remains of macroinvertebrates (e.g., shells, chitinous exoskeletons, and head capsules), resistant eggs, and aestivating adults or immatures may offer a new tool for determining the presence or absence of wetland hydrology. Most macroinvertebrate taxa are specific to particular hydrologic regimes that define wetland classes (Wiggins et al. 1980, Pennak 1989, Schneider and Frost 1996, Euliss et al. 1999). Further, many taxa are ubiquitous in wetlands but are not found elsewhere. Moreover, their remains may persist in wetlands for long periods and are not easily destroyed by mechanical tillage. Hence, they provide time-integrated information on wetland hydrology useful in identifying wetlands.

To test the feasibility of using macroinvertebrate remains as a wetland identification tool, we initiated research in 1992 with the following objectives. First, we wanted to determine if recalcitrant remains of invertebrates could be used to identify temporary and seasonal wetlands and separate them from the adjacent upland within intensively tilled agricultural fields. We chose temporary and seasonal wetlands because these two wetland classes (Stewart and Kantrud 1971) are most commonly farmed. Second, we wanted to determine if macroinvertebrate recalcitrant remains could be used to separate with some confidence seasonal wetlands from temporary wetlands. We conducted our study on cultivated wetlands in the glaciated drift prairie of North Dakota, South Dakota, and Minnesota, an area of extensive agricultural activity.

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