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The prairie pothole region covers about 715,000 km², extending from north-central Iowa, USA to central Alberta, Canada (Figure 1). The landscape of the prairie pothole region is largely the result of Pleistocene glaciation. The landscape is dotted with numerous depressional wetlands caused by the uneven deposition of glacial sediments, the scouring action of glaciers, and the melting of buried ice blocks. The landscape has been substantially altered since European settlement beginning in the late 1800s. Economic incentives to convert natural landscapes to agriculture have been great resulting in the loss of over half of the original 8 million hectares of wetlands (Tiner 1984, Dahl 1990, Dahl and Johnson 1991). Land-use impacts on wetland biota include enhanced siltation, contamination from agricultural chemicals, altered hydrology, spread of exotic plants, and habitat fragmentation due to wetland drainage and conversion of native prairie grasslands into agricultural fields.

For this study, we selected ten seasonal and ten temporary wetlands (Stewart and Kantrud 1971) from 640-km² hexagons systematically overlain on the prairie pothole region of North Dakota as part of the U.S. Environmental Protection Agency's Environmental Monitoring and Assessment Program (U.S. Environmental Protection Agency 1993). For each wetland class (i.e., seasonal and temporary), we selected at random five wetlands located within tilled agricultural landscapes (in production for a minimum of ten years) and five additional wetlands located within undisturbed grassland landscapes. For purposes of this study, we limited our study sites to within those hexagons occurring within the glaciated plains of North Dakota (Figure 2), an area of intensive agricultural activity. Seasonal wetlands thus selected ranged in size from 0.1 ha to 0.8 ha and temporary wetlands ranged from 0.1 ha to 0.3 ha.