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Surveys of Calling Amphibians in North Dakota

Study Areas

We used a systematic sample of areas in North Dakota east of the Missouri River (Fig. 1). They covered the portion of the state covered by Wisconsin-age glaciation, which retreated about 12,000 years ago (Bluemle 1991). The retreat left a huge number of depressions, called prairie potholes, which contain water for various lengths of time in most years, and support a diversity of wetland-dependent wildlife (Johnson 1996). Prior to settlement, the area was covered almost exclusively by prairie grasses and forbs. Since settlement, much of the upland has been cultivated for the production of annual crops, notably small grains. While nearly half of the wetlands in the state have been drained (Dahl 1990), many remain; Cowardin et al. (1981) found for central North Dakota that wetlands covered 9.7 percent of the area.

The study areas had been chosen for other studies, designed to develop and test methods of monitoring environmental quality in the Prairie Pothole Region. These studies, for U.S. Environmental Protection Agency's (EPA) Environmental Monitoring and Assessment Program, were conducted jointly by EPA, the Northern Prairie Science Center, and other cooperators. The sample units were 44 hexagons, each 40 ha in size.

Within each hexagon, we selected a route consisting of secondary roads or trails. We endeavored to sample from throughout the hexagon. Stations were located at half-mile intervals along the route, usually along section lines or half-section lines. We tried to make routes continuous and at least 10 miles long, but limited road networks in some areas precluded meeting these criteria. Actual route lengths ranged from 7 to 14 miles. Because of heavy precipitation preceding and during the survey period, roads were frequently washed out, which necessitated altering routes to reach certain stations.

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