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Surveys of Calling Amphibians in North Dakota

Methods


Surveys were conducted by a single observer (RDB) between 4 May and 30 June 1995. He typically began each survey at about sunset and continued until early morning. Again, weather conditions often required some variances from planned schedules.

Stations were located every 0.5 mile (0.8 km). At each station, the observer spent 10 minutes and recorded both birds and calling amphibians. Shirose et al. (in press) found that most anuran species were detected during the first two minutes of 30- or 60-minute survey periods. At the beginning and end of each route, he recorded temperature, relative humidity, wind speed, cloud cover, precipitation, and moonlight (Table 1). For analysis by station, estimates of those variables at a station were made by linear interpolation in time from the values at the first and last stations. For the categorical variables wind speed and cloud cover, we first converted coded values to their midpoints of intervals (Table 1), then interpolated, and finally converted back to nearest coded values.

For the first objective, we compared the distribution of species recorded with those published by Hoberg and Gause (1992). These compared closely with ranges in Conant and Collins (1991). We noted any extralimital observations.

For the second objective, we compared the distribution of observations under specified conditions (e.g., temperature, time of day) to the distribution of time spent by the observer in surveys under those conditions. For each species, we restricted attention to routes within the geographical ranges presented by Hoberg and Gause (1992), unless we detected the species in a hexagon outside that range. In that case, we also included that hexagon.

We compared our results to those obtained in two other studies. Bowers et al. (1997) conducted similar surveys in 20 of the same sample units in North Dakota, using both roadside transects and survey points adjacent to wetlands. Their report presents information only on detections, however, and does not account for the amount of time spent surveying under specified conditions. Mossman and Hine (1984, 1985) provided some preliminary results from the Wisconsin frog and toad survey. Bishop et al. (1996) reported on results of extensive monitoring of anurans in Ontario. They provided some comments on seasonal phenologies of the species encountered and variation in detections according to time of night, air temperature, wind speed, and precipitation.

[NOTE: WE WOULD APPRECIATE INFORMATION FROM OTHER INVESTIGATORS ABOUT THE INFLUENCES OF VARIABLES ON CALLING RATES OF THESE SPECIES. PLEASE CONTACT DOUGLAS JOHNSON AT Douglas_H_Johnson@usgs.gov]

For the third objective, we present some recommendations based on this effort and analysis.


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