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314 In North America, approximately 55 of the 103 species of anurans can be surveyed readily by using counts of vocalizing males as an index to their presence or population size. Such surveys are most applicable to eastern and northern parts of the continent, where almost all species regularly vocalize and breeding seasons extend over several weeks. In the West, members of the chorus frog (Pseudacris) complex and American bullfrogs (Rana catesbeiana) consistently call during their breeding seasons, but many of the other species call either infrequently, quietly, or sporadically only following heavy rains and are thus unsuited to be surveyed using calling count techniques. Calling surveys of amphibians are designed to provide an index to changes in amphibian populations. The technique can be applied in monitoring sampling designs to provide estimates of abundance and change at scales ranging from continental down to single wetlands. A powerful attribute of a uniform, randomly allocated system of calling surveys is that they can be repartitioned in many meaningful ways. The estimates of trends from the same system of surveys can be developed for watershed, political, physiological, or survey-wide units of scale (depending upon research and management requirements) in a way similar to the North American Breeding Bird Survey (Droege, 1990). By correlating observed trends and distribution patterns with ancillary variables, it is also possible to gain insight into the factors affecting population changes over time (e.g., declines may be associated with agricultural regions, while increases may occur in regions where wetlands have increased). Calling Survey Protocol Most calling survey protocols in North America use a simple index to population size based on the one developed for the Wisconsin Frog and Toad Survey (for details see Mossman et al., 1998; Weir and Mossman, this volume). Surveys are usually run three to four times per year in synchrony with the mating and peak calling seasons of the local species. Surveys are often strung together as a “route” (a series of stops) at wetlands along rural roads. Starting time is 30–60 minutes after sunset, and a route typically takes about 2 hours to complete. To reduce disturbance effects, an observer usually waits one minute at each wetland stop, then starts a three- to five-minute listening period. The observer records which species were heard along with a calling index value. Variation Land managers, researchers, states, and provinces initiate calling survey programs with the idea that these counts act as an index to the true number of frogs and toads present in the area being sampled. The assumption is that this index is both an accurate (unbiased) and precise (little variation) reflection of what we really want to track—the rate of change of the amphibians in the study areas. In this section, we explore the known and suspected factors that affect calling surveys and reflect on what impact they may have on the interpretation of the resulting trends. Any monitoring program’s ability (power) to estimate a change (within a given level of precision) is determined by the amount of year-to-year variation in the counts, with high variation in those counts tending to diminish the ability to estimate trends. The intrinsic variability of a monitoring program’s counts also influences the sampling effort (e.g., the number of routes, the number of stops, the time spent at a stop, and the number of replicates of stops) needed to detect trends (if any) of the magnitude set by the organizers of the count; more routes are required when there is high variability. To investigate the variability in calling survey counts, we used analyses by John R. Sauer examining 14 years of data provided by Mike Mossman and Lisa Hartman from the Wisconsin Frog and Toad Survey. These analyses used the number of stops (out of 10) at which a species occurred on each route. The full 0–3 Wisconsin Index was not used because further work is needed in the analysis and interpretation of the Index in calculating trends. Consequently, this simplified index represents changes in the frequency of occurrence of amphibians at wetlands . The estimates of variation in counts calculated using the annual indices produced by Sauer’s analysis were then used to determine whether calling surveys have the ability to detect changes in populations of frogs and toads in Wisconsin. The currency we used for evaluating the variation in counts is a simple proportion called the coefficient of variation (CV). FO RTY-S I X Evaluating Calling Surveys SAM...

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