Detecting Frogs and Detecting Declines: An Examination of Occupancy and Turnover Patterns at the Range Edge of Blanchard's Cricket Frog (Acris Blanchardi)

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While geographically widespread, Blanchard’s Cricket Frog (Acris blanchardi) has been declining recently in the northern portions of its range. No cause or causes have yet been definitively associated with this decline and few data are available regarding its current status. To provide data on recent trends in occupancy in the area of decline, we monitored 312 aquatic sites in three areas of western Ohio from 2004−2008 using calling surveys (1,807 surveys total). These sites were positioned surrounding the easternmost known populations identified in 2004 to allow us to detect ongoing decline, stasis, or recovery. Using occupancy modeling techniques, we estimated occupancy and turnover (extinction-colonization) rates and identified factors influencing occupancy, turnover, and detection probability. Site occupancy varied strongly by region but was stable or increasing in all monitoring areas during the study (north: 29% to 31%; central: 5% to 16%; south: 14% to 21%). Further, the easternmost known populations shifted eastward in two of the three monitoring areas, which together we interpret as potential evidence for limited recovery. We also detected population turnover with estimated annual colonization and extinction rates averaging 4% and 7%, respectively. Extinction rates were highest in ponds, lowest in streams, and intermediate in lakes, suggesting a possible source-sink population structure. The probability of detection was most heavily influenced by time of day and whether a broadcast of a breeding vocalization was played during the survey. Although these populations appear to be stable or expanding, they still occupy only a fraction of the species’ historic range in Ohio. We advocate continued monitoring to detect future distributional shifts in this formerly common species.


amphibian declines, metapopulation dynamics, monitoring, occupancy modeling, source-sink dynamics

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