Population dynamics of lowland leopard frogs in the Rincon Mountains, Arizona

Zylstra, Erin; Swann, Don; Steidl, Robert

Amphibians have declined in abundance and distribution worldwide, including populations of ranid frogs in Arizona. Identifying the mechanisms responsible for these declines requires that we distinguish between systematic, long-term declines and natural, short-term fluctuations, which can be challenging for species that inhabit environments where conditions vary markedly, both seasonally and annually. We leveraged 16 years of survey data for lowland leopard frogs (Lithobates yavapaiensis) in montane canyons of southern Arizona—an ecosystem where environmental conditions are highly dynamic—to assess status and trends of frog populations and to identify factors that govern dynamics in distribution and abundance over time. We characterized patterns of occupancy at two geographic scales, drainage (n = 9) and pool complex (n = 71), which we defined as collection of pools separated by ≥120 m; we characterized patterns of abundance at the pool-complex scale for a subset of complexes (n = 34). Between 1996 and 2011, park staff and volunteers completed 470 visual encounter surveys during spring (1 May – 15 July) and fall seasons (1 Oct – 31 Dec). Leopard frogs were observed at least once in >95% of the drainage reaches and tanks surveyed (n = 21), which were chosen because they were known to harbor frogs historically; in general, counts of frogs were highest in south-facing drainages with a high density of pools below 1200 m elevation. Probability of occupancy for adult frogs at both drainage and complex scales increased with the number of pools and as the amount of water available during the previous season increased, and was not associated strongly with distance between local populations. Abundance of adult frogs within complexes varied seasonally, with higher numbers in fall than in spring. Similar to occupancy dynamics, rates of recruitment and survival of populations increased with availability of surface water. We found no evidence of systematic trends in populations of lowland leopard frogs in and near the park. Dynamics and persistence of frog populations in these arid environments, however, depend critically on the amount of surface water available, which is likely to decline in response to increasing pressure from anthropogenic threats, including local withdrawal of groundwater, and changes in climate predicted for this region.

Population dynamics of lowland leopard frogs in the Rincon Mountains, Arizona

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