HUMAN INFLUENCES ON ANURAN DISTRIBUTION CHANGES IN THE SOUTHEASTERN U.S.A.
Luce, Michelle Diane
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As human populations continue to expand and grow, ecosystems and the species that inhabit them face ever-increasing stress. To conserve these ecosystems, scientists use indicator species to try to measure the impact of human-induced pressures on the environment. Anurans are exceptional indicator species because they are often the first vertebrates to respond to the direct and indirect effects of land use by humans. Sparse historical data have made it difficult to map the ‘natural’ distribution of many species and, therefore, to map any changes in distribution range as a result of human expansion. This study sought to address the lack of data coverage by using a predictive model to map the distribution of anuran species across the southeastern U.S.A. based on their response to environmental parameters. Historical (prior to 1960), transitional (1960-1989), and modern (1990-2013) distributions were mapped for each anuran species, and the expansions and contractions of distribution ranges were calculated between time blocks. Species life-history traits were analyzed as potential predictor variables to determine whether a species had increased or decreased its total range size in recent history. Results suggest that six species (60%) had a large decrease in range size from their historical distributions; three species (30%) had a large increase in range size, and one species (10%) had almost no change in range size. Most species that gained range size, from the historical to the modern block, expanded to the interior of the southeastern U.S.A. during the transitional-time block. Model results indicate that precipitation, temperature, and elevation are more important for determining frog distributions than human population density across all time blocks. Trait analysis did not indicate any traits were able to determine whether the species had range expansions or contractions. Distribution range trends suggest that the Atlantic Multidecadal Oscillation (AMO), a low-frequency climatic oscillation, may have a role in distribution expansions and contractions, but more long-term data are required to explore this possibility properly. Overall the study shows that historical distributions can be accurately mapped, despite the lack of landscape-scale historical surveys of individual species. Whether anuran species increase or decrease in range size from their historical distribution cannot yet be explained using species traits. Compounded effects from a changing climate cannot be ruled out.