Effect of anthropogenic disturbance and landscape structure on body size, demographics, and chaotic dynamics of Southern High Plains amphibians
Gray, Matthew James
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Amphibian populations are declining globally. Anthropogenic disturbance of landscapes surrounding wetlands may affect fitness, demographics, and dynamics of amphibian populations. Spatial positioning and relative connectedness of wetlands also may influence population demographics. Thus, I examined the effect of anthropogenic landscape use (cultivation vs. grassland) and structure on postmetamorphic body size (a fitness correlate), demographics, and dynamics of amphibians at 16 playa wetlands on the Southern High Plains (SHP) of Texas during 1999 and 2000. Amphibian populations were monitored using drift fence and pitfall traps, landscape structure was quantified using spatial analysis software, and dynamics were assessed using difference equations. Postmetamorphic body size of all amphibian species and age classes generally was greater at grassland than cropland playas, and in 1999 (i.e., a wetter year) than 2000. Abimdance of New Mexico and plains spadefoots (Spea multiplicata and S. bombifrons) generally was greater at cropland than grassland playas, and greater for barred tiger salamanders (Ambystoma tigrinum mavortium) in 1999 than 2000. Mean daily abundance of amphibians also was positively related to landscape structure indices representing geometric complexity and spatial positioning of wetlands. In general, as landscapes became more complex (e.g., numbers of edges increased) and inter-playa distance decreased, mean daily abundance of amphibians increased. Additional demographic analysis indicated that temporal niche partitioning existed in SHP amphibian populations; however, no differences existed between landuses. Lastly, biological chaos in the amphibian assemblage existed at 1 of 8 cropland and 7 of 8 grassland playas. A stochastic density-dependent Ricker function predicted chaotic dynamics most accurately. Anthropogenic disturbance surrounding wetlands affects body size, demographics, and dynamics of SHP amphibians. Spatial positioning of wetlands and landscape complexity may be as or more important than general landuse in affecting amphibian demographics. Annual differences in body size and abundance suggest rainfall may be important in influencing amphibian populations. Although spadefoot abundance was positively influenced by anthropogenic disturbance, I recommend retention and restoration of grasslands surrounding playa wetlands because landscape cultivation decreased body size and altered amphibian demographics and dynamics from an undisturbed state. These results have important implications in conservation biology, landscape ecology, and basic ecological and mathematical theory.