Community assembly of xeric-adapted anurans at multiple spatial scales

Abstract

The distribution and abundance of organisms is influenced by historical, abiotic, and biotic factors. The goal of my dissertation was to determine the distribution of anurans in the Big Bend region of the Chihuahuan Desert and to examine how abiotic and biotic factors shape the composition and structure of anuran communities at multiple spatial scales. My approach relied on extensive field surveys, laboratory and field experiments, and GIS modeling. Results from field surveys and reciprocal transplant studies of tadpoles indicate that abiotic conditions of the breeding site most likely do not play a significant role in causing the segregation of species among individual breeding pools. I used laboratory and mesocosm experiments to test for indirect and direct effects of predators on growth and survival of S. couchii tadpoles. I found that S. couchii tadpoles do not alter their behavior in the presence of predators and are very susceptible to predation. Although tadpoles reared with predators suffered high mortality rates, they metamorphosed significantly faster than tadpoles reared without predators. The reduced time to metamorphose is likely a result of the thinning of intraspecific competitors. Because the primary cause of death for S. couchii tadpoles is desiccation due to pond drying, predators may play an important role in facilitating metamorphosis by decreasing competitors and thus increasing per capita resources, therefore decreasing time to metamorphosis for the surviving tadpoles. At the landscape level anuran distributions seem to be influenced by environmental factors that influence the survival of the adult stage. At the level of the breeding site, microhabitat and abiotic components of the aquatic environment do not seem to play an important role in influencing breeding site use by different species. Rather, it seems likely that predation on tadpoles by predators is important in limiting the distribution of some species and that the fast-developing S. couchii may exclude other species from using sites via oophagy and predation on small tadpoles. My research elucidates the fact that in order to understand factors important in regulating ecological communities it is important to examine both abiotic and biotic factors at multiple spatial scales.