Abiotic and biotic factors affecting nonrandom distributions of Chihuahuan Desert anurans

Abstract

Distributions of Texas toad (Bufo speciosus), Couch's spadefoot toad (Scaphiopus couchii), narrow mouthed toad (Gastrophryne olivacea), red spotted toad (Bufo punctatus), and western green toad (Bufo debilis) are nonrandomly distributed across the Chihuahuan Desert landscape. I found significant differences between observed and expected numbers of amphibians associated with availability of soil and vegetation types. These results suggest large-scale habitat associations may act as an initial filter for presence of all species. Although abiotic factors may explain large-scale habitat associations by amphibians, biotic processes may operate at small scales. Several studies have shown that competition between and among larvae of amphibian species and susceptibility to predation affect local distributions of amphibian assemblages. However, few studies have examined mechanisms affecting distribution patterns and site use of anurans adapted to desert ephemeral habitats, such as temporary pools used by amphibians. I examined hypotheses about competition and predation as mechanisms creating non-overlapping patterns of breeding site use by 4 anuran species that breed in desert ephemeral habitats: S. couchii, G. olivacea, B. speciosus, and B. punctatus. These 4 species show a significantly nonrandom pattern of co-occurrence at breeding sites. Only 12% of 95 ephemeral breeding sites surveyed were occupied by more than one species at one time. In laboratory experiments I tested the hypotheses that activity rates of tadpoles, which in turn reflected their competitive ability and susceptibility to predation, are potential mechanisms creating non-overlapping use of breeding sites. Tadpoles of S. couchii were significantly more active and more susceptible to predation than tadpoles of G. olivacea, B. speciosus, and B. punctatus. Mass of G. olivacea, B. speciosus, and B. punctatus tadpoles was less, on average, when reared with tadpoles of S. couchii, demonstrating possible competitive dominance of S. couchii. These results suggest competitive ability of S. couchii may play a role in excluding G. olivacea, B. speciosus, and B. punctatus from desert ephemeral breeding sites, and that susceptibility to predation could play a role in excluding S. couchii from breeding sites of longer duration that are more likely to be colonized by aquatic predators.