Competition, predation, and the distributions of four desert anurans.

Several studies have shown that larval competition and susceptibility to predation affect distributions of amphibian assemblages across ephemeral and perennial habitats. However, few studies have examined mechanisms affecting distribution patterns and site use of anurans adapted to highly ephemeral habitats. This study examines hypotheses about competition and predation as mechanisms creating non-overlapping patterns of site use in four anurans that breed in highly ephemeral habitats: Scaphiopus couchii (Couch's spade-foot toad), Gastrophryne olivacea (narrow-mouthed toad), Bufo speciosus (Texas toad), and Bufo punctatus (red-spotted toad). These four anurans showed a significantly nonrandom pattern of co-occurrence. Only 12% of 95 ephemeral breeding sites surveyed were occupied by more than one species. We tested the hypotheses that non-overlapping use of breeding sites was due to activity rates of their tadpoles that in turn reflect their competitive ability and susceptibility to predation. Tadpoles of S. couchii were significantly more active and more susceptible to predation than were tadpoles of G. olivacea, B. speciosus, and B. punctatus. The masses of G. olivacea, B. speciosus, and B. punctatus were less when they were reared with S. couchii, demonstrating the possible competitive dominance of S. couchii. These results suggest that the competitive ability of S. couchii may play a role in excluding G. olivacea, B. speciosus, and B. punctatus from very 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.