The interaction of predation, competition, and habitat complexity in structuring an amphibian community

We examined effects of predation and competition on population dynamics and community structure in an aquatic system in central Arizona by measuring how refugia for tadpoles (Hyla eximia) influence predation by larval salamanders (Ambystoma tigrinum). Using field enclosures with three densities of A. tigrinum and two of H. eximia and two degrees of habitat complexity, we measured the effects of predation, competition, and habitat complexity on four components of fitness in larval amphibians: mass at metamorphosis, growth rate, length of larval period, and survival. Hyla eximia had low survival where salamanders were present. Tadpoles in treatments with two or four salamanders had one-fifth the survival of tadpoles in treatments with no salamanders. We found some evidence of density-dependent effects in Ambystoma. Mean salamander mass was negatively affected by salamander density. Density-dependent effects in H. eximia were weak. Habitat complexity alone did not affect any response variable but interacted with salamander density and tadpole density in a complex, nonadditive fashion. Many studies that have examined the relative importance of predation and competition in structuring amphibian communities have ignored habitat complexity. Our experiment, which examined different degrees of habitat complexity, suggests that the relationship between predation and competition may be nonadditive depending on the degree of habitat complexity.