Background/Question/Methods      Intermediate predators not only compete with top predators for food resources but they can also be eaten by top predators. With such pressures being exerted on intermediate predators, it is surprising that these predators can coexist at all. Coexistence could be promoted, however, if the abundance of alternative prey species (shared and unshared) is sufficient to reduce the intensity of interspecific competition exerted on intermediate predators and/or by providing different prey for top predators to consume.  We conducted an experiment in artificial ponds to evaluate how the abundance of larval Pseudacris crucifer (prey eaten by top and intermediate predators), microinvertebrates (prey eaten by intermediate predator) and larval overwintered Rana (prey eaten by top predator) affected fitness components of larval Ambystoma opacum (intermediate predator) in the presence of larval Anax spp. (top predator). We hypothesized that the effect of Anax on the fitness (survivorship and mass at metamorphosis) of A. opacum would be altered by the abundance of alternative prey (either shared or unshared).
Results/Conclusions      We found that Anax caused a dramatic reduction in A. opacum survivorship but the effect of Anax on A. opacum survivorship was not altered by varying the abundance of resources (shared or unshared). By reducing the number of A. opacum present in a pond, however Anax also caused an increase in the growth of surviving A. opacum. The abundance of unique prey resources did not alter the growth of A. opacum or the effect of Anax on A. opacum growth. The presence of shared prey, however, altered the effect of Anax on A. opacum growth when shared and unique prey abundances were relatively high but not when they were relatively low. Specifically, Anax had a greater positive effect on A. opacum growth when a relatively high abundance of shared prey was present in environments with a high abundance of unique prey than when shared prey were absent from these environments.  Our results suggest that alternative prey will not promote the survival of larval A. opacum when a top predator is present. Since larger A. opacum have greater reproductive potential, however A. opacum will be more likely to persist in environments with Anax when resource availability is relatively high.