Ecological and Behavioral Interactions Between Two Closely Related North American Frogs (Rana clamitans and R. catesbeiana)

Abstract Resource partitioning within the ecological niche space in which there is a high level of overlap between species can alleviate the tendency toward competitive exclusion. When competitive ability is asymmetrical due to predation or other ecological factors, it may be more effective for the less competitive species to lessen direct competition by contracting their use of local resources. Species occurring in mixed assemblages may come into direct contact with each other throughout their respective breeding seasons. Where competition for breeding habitat and acoustic space exists, the level of interference is expected to vary widely, depending upon the ecological and breeding similarities between the species involved and the relative importance of the resource. In this study, I investigated the breeding season interspecific interactions of two species of ranid frog in eastern North America, the American bullfrog (Rana catesbeiana) and the green frog (R. clamitans). The ecological and behavioral similarities between these species combined with phylogenetic relatedness and comparable natural distributions make them an ideal system for studying interspecific dynamics related to their breeding ecology. Specifically, I examined the influence bullfrogs have on the breeding behavior of green frogs over several timescales, including physical avoidance of encounters through counter movements of green frogs away from bullfrog territories, an adjustment of green frog microhabitat use, and green frog avoidance of acoustic masking by bullfrogs. I found that green frogs defend territories and lay eggs closer to shore than bullfrogs. Also, both green frog territories and eggs are under heavier overhead cover than bullfrog eggs and territories. I found green frogs respond to bullfrog chorusing on a fine temporal scale by placing their calls between the notes of bullfrog calls