Potential mechanisms underlying the displacement of native red-legged frogs by introduced bullfrogs

The bullfrog (Rana catesbeiana) is a widespread invasive species that may displace several species of native ranid frogs throughout its introduced range. Although this pattern is well known. the underlying mechanism of displacement remains unclear. Previous work has suggested that interactions with bullfrogs may contribute to the population decline of native red-legged frogs (Rana aurora) in Oregon, USA. Interactions between these species appear to be strongly context dependent and potentially influenced by habitat modification. To gain a more detailed understanding of this displacement, we studied the effects of food-resource distribution, a factor that can be influenced by human habitat alteration, on competitive interactions between larval red-legged frogs and larval bullfrogs. The presence of bullfrog larvae had strong negative effects on the performance of red-legged frog larvae. However, this effect was dependent on whether food resources were clumped or scattered. Survivorship to metamorphosis and mass at metamorphosis were reduced when red-legged frog tadpoles were exposed to bullfrogs in clumped-resource ponds. In contrast, the presence of bullfrogs had a negligible effect on larval performance of red-legged frogs in scattered-resource ponds. Behavioral observations indicate that a passive interference mechanism is likely to be responsible for the outcome of interactions between bullfrogs and red-legged frogs. Our results suggest that clumped resources can intensify interspecific competition, and this may influence the success of exotics when human-induced habitat alteration affects resource distribution, Understanding the context-dependent nature of interactions will be necessary if we are to predict invasion success and control the impact of exotics on natives.