Bullfrogs, disturbance regimes, and the persistence of California red-legged frogs

The introduction and spread of bullfrogs (Rana catesbeiana) in western North America may have played a central role in the declines of native ranid frogs. Specifically, a positive correlation exists between the absence of California red-legged frogs (Rana aurora draytonii) and the presence of introduced bullfrogs, but coexistence does occur in some environments. Enclosure experiments and diet studies have shown that bullfrogs prey on larval and juvenile California red-legged frogs. We used a modeling approach to quantify the threat of bullfrog predation on California red-legged frog populations. We created age-structured population models for both species. We used these models to (1) explore the sensitivity of red-legged frog populations to changes in the intensity of bullfrog predation; (2) explore the hypothesis that high flood frequencies increase the probability for coexistence in southern California streams; and (3) examine the efficacy of bullfrog management strategies, such as shooting adults and draining livestock grazing ponds. Our model simulations indicated that winter floods, which strongly increase mortality of bullfrogs but not red-legged frogs, facilitate coexistence if they occur more than once every 5 years. We found that increasing adult bullfrog mortality through shooting would benefit red-legged frogs only with extreme effort. Conversely, the draining of livestock grazing ponds can be effective in bullfrog management if draining occurs at least every 2 years. Shooting and draining in tandem were successful at decreasing bullfrog densities. Finally, our model provided a quantitative measure of bullfrog predation on California red-legged frogs that can potentially be used to assess the impact of bullfrogs on a site-by-site basis. Our model, plus experimental studies that link specific environmental factors to the bullfrog predation rate, can provide managers with a useful tool for controlling populations and facilitating conservation efforts for the California red-legged frog.