Palatability and Antipredator Response of Yosemite Toads (Anaxyrus canorus) to Nonnative Brook Trout (Salvelinus fontinalis) in the Sierra Nevada Mountains of California

Abstract

In the Sierra Nevada Mountains of California, Yosemite Toads (Anaxyrus canorus) have declined throughout their range where Brook Trout (Salvelinus fontinalis) have been widely introduced. Amphibians that evolved in fishless habitats often lack the necessary chemical defenses and behavioral responses to avoid predation. True Toads (family Bufonidae), however, possess noxious chemicals that often deter predators. Our goal was to test whether eggs, tadpoles, and postmetamorphic toads of A. canorus are a palatable food source for S. fontinalis, assess the antipredator behavior of tadpoles of A. canorus by exposing them to various predator chemical cues, and determine sublethal effects on three lifestages of A. canorus from sampling by S. fontinalis. We found that eggs, tadpoles, and postmetamorphic toads of A. canorus exhibit absolute unpalatability to S. fontinalis. In addition, tadpoles of A. canorus did not exhibit a significant difference in behavior when exposed to chemical stimuli. Moreover, observational experiments revealed that trout would sample, then reject lifestages of A. canorus unharmed, while trout readily consumed tadpoles of Pacific Chorus Frog (Pseudacris regilla). We therefore infer that early lifestages of A. canorus likely possess existing chemical defenses as a result of interactions with native predators that adequately protect them from nonnative trout predation. Although trout removal often leads to the recovery of other Sierra Nevada amphibian populations, trout removal would likely have no effect on populations of A. canorus given our results. Therefore, we suggest that other factors thought to be contributing toward A. canorus decline should be investigated before efforts are concentrated on removing trout from toad habitats.