Numbers, neighbors, and hungry predators: What makes chemically defended aposematic prey susceptible to predation?

Many chemically defended aposematic species are characterized by relatively low toxin levels, which enables predators to include them in their diets under certain circumstances. Knowledge of the conditions governing the survival of such prey animals-especially in the context of the co-occurrence of similar but undefended prey, which may result in mimicry-like interactions-is crucial for understanding the initial evolution of aposematism. In a one-month outdoor experiment using fish (the common carp ) as predators, we examined the survival of moderately defended aposematic tadpole prey (the European common toad ) with varying absolute densities in single-species prey systems or varying relative densities in two-species prey systems containing morphologically similar but undefended prey (the European common frog ). The density effects were investigated in conjunction with the hunger levels of the predator, which were manipulated by means of the addition of alternative (nontadpole) food. The survival of the tadpoles was promoted by increasing their absolute density in the single-species prey systems, increasing their relative density in the two-species prey systems, and providing ample alternative food for the predator. Hungry predators eliminated all individuals regardless of their proportion in the prey community; in treatments with ample alternative food, high relative density supported survival. The results demonstrated that moderately defended prey did benefit from high population densities (both absolute and relative), even under long-term predation pressure. However, the physiological state of the predator was a crucial factor in the survival of moderately defended prey. While the availability of alternative prey in general should promote the spread and maintenance of aposematism, the results indicated that the resemblance between the co-occurring defended and undefended prey may impose mortality costs on the defended model species, even in the absence of actual mimicry.