Prey responses to fine-scale variation in predation risk from combined predators
Organisms are embedded in ecosystems characterized by a broad array of environmental gradients. While it is well documented that organisms can express phenotypic plasticity in response to single gradients of environmental variation, our understanding of how organisms integrate information along multiple environmental gradients is limited in many systems. Using the snail Helisoma trivolvis and two predators (water bugs [Belostoma flumineum] and crayfish [Orconectes rusticus]), we explored how prey integrate information along multiple predation risk gradients (i.e. caged predators fed increasing amounts of prey biomass) that induce opposing phenotypes. First, we quantified reaction norms to assess: 1) the response threshold for each predator alone and 2) whether increased cue concentrations caused continual increases in prey defenses or whether prey defenses exhibited a response that reaches a plateau. Second, we determined how snails integrate information on the amount of predation risk from combined predators to form inducible defenses. Because H. trivolvis faces a phenotypic tradeoff between investment in shell thickness and shell width in response to each of the predators, we expected that the magnitude of phenotypic responses to one predator would decrease as the amount of predation risk associated with the other predator increased.
When exposed to predator cues from a single predator, we detected threshold responses; intermediate amounts of cue induced phenotypic responses, but higher amounts of cue induced little additional induction. For instance, the water bug response threshold for shell width was between 25 and 100 mg of consumed prey, but the magnitude of the response did not increase with higher amounts of cue (i.e. 400 mg of consumed prey). This suggests that additional increases in predator-induced traits with greater predator risk offer minimal increases in fitness or that a limit in the response magnitude was reached. When we combined gradients of cues from both predators, we found that the magnitude of response to one predator was often reduced when cues from the other predator were present. Interestingly, these effects were often detected at cue concentrations that were lower than the threshold concentration necessary to elicit a response in the single-predator treatments. These results reveal that organisms experiencing multiple environmental gradients can integrate this information to make phenotypic decisions and demonstrate the novel result that an exposure to multiple species of predators can lower the response threshold of prey.