The effects of predation risk on prey and emergent indirect effects on basal resources are shaped by how prey balance conflicting demands for food and safety. By limiting their foraging activity, prey can reduce their vulnerability to predators, but this comes at the cost of reduced energy intake and growth. Because of these associated costs, the decision to engage in anti-predator behaviors should be sensitive to the degree of predation risk prey perceive. For example, prey may respond more strongly to a predator that is actively feeding on conspecific prey. However, the ability of prey to detect/evaluate predator risk cues and fine-tune their responses relative to perceived threat may depend on how the prey and predator have coevolved. On rocky shores in New England, intertidal snails have a variety of predators, including native Cancer spp. crabs (C. borealis and C. irroratus) and the invasive green crab, Carcinus maenas. Using a mesocosm experiment, I examined how risk cues from each of these three predatory crabs affected the foraging and growth rates of their prey, the carnivorous snail Nucella lapillus, and if these effects depended on whether the crab was actively feeding on conspecific snails.
Results/Conclusions
The presence of a predatory crab actively feeding on conspecific prey caused the foraging, growth, and growth efficiency of N. lapillus to decline regardless of crab identity, but the presence of crushed conspecific snails alone had no effect. When crabs were not actively feeding on N. lapillus, the negative effects of both Cancer borealis and C. irroratus declined, but the negative effects of Carcinus maenas persisted regardless of its feeding habits. These results suggest that N. lapillus may exhibit more finely-tuned responses to Cancer spp., which are native, than to Carcinus maenas, which is invasive. However, there are several alternative explanations for these results, including differences in each crab’s habitat overlap with N. lapillus, their relative predation rates on N. lapillus, and the historical ecology of these predator-prey interactions in the North Atlantic. Despite these alternative explanations, this study highlights the context-dependent nature of prey responses to predation risk and the resulting indirect effects on ecological communities.