Disentangling the fitness payoffs and costs of risk induced trait responses

Background/Question/Methods

Predators can alter prey traits, influence demography, and ultimately drive ecosystem function. While the direct consumption of prey has been exhaustively studied in ecology, it’s only in recent decades that predation risk (i.e., nonconsumptive) effects have begun to be explored. While most work examines the cost of risk-induced trait responses, it’s generally assumed that such responses increase prey’s survival. Yet, there is a general lack of empirical evidence to support such assumptions given the difficulty in disentangling risk-induced responses, consumption, and survival. Here we tested the hypothesis that risk-induced trait responses increase survival against predators, even though they may come at a cost to prey. We tested this hypothesis using in a marine intertidal system of dog whelk snails (Nucella lapillus) and predatory green crabs (Carcinus maenas), with blue mussels (Mytilus edulis) added as a food source. To disentangle prey risk-responses from consumptive predator effects (i.e., prey survival) we first exposed dog whelks to a non-lethal (banded) green crab over 28 days and measured their risk-induced behavioral responses and growth. We then introduced a lethal crab for the following 15 days and measured snail survival. Snails were fed ad libitum throughout the experiment and were provided with a refuge tile.

Results/Conclusions

Consistent with our predictions, we found that predation risk from green crabs increased risk aversion in snails and decreased overall shell growth. Additionally, the more risk averse snails behaved, the better they survived. However, this increase in survival corresponded to a decrease in shell growth, suggesting that while the behavioral response to predation risk is adaptive, it is not without significant energetic cost. This study is one of the first to clearly show that risk-induced trait responses increase prey survival in the face of predation, but also come with consequences to their fitness.