Playing it safe; risk-induced trait responses increase survival in the face of predation.

1. Predation risk effects are impacts on prey caused by predators that do not include consumption. These can include changes in prey behavior, physiology, and morphology (i.e., risk-induced trait responses), which can have consequences to individual fitness and population dynamics (i.e., non-consumptive effects). While these risk-induced trait responses can lower individual fitness as compared to prey not exposed to risk, they are assumed to increase fitness in the presence of predators. While much work has been built upon this assumption, most evidence occurs in consumptive experiments where the trait values of consumed prey are unknown. We have little evidence showing individuals with a greater magnitude of risk-induced trait response have greater survival. 2. Here, we tested the hypothesis that risk-induced trait responses increase survival in the presence of predators, but come at a cost to growth. We tested this hypothesis using Nucella lapillus as prey and Carcinus maenas as a predator, and including mussels as a basal resource in a 2-phase mesocosm experimental set-up. In phase 1, Nucella were placed into either a control or risk treatment (exposure to non-lethal Carcinus) for 28 days and their behaviour and growth measured. In phase 2, a lethal Carcinus was added to all mesocosms (non-lethal crabs were removed), and survival was recorded for 15 days. 3. At the treatment (group) level, we found that Nucella exposed to predation risk in phase 1 had significantly greater risk aversion behaviour (summed score of risky vs. safe behaviour) and significantly lower growth. In phase 2, we found that Nucella exposed to predation risk had greater survival. 4. At the individual level (regardless of treatment), we found that Nucella with greater risk aversion scores in phase 1 had significantly higher survival in phase 2 when exposed to a lethal predator, but this came at a cost to their growth. 5. This study provides some of the first empirical evidence, at both the group and individual level, testing a long-held assumption that predation risk-induced behavioural responses increase survival in the face of direct predation, but that these responses come at a cost to the prey. These results add to our growing understanding of the benefits of risk-induced trait responses to individual fitness and to non-consumptive effects generally.