The influence of predator lethality on prey behavior and growth

Predators are known to impact prey populations through both consumptive and nonconsumptive effects, and the magnitude of prey responses has long been assumed to depend on the lethality of the predator. Yet, there are few, if any studies, that have empirically tested this assumption. While there are studies that have approximated lethality and provide support for this assumption, difficulties arise given there is a need to estimate encounter rates and subsequent predation events while simultaneously separating prey risk responses from such events to truly determine predator lethality. Without the separation of risk responses from predation events, predator lethality may be erroneously assigned; for example, a highly lethal predator may elicit strong and early antipredator responses from prey, reducing actual capture and consumption, making the predators 'seem' less lethal than they actually are. We used a marine crab-snail system consisting of Carcinus maenas or Hemigrapsus sanguineus (crab predators) with Nucella lapillus (snail prey), pairing large and small crabs with large and small snails in a two-phase mesocosm experiment. We first eliminated prey's behavioral risk responses to determine predator lethality and second estimated prey's risk responses after rendering the predators nonlethal. We found that the specific lethality of a predator for a given prey did not drive prey risk responses. Our most lethal treatments (large crabs paired with small snails) had similar risk responses to our least lethal treatments (small crabs paired with large snails). The greatest risk responses came from treatments with large crabs paired with large snails (only medium lethality). Thus, while our results do not support the assumption that lethality itself drives prey risk responses, we did find that overall snails responded the most to the largest predators. This is likely due to the ability of these predators to consume prey over a wide size range and prey's inability to recognize their own relative size. Furthermore, prey state (body size) is clearly an equal if not greater contributor to prey risk responses than a predator's ability to kill.