Abstract
Predators are known to affect prey populations through both consumptive
and non-consumptive effects, and the magnitude of prey responses has long
been assumed to depend on predator lethality. 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; e.g., 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 maenus 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
behavioural risk responses to determine predator lethality, and second
estimated prey's risk responses after rendering the predators
non-lethal. 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 the 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.