Friend or foe: Conflicting demands and conditional risk taking by opportunistic scavengers

To optimize individual fitness, prey should balance predation threats with their own foraging demands. For prey that have additional competitive or positive interactions with their predators, however, little is known about the dynamics that regulate risk management behaviors. Our study focused on common marine gastropods (oyster drills) and one of their major predators: stone crabs. Notably, stone crabs not only prey on drills, but also serve as an ally by breeching the shells of eastern oysters. Subsequently, the foraging success of drills on oysters can be enhanced in the presence of crabs via scavenging on damaged oysters. We manipulated predator (crab) presence, resource (oyster) availability and prey (drill) hunger to explore how complex food-web interactions affect the risk management of predatory and scavenging gastropods. In mesocosm trials, we found that: (1) drills selected habitat closer to a central risk-reward patch as resource availability increased, although the presence of a stone crab generally stifled this movement toward the risk-reward patch; (2) in treatments without any live oysters provided as a foraging resource, drills were attracted by the presence of stone crabs despite the lack of an obvious incentive for this risk-enhancing behavior; (3) drills starved for 14 days prior to entering mesocosms selected habitat without regard to predation threat, while drills that were not starved responded strongly to predator presence; and (4) drills became more aggregated as resource availability decreased, predation threat increased, or starvation level decreased. These results indicate that in resource-poor environments, drills demonstrate riskier behaviors than predicted by simple predator–prey models due to additional, positive interactions with their crab predators.

► Some prey have additional positive interactions with their predators. ► Little is known about how this affects risk management behaviors. ► Our study focused on oyster drills and one of their major predators, stone crabs. ► Crabs also serve as an ally for scavenging drills by breeching oysters shells. ► In certain scenarios, drills accept greater risk due to this facilitation.