Marginal predation: do encounter or confusion effects explain the targeting of prey group edges?

Marginal predation, also known as the edge effect, occurs when aggregations of prey are preferentially targeted on their periphery by predators and has long been established in many taxa. Two main processes have been used to explain this phenomenon, the confusion effect and the encounter rate between predators and prey group edges. However, it is unknown at what size a prey
16 group needs to be before marginal predation is detectable, and to what extent each mechanism drives the effect. We conducted two experiments using groups of virtual prey being preyed upon by three-spined sticklebacks (Gasterosteus aculeatus) to address these questions. In Experiment 1 we show that group sizes do not need to be large for the edge effect to occur, with this being detectable in groups of 16 or more. In Experiment 2 we find that encounter rate is a more likely explanation for marginal predation than the confusion effect in this system. We find that whilst confusion does affect predatory behaviours (whether or not predators make an attack) it does not affect marginal predation. Our results suggest that marginal predation is a more common phenomenon than originally thought as it also applies to relatively small groups. Similarly, as marginal predation does not need the confusion effect to occur, it may occur in a wider range of predator-prey species pairings, for example those where the predators search for prey using non-visual sensory modalities.