When your mates get the munchies: Cannibalism as a consequence of flood regime

Flows of energy across ecosystem boundaries can have dramatic top-down effects on recipient communities by altering the magnitude and direction of strong biotic interactions. However, the influence of these cross-ecosystem prey subsidies is likely to be modified by the availability of in-situ consumer resources (e.g., habitat) that control the ability of the consumer to respond to the subsidy. We investigated intraguild predation, particularly cannibalism, rates of a cursorial, riparian spider across a flood disturbance gradient that altered both the magnitude of the subsidy to the spider (winged aquatic insects) and spider habitat availability (loose riverbank rocks). The distributions of these two resources across the disturbance gradient caused high-density clumps of Dolomedes at stable and disturbed sites, and evenly distributed, low-density populations at intermediately disturbed rivers. Both stable isotope analysis of spiders and a mesocosm experiment indicated that relative rates of cannibalism were markedly higher at both ends of the disturbance gradient, where small scale spider densities were highest. The effects of prey from an adjacent ecosystem on intraguild predation rates of a consumer were modified by the availability of an in-situ resource, consumer habitat. Thus, predicting the outcome of energy flows across ecosystem boundaries depends on the magnitude of the subsidy in relation to in-situ resource availability for recipient consumers.