Disturbance across an ecosystem boundary drives cannibalism propensity in a riparian consumer

Biotic and abiotic interactions between adjacent ecosystems are common and can take the form of resource subsidies or spillover effects of disturbance. These cross-ecosystem exchanges may have significant impacts on the recipient ecosystem by altering the occurrence of strong biotic interactions, such as cannibalism. Cannibalism is ubiquitous and has the potential to be a defining feature in many food webs through impacts on population and community structure. However, there is little empirical evidence detailing how environmental gradients in adjacent ecosystems may alter cannibalism propensity of predators that live on the ecosystem boundary. We investigated cannibalism propensity of a riparian spider across a flooding gradient that altered both the magnitude of an allochthonous prey subsidy to the spider (winged aquatic insects) and spider habitat availability (loose riverbank rocks). Spider density was affected by the interaction of prey and habitat availability across the environmental gradient with small-scale spider densities highest at both stable and disturbed rivers and intermediate at others. Stable isotope analysis of spiders and a mesocosm experiment indicated that cannibalism was higher at stable and disturbed rivers. This demonstrates that an environmental gradient in one system can indirectly alter the propensity for strong biotic interactions of a consumer in an adjacent system through interactive effects of allochthonous and in situ resources.