Predator-free space, functional responses and biological invasions

Predator-prey interactions are mediated by the structural complexity of habitats, but disentangling the many facets of structure that contribute to this mediation remains elusive. In a world replete with altered landscapes and biological invasions, determining how structure mediates the interactions between predators and novel prey will contribute to our understanding of invasions and predator-prey dynamics in general. Here, using simplified experimental arenas, we manipulate predator-free space, whilst holding surface area and volume constant, to quantify the effects on predator-prey interactions between two resident gammarid predators and an invasive prey, the Ponto-Caspian corophiid Chelicorophium curvispinum. Systematically increasing predator-free space alters the functional responses (the relationship between prey density and consumption rate) of the amphipod predators by reducing attack rates and lengthening handling times. Crucially, functional response shape also changes subtly from destabilizing Type II towards stabilizing Type III, such that small increases in predator-free space to result in significant reductions in prey consumption at low prey densities. Habitats with superficially similar structural complexity can have considerably divergent consequences for prey population stability in general and, particularly, for invasive prey establishing at low densities in novel habitats.