Disturbance-mediated consumer assemblages determine fish community structure and moderate top-down influences through bottom-up constraints

Disturbance is a strong structuring force that can influence the strength of species interactions at all trophic levels, but controls on the contributions to community structure of top-down and bottom-up processes across such gradients remain poorly understood. Changes in the composition of predator and consumer assemblages, and their associated traits, across gradients of environmental harshness (e.g. flooding), are likely to be a particularly important influence on the strength of top-down control and may drive bottom-up constraints. We examined how consumers with particular traits, and the predators that consumed them, varied across a gradient of stream flooding disturbance and used experiments to assess the predation impact on those contrasting consumer communities (ultimately quantifying how flood disturbance altered the strength of top-down control). Consumer community composition and mobility were strongly related to flood disturbance; the biomass and drift of protected primary consumers (i.e. those with morphological defences) decreased with increasing flood disturbance. Predatory fish species had different disturbance niches, and path analysis identified that both direct flood disturbance effects and indirect bottom-up constraints of flood disturbance on consumers influenced predatory fish composition and biomass. Fish generally fed most effectively on consumer types associated with their particular niche, but all fish were strongly size-selective when feeding on protected consumers. Although protected consumers did not grow large enough to escape predation, an in situ experiment showed protected consumers were at a reduced risk of predation as disturbance increased compared to unprotected consumers. Overall, top-down control declined with flood disturbance, but the effect depended on consumer traits. Predatory fish were only capable of exerting top-down control on protected consumers in benign habitats but impacted unprotected consumers across a larger range of the disturbance gradient. Collectively our findings suggest that a shift towards a more disturbed state will probably result in reduced predator impacts and a weakening of top-down control. Moreover, predicted increases in the frequency and intensity of climatic events causing disturbance, such as flooding, are likely to result in a community shift that disproportionately impacts protected consumers and the predators that utilize them as prey through the subsequent bottom-up constraints.