Synthesizing drivers of fish functional responses across species

Fishes are subject to numerous stressors, including climate change, fishing and impacts by alien species. One of the challenges in understanding species and community responses to these stressors is identifying how they modify predator–prey interactions, a key process shaping aquatic food webs. Here, we aim to synthesize how species traits, such as size, activity level and alien status, and environmental factors, such as water temperature, shape the functional response: the change in predator consumption rate in relation to changes in prey density. We compiled over 300 fish functional responses and examined sources of variation in two key parameters dictating its shape: handling times and space clearance rates. We found that compared to cold waters, warmer waters were characterized by decreased handling times and increased space clearance rates for smaller predators, but had an opposite effect for bigger predators, suggesting that, across species, altered predation rates may underlie the decrease in size at higher temperatures (the temperature-size rule). We also found that the negative effect of increased temperature on the functional response of larger predators is more pronounced in active species. Finally, we found that known alien species do not exhibit different functional response parameters when examined on their native prey, suggesting that alien species are not primed for invasion via their high functional response. Together, these asymmetric changes imply that, across species, warmer waters may alter predator–prey relationships differentially according to predator size, prey size and activity levels.