Using functional responses to quantify interaction effects among predators

1. Predator diversity alterations have been observed in most ecosystems as a result of the loss and/or addition of species. This has implications for predator-prey dynamics as non-trophic interactions among predators, so-called multiple predator effects (MPE), are known to influence predation success. In addition, there is often a density-dependent relationship between prey availability and prey consumption (functional response). While MPE investigations are common in the literature, functional responses have rarely been incorporated into this field of predation ecology. 2. Here, we outline an experimental procedure that incorporates functional responses into multiple predator effect studies. Using three fish species with different functional traits as model predators (bluegill Lepomis macrochirus, southern mouthbrooder Pseudocrenilabrus philander and banded tilapia Tilapia sparrmanii), we assess intra-and interspecific predator interaction outcomes on predator-prey dynamics. This was done by contrasting observed functional responses of heterospecific and conspecific combinations of predators with expected responses based on those of individual predators. 3. Multipredator combinations produced variable results. Bluegill were the only species in which observed conspecific multipredator functional responses matched those of expected based on individual performance (prey risk neutral effects). In contrast, prey risk reduction was observed for both mouthbrooder and tilapia conspecific multipredator trials. Heterospecific combinations revealed strong prey risk reduction effects for mouthbrooder-tilapia and bluegill-tilapia trials, while mouthbrooder-bluegill multipredator functional responses combined additively. These results are discussed within the context of behavioural traits of the species and the development of a trait-based predictive framework. 4. Using a functional response approach allowed for the assessment of multiple predator effects across a range of prey densities. We propose that the incorporation of within-guild predator combinations into classic functional response investigations will enhance predictive capacity development in competition and predation ecology.