Multiple Predator Effects: Predicting Combined Functional Response of Stream Fish and Invertebrate Predators

The effects of different types of predators may combine in complex ways to impact prey populations. In a series of microcosm experiments I empirically derived the functional response curves for two common types of stream predators, sculpins (Cottus bairdi) and stonefly larvae (Agnetina capitata), on two behaviorally and morphologically distinct types of mayfly prey (Baetis tricaudatus and Ephemerella subvaria). Data from these separate trials were combined using simple additive models to generate predicted combined functional responses for both types of predator. For both types of prey, actual combined functional response curves (both types of predators together in a treatment) differed significantly from predicted combined functional response for some ranges of prey densities. When Baetis was used as prey, significant departures from the predicted values occurred at moderate and high prey densities and were in a negative direction (interference between predators). When Ephemerella was the prey, significant departures occurred at low and moderate densities and were in a positive direction (facilitation between predators). Additive models for predicting combined predator effects cannot account for non—linearities in combined functional response introduced by non—trophic (behavioral) interactions that occur between fish and stoneflies, and between these predators and their prey. Other aquatic and terrestrial food webs contain similar linkages between vertebrate and invertebrate predators and their prey. Inclusion of behavioral interaction terms seems likely to be a necessary part of any general predicting combined effects of predators on prey populations in these systems.