Quantifying predator dependence in the functional response of generalist predators

Abstract

A long-standing debate concerns how functional responses are best described. Theory suggests that ratio dependence is consistent with many food web patterns left unexplained by the simplest prey-dependent models. However, for logistical reasons, ratio dependence and predator dependence more generally have seen infrequent empirical evaluation and then only so in specialist predators, which are rare in nature. Here we develop an approach to simultaneously estimate the prey-specific attack rates and predator-specific interference (facilitation) rates of predators interacting with arbitrary numbers of prey and predator species in the field. We apply the approach to surveys and experiments involving two intertidal whelks and their full suite of potential prey. Our study provides strong evidence for predator dependence that is poorly described by the ratio dependent model over manipulated and natural ranges of species abundances. It also indicates how, for generalist predators, even the qualitative nature of predator dependence can be prey-specific.

Consistent temperature dependence of functional response parameters and their use in predicting population abundance

“…There are some indications that laboratory experiments may grossly overestimate field‐based feeding rates, although explicit tests are still scarce (Aljetlawi, Sparrevik, & Leonardsson, ; Wilhelm, Schindler, & McNaught, ). In situ functional response experiments are difficult to carry out and, consequently, they are rare (e.g., Jost, Devulder, Vucetich, Peterson, & Arditi, ; Barrios‐O'Neill, Dick, Ricciardi, MacIsaac, & Emmerson, ; Novak, Wolf, Coblentz, & Shepard, ), so data are often combined from a range of locations with the assumption of spatial and temporal consistency (Angerbjorn, Tannerfeldt, & Erlinge, ). Comparative field and laboratory studies are needed to test consistency across contexts (laboratory vs. field) or time (within vs. between years), but the few studies done on this topic are still confounded with the spatial scale of the research (O'Neil, ; Wang & Ferro, ; Xia, Rabbinge, & Werf, ).…”

What drives interaction strengths in complex food webs? A test with feeding rates of a generalist stream predator

“…This observation has fueled debate over whether and how functional responses should incorporate predator density, but few studies have assessed its importance in the field (Novak et al. ). Our results provide some support for the presence of predator interference in our study system.…”

“…For instance, manipulations that extend the range of predator densities beyond those that we observed in our surveys would be informative for assessing the utility of ratio‐ vs. more general predator‐dependent models for describing the feeding rates of generalist predators, and for assessing whether the constancy of per capita terms assumed in these models is in fact appropriate (Novak et al. ).…”

“…), the size of the arena used for feeding trials (Uiterwaal and DeLong ), or unrealistic community structure (e.g., low prey richness, Novak et al. ) have potential to strongly influence predator feeding rates. Additionally, scaling laws based on temperature and body mass have been applied to infer interaction strengths in food webs, despite sometimes equivocal support for the generality of their predictions (Wootton and Emmerson , Rall et al.…”

“…Our approach represents a generalization of that described in Novak et al. from predators that feed on a single prey item per observable feeding event to predators that feed on multiple prey items per observable feeding event (i.e., predator stomachs containing multiple prey items; see also Woodward et al. , Novak and Wootton and Wolf et al.…”