A longstanding debate concerns how predator functional responses are best described. Theory, for example, 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 observational approach to simultaneously estimate the prey-specific attack rates and predator-specific interference rates of predators interacting with arbitrary numbers of prey and predator species.
We apply the approach to surveys and manipulative field experiments involving two intertidal whelks and their full suite of potential prey. Our study provides strong evidence for the presence of weak predator dependence that is poorly described by the ratio dependent model over both natural and manipulated ranges of species abundances. Our study also indicates how, for generalist predators, even the qualitative nature of mutual predator effects can be prey-specific.