The Fallacies of "ratio-dependent" Predation1

It has recently been claimed that the functional response of predators depends on the numbers of prey and predators only via their ratio. Contrary to this claim, I argue that such "ratio-dependent" functional responses are very unlikely to occur on theoretical grounds and have not been shown empirically. Previous justifications for ratio-dependent functional responses ignore the role of these responses in determining prey population dynamics. Patterns in the abundances of trophic levels and experimental results that have been cited as support for "ratio-dependent" predation are consistent with numerous other explanations. These other explanations do not suffer from the pathological behaviors and lack of a plausible mechanism that afflict ratio-dependent models. Furthermore, ratiodependent functional responses, because of their lack of a mechanistic basis, do not provide a good basis for developing more detailed models. Nor can the use of ratio-dependent models be justified as a limiting case of more general predator-dependent models.