Associate Professor Oregon State University Corvallis, Oregon, United States
Abstract: The attributes of predator-prey interactions that enable coexistence, inhibit population cycles, intensify trophic control, and promote resilient responses to environmental perturbations have undergone intense scrutiny. The strength and nonlinear form of predator functional responses — how fast predators feed and how quickly they become saturated as the density of prey increases — have long been considered of critical importance. Intuition for their effects comes, in part, from the tendency of consumer-resource models to show destabilization when attack rates and handling times are increased; changes in either can increase feeding rates and bring predator individuals closer to saturation in manipulative functional-response experiments, implying that predator populations in which a high proportion of individuals are busy handling captured prey exert greater trophic control but are less able to limit destabilizing prey growth from enrichment. Here, I show how this intuition fails. By recasting the canonical Rosenzweig-MacArthur “paradox of enrichment” model in terms of the proportion of feeding predators, I derive simple analytical expressions which show that (i) population cycles ensue when the proportion of feeding predators falls below a threshold level rather than when saturation is high, (ii) trophic control increases as the proportion of feeding predators decreases (iii) more saturated predator populations are at increased risk of deterministic extinction, and (iv) coexistence and dynamical resilience to environmental perturbations is maximized at intermediate levels of saturation. I elucidate these insights with modern consumer-resource theory, relate them to the recently-published inference that most predators studied in isolated laboratory-based functional-response experiments are far from being saturated by prey in nature, and report on preliminary data on the empirical frequency distribution of saturation obtained from a global literature compilation of field-based predator diet surveys.
