Most consumer species exhibit saturating (Type II) functional responses. Saturation occurs because of the time involved in handling and processing prey, a biological reality for the majority of heterotrophs. The resulting time delay generates positive feedback in species' per capita growth rates, leading to oscillations that can cause extinction unless counteracted by negative feedback processes such as self-limitation. Consumer fitness is maximized when the attack rate is high and the handling time is short, but resource species' fitness is maximized when the attack rate is low and the handling time is long. Data from real species show only a small percentage of interactions exhibiting these combinations. Most consumer-interactions are characterized by relatively low attack rates and short handling times. I hypothesize that the conflict of interest between resource and consumer species combined with the propensity for oscillations driven by long handling times drives the evolution of functional responses. I test this hypothesis using a mathematical framework that combines quantitative genetics of trait evolution with dynamical models of consumer-resource interactions.
Results/Conclusions
I find that stochastic extinction during periods of low abundances constrains the combination of attack rates and handling times to a distribution that closely mirrors empirical data. This suggests that functional response evolution must occur under the constraint imposed by oscillatory ecological dynamics. The eco-evolutionary model confirms this finding. In the absence of ecological dynamics, attack rates higher, and handling times longer, than those observed in the data can evolve given sufficient genetic variation. In the presence of oscillatory dynamics, consumer-resource interactions in which individuals exhibit such combinations go extinct during periods of low abundances, and attack rate-handling time combinations that evolve in interactions that persist closely resemble the distribution observed in the data. The important implication is that ecological dynamics provide both a selective force as well as a demographic constraint for the evolution of consumer functional responses, highlighting the importance of considering both selection and constraints in the study of phenotypic evolution.