Adaptive behavior and interference in the functional response of predator to prey

Abstract

The behavior of individuals in predator-prey interactions plays a fundamental role in population ecology and evolution. Accordingly, the development and assessment of theory that provides a conceptual framework for understanding and predicting behavior in predator-prey systems is of basic importance. This dissertation is focused on developing and assessing theory that provides a quantitative structure for predator-prey interactions.Chapter 1 provides a theoretical treatment of the problem of modeling the behavior of an animal facing two- and three-way tradeoffs among growth, mortality and reproduction, three components of Darwinian fitness. In this situation benefits in terms of one fitness component can be obtained only with a cost in terms of another fitness component. Applying a dynamic optimization method, the theory describes the behavior of phenotypes that adaptively balance growth, mortality and reproduction in a size-structured population. One simple prediction of the theory is that non-growing adults in a stable population should behave so as to minimize the ratio of mortality rate to birth rate. A further extension of the theory using the idea of ideal-free distributions results in a prediction of a positive and linear relationship among the rates of growth, mortality and birth when measured across different habitat patches. These and other results lead to testable predictions and I discuss empirical examples of how subsets of the theory have been and can be assessed. Chapter 2 is an empirical assessment of four alternative models of the functional response of predator to prey (the per capita feeding rate of a predator) using 19 data sets from the literature. The results show that predator-dependent functional responses (i.e., forms that are functions of predator and prey abundance because of predator interference) provide better descriptions of 18 of the 19 data sets than the Holling Type II model, a commonly used prey-dependent functional response (i.e., a model that depends only on prey abundance). Hence some form of predator interference is common in these data. However, no single functional response can best describe all of the data sets. A key result is that the best-fitting predator-dependent model depends on the presence or absence of predator-dependence when prey are very abundant. Accordingly, I suggest use of the Beddington-DeAngelis or Hassell-Varley model when predator feeding rate becomes independent of predator density at high prey density, and use of the Crowley-Martin model when predator feeding rate is decreased by higher predator density even when prey density is high. These results suggest that predator-dependent functional responses should be more widely considered in the literature. Chapter 3 is an empirical assessment of four alternative optimization models of behavior for animals facing a feeding-mortality tradeoff (i.e., increases in feeding rate can be obtained only at a cost of increasing mortality rate by predation). Using lab experiments, I measured the feeding and mortality of stream fish (bluehead chubs) in the presence of varying levels of food availability and green sunfish predators. The models vary in their specifications as to how bluehead chubs might value, in terms of fitness, rates of growth and mortality. Two models that emphasize the importance of growth or mortality alone fit the data poorly, whereas the two models that balance growth and mortality adaptively provide a much better description of the data because bluehead chubs continue to feed, albeit at a reduced level, in the presence of the predator-maintained mortality hazard. A comparison of the two best-fitting models leads to the inference that reproductive value in the bluehead chub should be thought of as a function of body size and age rather than a function of body size alone. A key implication of this result is that risk-taking behavior in these fish can be quantitatively estimated using a parameter that incorporates the benefits of growth and the costs mortality into a single fitness-based metric. The metric, called the marginal rate of substitution of mortality rate for growth rate, provides a link between behavior and life history in the bluehead chub. In summary, this dissertation provides some solutions to the challenge of how to conceptualize and predict animal behavior in predator-prey settings. The emphasis is on developing and assessing alternative hypotheses in a quantitative framework.