Predation and Risk in Foraging Minnows: Balancing Conflicting Demands

When foraging in habitat patches that simultaneously vary in food abundance and predation risk, foragers confront the conflicting demands of efficient foraging and predator avoidance. We hypothesized that foragers will balance these conflicting demands, taking proportionately greater risks when benefits are high. To test the balancing hypothesis we predicted that (1) prey would choose patches of high food abundances when all other variables are constant; (2) prey would avoid predator locations when all other variables are constant; and (3) when food and predators vary in combination, a significant statistical interaction would exist between the two effects. We used adult Semotilus atromaculatus as the predators and juvenile Rhinichthys atratulus as the prey, to test these predictions in a seminatural, artificial stream. When specific locations in the stream were varied in all possible combinations of food level (high, low) and predators (present, absent) we found that the prey responded positively to high food locations (prediction 1) and negatively to those containing predators (prediction 2). However, prey did not take proportionately greater risks when the benefits were high (prediction 3). As an alternative to balancing we propose the patch choice model as a working hypothesis that relates net energy intake to time spent in patches (patches vary in food level and predator risk as above) where predators, if present, reduce time spent foraging by their prey. In accord with our data, this model predicts that the relative benefit of high food patches is independent of the presence of predators.