Combined Effects of Predation Risk and Food Reward on Patch Selection

• Published in: Ecology (Durham) Vol. 69; no. 1; pp. 125 - 134
• Format: Journal Article
• Language: English
• Published: Ecological Society of America 01.02.1988
• ISSN: 0012-9658; 1939-9170
• DOI: 10.2307/1943167

Experiments were performed to examine the effects of predation risk on selection of food patches by a mobile forager, young black surfperch (Embiotoca jacksoni). The goals were to investigate (1) how predation risk affected relative selectivity among patches that differed in density of food, and (2) whether the forager's response depended on how variations in food level and predation risk were encountered. In the experiments, density of food (crustaceans on algae) was manipulated among spatially uniform patches of a foliose alga. Safety of the forager was varied by manipulating the presence in patches of kelp bass (Paralabrax clathratus), a principal predator of young surfperch. Patch choice was determined for an array of situations where food and risk were varied separately and together in space and time. The response of foragers to variations in food level and risk depended upon the specific manner in which the factors were encountered. When patches with a high food level contained a predator, foragers shunned the risky food—rich areas for safe food—poor patches. When food was equal between safe and risky patches, the degree to which patches with a predator were avoided remained constant over a range of absolute food densities. By constrast, selectivity between patches of differet food densities was dampened when all available patches contained a predator. Finally, predators enhanced relative selectivity for food—rich areas when foragers had simultaneous access to the complete combination of variation in rish and food level. Heightened selectivity in the presence of predators resulted from two hierarchical responses: foragers always took less risk to feed in the food—poor patch, and, when the difference in comparable value of available food patches was sufficiently great, they took more risk to eat where the density of food was higher. Overall, these results indicate that juvenile black surfperch possess an array of responses to food and predators, which together may act to minimize the sublethal effect of predators on feeding at a minimal increase in probability of death. Such plasticity may be a general characteristic of foragers that experience variation in food and risk at several spatial scales and temporal combinations.