Temporal dynamics in foraging behavior of a pelagic predator

Abstract

A paradox exists between common observations of highly complex food webs in nature and prevailing ecological theory suggesting that complex food webs are unstable. Qualities of food webs, such as dynamic trophic links, that were not previously considered in theoretical models have been suggested to stabilize complex food webs and resolve this paradox. Flexible foraging behavior can result in dynamic trophic linkages between a predator and its prey over short time scales. Using 4 years of data on diets of juvenile sockeye salmon (Oncorhynchus nerka) and the zooplankton community from Lake Washington (Seattle, Washington, USA), we determined that juvenile sockeye show dynamic foraging behavior that depends solely on the density of its preferred prey, Daphnia. Juvenile sockeye consumed only Daphnia when their density was greater than 0.4·L–1. Long-term patterns in Daphnia densities demonstrated that for the last 28 years, trophic links between sockeye and the less preferred copepods are eliminated for approximately 6 months of each year when Daphnia is above this switching density threshold. Our data provide a convincing empirical example of the activation and inactivation of trophic links occurring on short time scales as a result of dynamic foraging and help to explain the commonness of high-complexity food webs.