Prey Vulnerability and Size Selection by Chaoborus Larvae

The planktonic larva of Chaoborus trivittatus (Diptera: Chaoboridae) feeds in an opportunistic manner on all sizes of its major prey, Daphnia pulicaria. Nevertheless, medium—sized prey constitute a greater portion of the diet than would be expected on the basis of their relative abundance in the environment alone. The differential vulnerability of food items is responsible for this apparent size—selection. Vulnerability is defined here as the product of the encounter rate between predator and prey and the capture success of the predator. The rate of encounter increases as a function of prey size, whereas the capture success of the predator decreases with prey size. This leads to a vulnerability curve that is concave to the prey size axis; hence, medium—size items should be overrepresented in the diet when the predator feeds indiscriminately. A model of foraging behavior, which incorporates this concept of differential vulnerability, is used to predict the optimal breadth of diet for Chaoborus feeding on a mixture of prey sizes. Because Chaoborus is an a mbush predator, pursuit time is insignificant, and the profitability of prey size i(e i /t i ) can be approximately by the joules assimilated from eating an individual of size i divided by the handling time. The energy content of Daphnia increases as approximately the third power of body length. Handling time, defined as the time taken to swallow an item, increases exponentially as a function of prey size. The profitability curve is concave to the prey—size axis, peaking in the middle of the prey size range. However, the foraging model predicts that the larva should include all sizes of Daphnia in its diet within the range of prey densities commonly found in nature. Furthermore, an analysis of the crop contents of C. trivittatus in Lake McDonald and in laboratory feeding experiments supports the hypothesis that larvae are opportunistic feeders on Daphnia; the size composition of the diet can be explained by the relative abundance of prey sizes in the environment weighted by their relative vulnerability. Since vulnerability may be correlated with profitability for many predator—prey combinations, it is possible that many apparent examples of o ptimal behavior can be explained simply on the basis of the differential vulnerability of prey.