Diet variation among individuals appears to be common, even among apparently identical individuals. Given this supposed commonality, ecologists are seeking to understand the mechanisms driving diet variation and its potential effects on consumer-resource interactions and communities. One of the most common proposed mechanisms is the effect of intraspecific competition on the composition of prey communities. Less recognized, is that the evenness of the prey community can also drive diet variation. This can occur if individuals’ preferences change with previous experience (e.g. through learning or ingestive conditioning) or if all individuals have the same preferences but encounter prey randomly. We tested the relative role of intraspecific competition and prey evenness in driving diet variation in the whelk, Nucella ostrina, in the rocky intertidal of Oregon. To do so, we manipulated the relative number of prey per predator in cages that differed naturally in their prey community compositions. To estimate diet variation across cages we used repeated observations of individuals feeding over four months. We also estimated the effects of variation on the strength of the predator-prey interactions by comparing observed total feeding rates in cages to the expected feeding rates if all individuals had the average feeding rate.
Results/Conclusions
After accounting for biases due to relative prey abundances and probabilities of observing feeding events, variation in prey preferences showed no relationship with prey density per predator, a proxy for intraspecific competition. The ratio of abundances of the two most preferred prey of the whelk (the barnacle Balanus glandula and the mussel Mytilus trossulus), however, was associated with individual variation in diet preferences across cages. Estimates of total feeding rates differed between the case in which variation among individuals was included and the case in which all individuals were assumed to have the average feeding rate within a cage. Furthermore, the discrepancy in feeding rate estimates increased with the amount of diet variation among individuals. These results suggest that individual diet variation in this system is largely driven by the composition of the prey community, in particular its evenness, and that this variation alters the perceived strength of predator-prey interactions relative to the case in which diet variation is ignored. This suggests that regardless of the mechanism underlying diet variation, it can have important effects on our estimates of the interactions among species. These results also highlight the importance of accounting for likely biases when estimating individual diet variation.