Size-dependent predation alters interactions between parasites and predators

Abstract

Increasing evidence indicates that parasites play an important role within many systems as prey for higher trophic levels. Predation on parasites can decrease their numbers and may affect host infection rates. Cercariae, a free-living infectious stage of trematode parasites, are abundant in freshwater systems and are directly consumed by a number of freshwater predators. However, few studies have tested whether predators exhibit a preference for cercariae when alternative prey are available or how these preferences vary across predator body sizes. We assessed whether dragonfly larvae (dot-tailed whiteface, Leucorrhinia intacta (Hagen, 1861)), top predators in freshwater systems without fish, foraged preferentially when presented with two prey types, cercariae and zooplankton, and whether foraging preferences changed across predator body size. Body size of larval dragonfly predators was found to be significantly, and negatively, related to the fraction of cercariae in the diet. Larger bodied dragonfly larvae shifted their diet choice from cercariae to zooplankton. Changes in foraging selectivity as body size increases across a predator’s ontogeny can alter the strength of predator–prey interactions. Further investigation into size-selective foraging on parasites may provide new insights into the effects of predation on parasite abundance and transmission in natural systems.