Size-dependent foraging efficiency, cannibalism and zooplankton community structure.

To examine size-dependent food web interactions in systems with cannibalism, we compared the abundances of zooplankton and phytoplankton over 2 years in four lakes with cannibalistic perch (Perca fluviatilis) of which two also supported the top predator pike (Esox lucius). The abundance of perch 2 years and older was lower in lakes with pike than in lakes with only perch. In contrast, the abundance of small perch (young-of-the-year and 1-year old) was lower in lakes with only perch suggesting that intense cannibalism reduced these size classes to low levels in lakes lacking pike. Functional response experiments with differently sized perch and zooplankton showed that the attack rate of small perch susceptible to cannibalism was much higher than that of large cannibalising perch. The optimal body size of perch with respect to attack rate was also lower for small zooplankton prey than for large zooplankton. The zooplankton communities in lakes with only perch were dominated by the relatively small species Ceriodaphnia quadrangula and Bosmina spp. and total zooplankton biomass was higher in these lakes than in lakes with both pike and perch. In contrast, the mean size of cladoceran zooplankton was largest in lakes with both pike and perch owing to a dominance of the large zooplankton species Holopedium gibberum in these lakes. We relate these patterns to (1) the low foraging efficiency of large perch on small zooplankton and (2) the low abundance of small zooplanktivorous perch (due to cannibalism) in lakes with only perch. The differences in zooplankton community structure also resulted in different seasonal dynamics of phytoplankton between lakes. Cannibalism introduces a vertical heterogeneity to food webs that causes consumer-resource dynamics that are not predictable from linear food chain models.