Effects of habitat complexity on intraguild predation and cannibalism in an assemblage of size-structured predators

Intraguild predation (IGP) and cannibalism substantially affect community dynamics but often are poorly documented in food web studies. Feeding choices of generalist predators often relate more to size of prey than to species identity, leading to IGP and cannibalism. Size-structure of predator populations and physical habitat complexity further complicate predator-prey interactions. Densely-vegetated habitats provide refuge for prey and diminish predator hunting success. This experimental research examined effects of habitat complexity and population size-structure by intraguild larval aquatic beetles (Cybister fimbriolatus) and larval dragonflies (Anax junius), common predators in fishless ponds. In mesocosms, predation for all pairwise combinations of large and small top predator, C. fimbriolatus, and large and small intermediate predator, A. junius, was measured. Predation in replicate trials of these six predator/size combinations was measured at high and low habitat complexity. High habitat complexity decreased predator success. In some cases, the size of competing predators influenced a predator’s success. In a choice test, predator preferences to consume either conspecifics (cannibalism) or heterospecifics (IGP) were also tested. C. fimbriolatus consistently consumed a conspecific before a heterospecific, while A. junius consumed a heterospecific before a conspecific. Preferential cannibalism by the top predator C. fimbriolatus, coupled with low cannibalism by an intermediate predator, A. junius, could help explain sustained coexistence of these predators.