COS 81-3 - Predation and parasitism of dietary specialist and generalist caterpillars in fragmented forests

Thursday, August 15, 2019: 8:40 AM
L016, Kentucky International Convention Center
Michael S. Singer and Riley Anderson, Biology Department, Wesleyan University, Middletown, CT
Background/Question/Methods Plant-herbivore-predator interactions occur in increasingly fragmented landscapes, which have the potential to change the dynamics of tri-trophic interactions. Trophic island biogeography theory posits that reduced habitat patch size renders its strongest negative population effects on species at higher trophic position and habitat specialization. Consequently, the local loss or decline of such species in a community is expected to weaken species interactions such as predation and parasitism. Furthermore, the enemy-free space hypothesis, which proposes that dietary generalist herbivores have limited abilities to use their host plants for anti-predator defense relative to dietary specialists, predicts that generalist predators will have stronger effects on dietary generalist herbivores relative to dietary specialists in the same community. Previous work has not integrated these theoretical frameworks by testing their predictions in a single experimental paradigm. To do this, we took advantage of existing fragmented forest patches of various sizes (3 – 1013 ha) to determine if smaller forest patches exhibit reduced predation by birds (top predators) and ants (omnivores) and reduced parasitism by parasitoids (habitat specialists) on caterpillar species (herbivores) of varying diet breadth. We used a predator-exclusion experiment replicated across two tree species, two field seasons, and 16 sites to measure the effects of bird and ant predation on dietary specialist and generalist caterpillars in relation to forest patch size. Caterpillars from this experiment were collected for rearing to determine mortality from parasitoids.

Results/Conclusions Top-down effects by birds, ants, and parasitoids were generally consistent with the enemy-free space hypothesis and trophic island biogeography theory. Bird predation was stronger on dietary generalist than specialist caterpillars, and the strength of bird predation did not change with forest patch size. Ant predation was weaker overall, stronger on dietary generalist caterpillars, and unaffected by forest patch size. The stronger effects of bird and ant predation on dietary generalist caterpillars, which were consistent for both tree species, support the enemy-free space hypothesis. By contrast with birds and ants, the probability of mortality from parasitoids was higher for dietary specialist caterpillars, and decreased in small forest patches for specialist caterpillars but not for generalist caterpillars. The contrast between landscape effects on parasitoids versus generalist predators supports the prediction from trophic island biogeography theory that tri-trophic interactions involving ecologically specialized predators are most vulnerable to disruption by landscape fragmentation.