Tritrophic Interactions at a Community Level: Effects of Host Plant Species Quality on Bird Predation of Caterpillars

Abstract

Effects of plant traits on herbivore-carnivore interactions are well documented in component communities but are not well understood at the level of large, complex communities. We report on a 2-year field experiment testing mechanisms by which variation in food quality among eight temperate forest tree species alters avian suppression of an assemblage of dietary generalist caterpillars. Plant quality and bird effects varied dramatically among tree species; high-quality plants yielded herbivores of 50% greater mass than those on low-quality plants, and bird effects ranged from near 0% to 97% reductions in caterpillar density. We also find evidence for two mechanisms linking host plant quality to bird effects. If caterpillar density was statistically controlled for, birds had relatively strong effects on the herbivores of low-quality plants, as predicted by the slow-growth/high-mortality hypothesis. At the same time, caterpillar density increased with plant quality, and bird effects were density dependent. Consequently, the net effect of birds was strongest on the herbivores of high-quality plants, a dynamic we call the high-performance/high-mortality hypothesis. Host plant quality thus changes highly generalized herbivore-carnivore interactions by two complementary but opposing mechanisms. These results highlight the interrelatedness of plant-herbivore and herbivore-carnivore interactions and thus the importance of a tritrophic perspective.

Herbivore diet breadth mediates the cascading effects of carnivores in food webs

“…To test the EFS hypothesis, we measured the effect of bird predation over 4 y on the 41 most abundant forest caterpillar species (84% of all individual caterpillars collected) in central Connecticut. These herbivores span a range in diet breadth from specialists, feeding on a single plant species, to broad generalists, feeding on multiple families of plants (29). For each of these caterpillar species, we compared caterpillar densities (individuals per square meter of foliage) on tree branches or entire saplings with netted exclosures (bird exclusion) to those on branches exposed to birds (control).…”

“…First, we considered the bitrophic possibility that herbivore proportion S might by itself determine levels of plant damage. Therefore, we analyzed the mean level of leaf damage per tree species (from bird exclusion branches only) as a function of herbivore proportion S. Host plant quality could also mediate predator effects on plants; we therefore regressed bird LRR plant on host plant quality, which was quantified as the average growth performance per tree species of the seven most abundant generalist caterpillar species in the community (29). High host plant quality was previously found to strengthen bird predation of generalist caterpillars (only) in this system (29), leading to the prediction of stronger predator effects on plant damage with increasing host plant quality.…”

Enemy-Free Space for Parasitoids

“…To provide an initial test of host plant-related EFS for parasitoids at a community level, Singer and colleagues compared the frequency of parasitism of an assemblage of dietary generalist caterpillars among tree species upon which the caterpillars are subject to a gradient in the risk of bird predation per caterpillar (Singer et al 2012). The EFS for parasitoids hypothesis predicts a negative relationship between the frequency of parasitism and the risk of bird predation.…”

Secondary metabolites in floral nectar reduce parasite infections in bumblebees

“…There is growing recognition that these compounds can mediate tritrophic interactions among plants, herbivores, and their natural enemies. For example, although secondary metabolites can reduce survivorship and reproduction, their consumption can also benefit herbivores by reducing parasitism or predation [2,3]. Similarly, plant secondary metabolite consumption slows the growth of some protozoan gut parasites of insects, including the causal agents of human diseases such as Chagas and leishmaniasis [4,5].…”