Tri-trophic interactions in complex communities: plant species effects

image002 (1)pic3Camponotus iwth caterpillar2

Insectivorous birds, parasitoids, and ants are major enemies of caterpillars in forest communities. Images show a black-capped chickadee with a caterpillar in its bill (left, photo by Christian Skorik), a parasitoid wasp cocoon near a parasitized caterpillar (center, photo by Michael S. Singer), and a carpenter ant taking a caterpillar (right, photo by Christian Skorik).

A major branch of my research program attempts to link mechanistic knowledge of tri-trophic interactions, mainly derived from evolutionary ecology studies of interacting traits of a few species conducted at small spatial and temporal scales, to larger-scale dynamics and structure of ecological communities and food webs. If the structure and dynamics of ecological communities were additive and not context-dependent, inferences from small-scale studies would easily translate into larger-scale inferences. But it is abundantly clear from numerous ecological studies that effects observed in simplified and isolated experimental communities can be altered by additional species present in natural communities as well as other ecological factors. Despite the conjoined obstacles of complexity and contingency that conspire against mechanistic understanding at this large scale, it is vitally important to conduct ecological research that informs the dynamics and structure of large-scale communities and ecosystems because this is the scale at which the predictive power of ecological theory is weakest and conservation and environmental management needs are greatest. Although there is a large body of research on biodiversity and ecosystem functioning, relatively little of it is mechanistic and multi-trophic.

This branch of my research (hereafter, “forest ecology project”) analyzes tri-trophic interactions in the context of a complex community, using the forest ecosystem Connecticut as a study system, and focuses on direct and indirect food web interactions among multiple species of forest trees, tree-feeding caterpillars, and carnivores (i.e., parasitoid wasps and flies, insectivorous birds, hunting spiders, and ants) that eat the caterpillars. As a result, the scope of this study system is immense and the complexity of species interactions is challenging to unravel mechanistically. However, this project, more than any other, captures my interest in integrating multiple levels of biological organization, distinct theoretical frameworks, and traits of organisms in their ecological context to understand the organization and dynamics of biodiversity.

One aim of the forest ecology project is comparing tri-trophic interactions among multiple plant species in the same local community. Doing this can uniquely place the known diversity of species interactions in a common context (i.e. community functioning). Deriving an understanding of tri-trophic community dynamics from the literature is difficult because of the atomized nature of study systems. Most studies look at insects associated with a single plant species or taxon. To get a more integrative community perspective, I chose a set of eight tree species that commonly co-occur in local forest communities, vary in phylogenetic relatedness, share a community of dietary generalist herbivores and carnivores, but differ in caterpillar density and other aspects of community structure. Among other things, this study system can reveal effects of tree species characteristics on their shared sets of generalist herbivores and carnivores. In series of studies that formed the basis of several graduate student thesis projects, we tested the effects of different tree species on three types of herbivore-carnivore interactions that had not previously been studied in the same community context.

Together, this body of research shows that tree species identity varies the strength of antagonistic interactions between forest caterpillars and insectivorous birds (Singer et al. 2012), parasitoids (Farkas and Singer 2013), and ants (Clark et al. in press), but through different mechanisms in each case. Our experimental exclusion of birds (Singer et al. 2012) has been the most informative aspect of this work so far because it revealed the role of host-plant food quality for caterpillars as a driver of differences in bird predation among tree species. That is, tree species that support the highest growth performance of dietary generalist caterpillars also harbor the highest densities of generalist caterpillars, in turn attracting overcompensating density-dependent predation from birds. The strength of ant predation on forest caterpillars varies among tree species and interactive effects of mutualistic sap-feeding herbivores, but not on caterpillar density at the scale of tree species or local geographic area (Clark et al. in press). Parasitoid attack of caterpillars varies among tree species, but it is unrelated to caterpillar density or the food quality of their host-plant species (Farkas and Singer 2013). Dr. Kailen Mooney and I presented a broader conceptual perspective on the topic of plant effects on herbivore-carnivore interactions in a chapter of the book, Trait-mediated Indirect Interactions, published by Cambridge University Press (Mooney and Singer 2012). Future goals include examining the chemical basis of host-plant quality for caterpillars, the induced volatile emissions of trees as cues for parasitoids, and plant mediation of interactions between sap-feeding herbivores and caterpillars.