Community organization of herbivores on milkweed

Understanding the relative importance of factors that generate community structure and allow for the coexistence of species is central to ecological research. Insect herbivores have been a model for such research because of their high diversity and the relative ease with which they can be manipulated. Despite tremendous historical influence and the heated debate that ensued, systematic studies of the relative importance of multiple factors that control herbivore diversity and coexistence are rare. Does strong predation pressure limit competition between herbivores? Modern analyses cannot be limited to predation and direct competition between herbivores, but must take advantage of the many proposed, and yet untested indirect mechanisms of coexistence. This research project, currently funded by a five year grant from NSF, is an integrated program to examine the mechanisms promoting coexistence of insect herbivores on milkweed, and combines approaches from ecological genetics, chemical ecology, and metapopulation biology. Postdoctoral fellow, Kailen Mooney, is currently working on various aspects of the project.

Most theory predicts that trade-offs in fitness enhancing traits are necessary for species to coexist in communities. We are evaluating multiple, non-exclusive hypotheses for trade-offs that maintain low populations of competitively dominant insects in the community of milkweed herbivores (ten species in five feeding guilds): high susceptibility to predation, low colonization ability, variable preference and performance on plant genotypes that vary in defense, and specificity in induced plant responses (and effects on herbivores). Though many of the herbivore species utilize different parts of the plant (from the roots, to the stems, leaves, phloem sap, and seed pods), we are particularly focusing on the three aphids. Among these species, Aphis nerii appears to have the "toxic" and aposomatic strategy, Aphis asclepiadis has the "mutualism" strategy as it is usually tented by ants in the field, and Myzocallis asclepiadis has the "fugitive" strategy as it is the only species of the three to always have winged adults.

We have also previously demonstrated that milkweed plants exhibit high levels of intraspecific genetic variation and phenotypic plasticity in six defensive and nutritional traits, suggesting that the resource base for herbivorous insect communities is highly dynamic. This variation is predicted to be a key factor in the maintenance of diversity in herbivore communities. The explicit hypothesis is that trade-offs in the exploitation ability of herbivores on different plant phenotypes (generated by genetic variation or plasticity) allow variation in plant populations to maintain herbivore coexistence. Ultimately, an assessment of trade-offs promoting coexistence will be evaluated on several axes.