Plants play an important role in structuring ecological communities; however, there is insufficient understanding of how intraspecific biogeographic variations in plant traits affect communities through plant-herbivore interactions. This research lies at the interface of biogeography, plant traits, and herbivore development, and its purpose is to elucidate the ways in which plant traits affect plant-herbivore interactions across broad latitudinal and climatic gradients. We focus on the interaction between common milkweed (Ascelpias syriaca) and the monarch butterfly (Danaus plexippus). First, we quantified the intraspecific variation in common milkweed functional, defensive, and demographic traits in milkweed from 55 populations across its entire range. We planted representatives from these sites in a common garden and remeasured these traits while controlling for environmental variation. We have used multivariate statistical techniques to quantify variation among populations. To determine how observed variation in common milkweed traits affects monarchs, we measured survival, fitness and performance of monarch larvae feeding on plants with varying traits from across the range.
Results/Conclusions
Traits differed significantly among populations (P < 0.0001) and several traits exhibited latitudinal trends. As latitude increases, plant height (R2 = 0.07, P = 0.04), foliar carbon (R2 = 0.09, P = 0.02), and herbivory intensity (R2: = 0.10, P < 0.0001) decline while cardiac glycoside concentrations increase (R2 = 0.14, P = 0.006). Principal component analyses indicate clustering of geographically near sites when spatial data is removed. Preliminary analysis indicates distinct clusters east and west of the Appalachian Mountains as well as west of the Mississippi river. Linking herbivore response to plant trait variation through latitudinal and climatic gradients may provide insight into how herbivores will be affected by plant range shifts due to a changing climate. Finally, learning more about how monarchs respond to different populations of milkweed as they make their northern migration every spring will provide valuable conservation insight for the management community.