PS 52-71
Herbivory and the common milkweed: A study of the effects of herbivory on plant fitness
The common milkweed, Asclepias syriaca, is a widespread North American perennial known for exhibiting a myriad of plant defensive traits. These traits include the production of cardenolides, chemical toxins known to compromise membrane function, as well as physical defensive attributes such as latex and trichomes. Despite these traits, A. syriaca is commonly attacked by several specialist herbivores that cause significant tissue damage and nutrient loss: aphids, leaf-miner flies, and stem-boring weevils. Given the nature of these ecological interactions, the purpose of this study is to assess how herbivores impact the fitness of the common milkweed and, consequently, the evolutionary trajectories of these species. Through a natural field experiment and an observational study, plant height and fruit number, proxies for plant fitness, were measured as a function of plant genotype and the presence or absence of herbivores. For the experiment, forty plants from four different genotypes were randomly assigned treatments (with or without herbivores), allocated positions in an eight by five grid, and response variables were measured once a week for the duration of the study. These same variables were measured and insect surveys taken twice a week on 180 observational plants.
Results/Conclusions
Both change in height and change in fruit number varied among genotypes. This genotypic variation was exhibited in the presence and in the absence of herbivores. Plants suffered a greater loss of fruit when in the presence of aphids (25.3%) and stem-boring weevils (44.9%) than when these herbivores were not present. Leaf-miner flies had no effect on change in fruit number. Additionally and conversely, milkweed plants exhibited a greater positive change in height (27.5%) when aphids were present compared to when they were not. Stem-boring weevil and leaf-miner fly presence did not correlate with change in height.
The notion that herbivores can influence the evolution of plant communities is predicated upon the assumption that they can cause differential fitness impacts within these communities. This study has shown that at least two herbivores, aphids and stem-boring weevils, are capable of acting as selective agents on the common milkweed. Although these results are not comprehensive, they indicate that these intricately linked ecological interactions have the potential to shape evolutionary trajectories. Given how rapidly ecological communities are changing due to global change, a clear understanding of the mechanisms by which adaptive change is fostered is essential.