Range-Wide Variation in Common Milkweed Traits and its Effect on Larvae of the Monarch Butterfly

Plants play an important role in structuring ecological communities from the bottom up through interactions with herbivores, and environmental variation can affect these interactions. We use the interaction between common milkweed (Asclepias syriaca) and the monarch butterfly (Danaus plexippus) to examine 1) the role of environmental variation in dictating plants traits, and 2) how those variations affect herbivores. We quantified intraspecific trait variation in 53 natural common milkweed populations, then remeasured these traits when population representatives were regrown in a common garden to control for environmental variation. We then measured growth, performance, and survival of monarch larvae feeding on these same plants. Our findings indicate distinct spatial patterns in traits throughout the range of A. syriaca, but these patterns dissipate when genets are regrown in a common environment. When monarch larvae are raised on these milkweeds, those fed on plants from the Northeast gain more weight than those fed on plants from the Northcentral and Southcentral regions. These results can better inform monarch conservation efforts; current conservation efforts have been focused on milkweed restoration in the Midwest, but an increased focus on milkweed restoration in the Northeast may be beneficial. Furthermore, we demonstrated plasticity in specific plant traits in response to environmental change, which could have theoretical implications in light of current and projected changes in climate. *The most recent version of this work is published in The American Journal of Botany in March 2021 (v. 108, iss. 3). Doi: https://doi.org/10.1002/ajb2.163