Background/Question/Methods
Climate change is altering environmental conditions globally, which impact ecosystem structure and function. Existing theory in the realm of biogeography provides a lens through which we may examine how organisms exist across, and adapt to, varying environmental conditions. According to conventional wisdom, herbivory pressure should be greatest at the equator and should decrease as latitude increases; plants should therefore exhibit similar patterns in defensive traits – an idea which has been widely debated. This study attempts to elucidate the environmental drivers underlying patterns of trait variation in common milkweed (Asclepias syriaca) and to link key plant traits to monarch butterfly (Danaus plexippus) growth and performance. We sampled 53 populations of common milkweed across its entire native range. We measured foliar functional traits and looked for patterns of trait variation that could be explained by latitude, longitude, or environmental variables. We performed a common garden experiment with field collected specimens to determine what role environment plays in driving biogeographic patterns in foliar functional traits. We performed a bioassay in order to compare monarch larvae growth and performance to the traits of the plants they were fed.
Results/Conclusions
We found that in natural populations of common milkweed, key plant traits (i.e. leaf mass per area, carbon, cellulose, nitrogen, defensive compounds, and leaf water content) exhibit patterns of variation along latitudinal or longitudinal gradients. While we did find greater herbivory pressure at lower latitudes, we found that defensive compounds had a pattern of increasing toxicity at higher latitudes – a finding that runs opposite to popular theory but which falls in line with findings that some secondary metabolites (e.g. phenolics) increase with latitude. Many of these patterns in trait values along latitude and longitude can be explained by variation in environmental variables (like temperature and precipitation). Furthermore, we found that milkweed plants are capable of rapidly (in less than 1 year) adjusting their traits to adapt to a novel environment, which could indicate that milkweed may readily adapt to any environmental changes that occur as a result of climate change. Finally, we found that some key plant traits (most notably nitrogen) correspond with increased monarch growth while others (leaf water content) correspond with decreased monarch growth. We expect that milkweed with higher nitrogen and lower leaf water content should result in greater growth of monarch larvae and potentially more successful adult migrants.