Climate change, especially increasing global temperatures, will have untold impacts on the long-term dynamics of populations. However, research often focuses on species in isolation and does not always consider how climate change will alter species interactions and, subsequently, the long-term dynamics of the species involved. Population projection models that account for changes in both abiotic and biotic interactions can provide a fruitful window into assessing how climate change will directly and indirectly affect population dynamics. The importance of changes in biotic interactions in determining population dynamics will be most prevalent when examining tightly linked interactions between species, such as the interaction of the monarch butterfly (Danaus plexippus) and its parasite, OE (Ophryocystis elektroscirrha). Yet, the impacts of climate change on wildlife parasites, and thus their interactions with their hosts, is by large, empirically untested. To this end, we asked how does elevated temperature and parasite infection affect monarch population dynamics and overall fitness. In both the field and lab, the development and survival of OE-infected and OE-uninfected monarch butterflies, reared in either a warmed or ambient environment, was used to build population projection models to assess changes in monarch fitness due to differing abiotic and biotic conditions.
Results/Conclusions
Warmer temperatures decrease monarch population growth, which is further exacerbated by parasite infection. Sensitivity and elasticity of matrix elements differed depending upon the treatment. For OE-infected butterflies, regardless of temperature, populations were most sensitive to changes in eclosion from pupae to an adult; whereas, for uninfected monarchs, late-instar larval transitions were particularly sensitive in warming treatments. Survivorship as a fifth instar larvae and pupae are the most elastic elements regardless of the abiotic or biotic conditions. In general, monarch survivorship decreased when reared in warmer temperatures (regardless of infection status), but differences in fitness metrics (e.g., development time, weight, melanism, and size across growth stages) between ambient and warm conditions were seen only in OE-infected monarchs. Thus, OE-infected monarchs reared at increased temperatures are lighter in color and have smaller overall dorsal areas, which can affect individual fitness due to changes in fecundity and/or migration success. Ultimately, the combination of climate-change induced increases in temperature with parasite infection pose a serious threat to monarchs. While monarchs display phenotypic responses to increased temperatures, this may not be enough to overcome the one-two punch combination of increased temperatures and infection.