Untangling the direct and indirect effects of climate change on monarch butterfly fitness

Background/Question/Methods

The impacts of global climate change on individual species depend upon both the direct and indirect effects of a changing climate.  This is particularly true for specialists that rely on a few specific resources to complete their life cycle.  For instance, monarch butterflies (Danaus plexippus) and other specialized migrants can be directly affected by climate change and indirectly affected through changes in resource quality. To understand the effects of climate change on interactions between specialists and their hosts, this study examines the impact of a warmer climate on the monarch butterfly in the presence of two chemically distinct host plants, a native milkweed (Asclepias incarnata) and an invasive milkweed (Asclepias curassavica). Host plants, A. incarnata or A. curassavica, were randomly placed in either warmed or control treatments. Warmed plots were passively heated by open top chambers (OTCs).  On each plant, two monarch caterpillars were placed. Monarchs were reared until pupation and brought into the lab to eclose into butterflies. Common metrics (e.g., forewing length, weight) were collected to quantify the effects on monarch fitness. Plant responses to the treatment were also measured to quantify the effects of increased temperatures and herbivory on the host.

Results/Conclusions

Both monarchs and host plants are adversely affected by a warmer environment. Butterflies reared in warmed plots experienced higher mortality and were less fecund. Furthermore, monarchs were smaller and weighed less when reared in warmed treatments. Monarchs were larger and had a greater chance of surviving on the invasive host plant, A. curassavica, when compared to the native host, A. incarnata. Both host plants produced less defenses against herbivores (i.e., latex and toxic cardenolides) in the presence of increased temperature and herbivory. The addition of a warmer environment had the greatest effect in reducing these defenses. Interestingly, previous research has shown that A. curassavica reduces monarch survival and reproduction by altering migratory tendencies; whereas, this study demonstrates that under current conditions, A. curassavica benefits monarch performance while in a warmer climate the invasive milkweed becomes a lower quality host.  These conclusions provide critical insight into how a warmer climate will directly and indirectly affect this migratory species and allow for better informed decisions regarding the conservation of monarchs. This study also adds essential knowledge on the pressing impacts climate change will have on large-scale, complex, multi-species interactions.