The eastern population of monarch butterflies (Danaus plexippus) is famous for its annual multi-generational round-trip migration from Mexico, through the U.S.A., to Canada. The monarch population has declined over the last two decades and suggested drivers of the decline of monarchs include habitat loss at the overwintering site, increased mortality during migration, and the decline of host plants in the summer breeding sites. However, the relative contribution of each of these drivers is contentious, and a more thorough understanding of monarch population dynamics is needed to confirm the relative importance of the various drivers. One set of tools that are used to better understand monarch population dynamics are endogenous biochemical markers, such as isotopes. Isotope compositions vary spatiotemporally with biogeochemical processes and give a natural “isotopic fingerprint” to different locations. These isotopic fingerprints are transmitted to biological tissues and, in the case of migrating monarchs, can be used to assess natal origin. Currently, hydrogen (δ2H) and carbon isotopes are used for monarch provenance studies, however, these isotope systems result in a low spatial resolution prediction and, therefore, their use is limited for ecological studies.
Results/Conclusions
Here, we combined the novel use of strontium isotope ratios (87Sr/86Sr) with frequently-used δ2H values to increase the precision of monarch natal origin assignments. We constructed a bioavailable 87Sr/86Sr isoscape, or map of isotope ratios, for the breeding area of the monarch butterfly eastern population using an empirical training dataset of plant 87Sr/86Sr ratios (including 137 new ratios) and spatial explanatory variables using random forest regression. Next, the natal origin of 35 monarch butterflies were predicted using the 87Sr/86Sr isoscape alone, a δ2H isoscape alone, and both isoscapes together. We found that the 87Sr/86Sr isoscape results in a more precise geographic assignment of natal origin than the δ2H isoscape, but that combining the two isotopic systems results in a better assignment than either technique alone. The addition of 87Sr/86Sr, combined with existing isotopic tracers, to the toolbox of ecologists allows us to provide high spatiotemporal resolution reconstruction of monarch population dynamics, as the assignment of 35 monarch butterflies shows. This tool may allow ecologists to finally disentangle the multiple drivers contributing to monarch decline and result in a more efficient and effective allocation of conservation resources.