Exploring current and future distributions of arid-adapted Asclepias spp. in the western U.S

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Natalie Melkonoff and Kevin R. Hultine, Research, Conservation and Collections, Desert Botanical Garden, Phoenix, AZ, Natalie Melkonoff and Kathleen Prudic, School of Natural Resources and the Environment, University of Arizona, Tucson, AZ

Background/Question/Methods
The American West is becoming inexorably more arid. Winter snows are lighter, and the spring melt much earlier. River flows are slowing, in some cases going entirely underground, while droughts are longer, and catastrophic wildfires are more frequent and intense. This aridity has profound impacts on many aspects of the biosphere, many of them negative. Yet some species and their ecological relationships may benefit from these new climatic circumstances. Here we examine the impact of a drying and warming climate on arid-adapted host plants and their specialist insect herbivore. We integrate data from three community science repositories and employ MaxEnt modeling coupled with ENMeval and blockCV packages in R to explicitly outline tuning and to incorporate a spatially independent evaluation design to minimize overfitting. We estimate a series of temporally explicit species distribution models to generate range predictions of five predominant arid-adapted host plant species (Asclepias angustifolia, A. asperula, A. erosa, A. linaria, and A. subulata) and for a highly dispersive specialist insect herbivore (Danaus plexippus) able to quickly respond to changes in host plant distributions.
Results/Conclusions
We find some arid-adapted milkweeds show a positive relationship with a changing climate expanding their range. The distributional changes in these host plants were complex and nuanced inviting further research and exploration. These model estimates span numerous habitat types and environmental factors that can be used to draw further conclusions about where additional populations might occur as well as where habitat conservation and restoration efforts should be directed as the climate continues to aridify. The specialist insect herbivore, the monarch butterfly, easily tracked the composite distribution of all milkweed species with a proclivity towards 1-2 specific milkweed species. These combined findings have the potential to improve conservation and restoration efforts of the monarch butterfly and its larval habitat across the American West by identifying potential host plants responding positively to aridification.