Jenna L. Braun, Chris Lortie

Aim: Climate change is changing species distributions globally, but predicting these impacts on assemblages and their spatial overlaps under future scenarios is an ongoing challenge. Here, we explore how climate change influences distributions among two mutualistic assemblages.

Location: The Mojave and Colorado Deserts, California, United States

Methods: We developed stacked species distribution models for the community of extrafloral nectar (EFN)-bearing plants and their mutualistic ant community and projected these models under two future climate scenarios. To assess the vulnerability of this mutualism due to spatial mismatches, we examined potential shifts in geographic overlap between the EFN-bearing plants and ant species under both scenarios. We analyzed the bioclimatic factors influencing species richness and distribution in both the plant and ant communities, as well as their responses to future climate change. We also tested whether environmental breadth and phylogeny could predict the responses of ants to climate change. Lastly, we evaluated the significance of the EFN community on ant species distributions by determining whether the inclusion of EFN plants in ant distribution models enhances their predictive accuracy.

Results: The species richness of both the EFN-bearing plant communities and ant communities decreased under both predicted climate change scenarios. The geographic overlap between EFN plants and ants significantly decreased under both future scenarios. The response of different ant species to climate change varied based on their environmental generalization but not their evolutionary relationships. Including the EFN plant community as a predictor in the species distribution models for ants improved their predictive performance.

Main conclusions: The EFN plant community is an important driver of their ant mutualists' geographic distribution and diversity. More environmentally generalized ant species benefit from the changing climate, whereas the EFN-bearing plants are uniformly and negatively impacted despite their environmental generalization. Despite the range increase of some ant species across the Mojave and Colorado Deserts, these mutualisms are vulnerable to climate change because of the decrease in geographic overlap between pairs of ant and EFN-bearing plant species.