Roles of hostplant availability and quality for the distribution and climate change response of a dietary specialist herbivore

Species distributions are recognized to be driven by abiotic factors, but the importance of biotic interactions that provide critical resources is less well understood, especially with respect to variation in critical resource quality. Disentangling the relative importance of these factors − abiotic environment, presence of critical resources and their quality − may be critical to predicting species response to climate change. We used species distribution models (SDMs) to address these questions for the western monarch butterfly (Danaus plexippus), a species that obligately feeds upon plants in the genus Asclepias, and for which hostplant quality in this region varies among species by an order of magnitude. We modeled the distribution of 24 Asclepias species to develop and compare three monarch distribution models with increasing levels of ecological complexity: (i) a null model using only environmental factors (a climate envelope model), (ii) a model using environmental factors and Asclepias spp. distribution, (iii) and a model using environmental factors and Asclepias spp. distribution weighted by hostplant quality assessed through a greenhouse bioassays of larval performance. Asclepias models predicted that half of the Asclepias spp. will both expand their ranges and shift their distribution towards higher latitudes while half will contract within the study region. Our performance analysis of monarch models revealed that the climate envelope model was the poorest performing. Adding hostplant distribution produced the best performing model, while accounting for hostplant quality did not improved model performance. The climate envelope model estimated more restrictive contemporary and future monarch ranges compared to both hostplants models. Although all three models predicted future monarch range expansions, the projected future distributions varied among models. The climate envelope model predicted range expansions along the Pacific coast and contractions inland while hostplants models predicted range expansions in both of these regions and, as a result, estimated 14 and19% increases in distribution relative to the climate envelope model, respectively. These results suggest that information on biotic interactions that provide critical resources is needed to predict future species distributions, but that variation in the quality of those critical resources may be of secondary importance.