Species distributions are driven by abiotic and biotic factors, but the importance of variation in the availability and quality of critical resources is poorly understood. Disentangling the relative importance of these factors – abiotic environment, availability of critical resources, and resource quality– will be important to modeling species current distributions and responses to projected climate change. We address these questions using species distribution models (SDMs) for the western monarch butterfly population (Danaus plexippus), whose larvae feeds exclusively on Asclepias species known for their heterogeneously distribution and variation in host-quality. We modeled the distribution of 24 Asclepias species to compare three monarch distribution models with increasing levels of complexity: (i) a null model using only environmental factors (climate envelope model), (ii) a model using environmental factors and Asclepias availability estimated as species richness, (iii) and a model using environmental factors and Asclepias’ availability weighted by hostplant quality as assessed through a greenhouse bioassay of larval performance. Asclepias models predicted that half of the Asclepias species will expand their ranges and shift towards higher latitudes, while half will contract. These patterns were uncorrelated with hostplant quality. Among the three monarch models, the climate envelope model was the poorest performing. Models accounting for hostplant availability performed best, while accounting for hostplant quality did not improve model performance. The climate envelope model estimated more restrictive contemporary and future monarch ranges compared to both hostplant 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. In contrast, the hostplant availability and quality models predicted range expansions in both of these regions and, as a result, 14 and 19% increases in distribution (respectively) relative to the climate envelope model. These models do not include other factors affecting monarch persistence. Nevertheless, our findings suggest that accounting for information on hostplant availability and response to climate change is necessary to predict future species distributions, but that variation in the quality of those critical resources may be of secondary importance.

 Milkweed (Asclepias genus) records for the western United States using R Studio from multiple open source databases using the R packages SPOCC, Ecoengine, rbison and by accessing species occurrences directly from GBIF and iNaturalist databases (GBIF 2019, iNaturalist 2019). We controlled for common issues in open-source databases such as incorrect coordinates, sampling biases, species missidentification, whenever possible. For example, we used filters that only retrieved records confirmed by experts and/or records classified as of research quality when permitted and spatial filtering to control for sampling biases. To focus on the western monarch population, we selected milkweeds records from states corresponding to this region: California, Nevada, Colorado, Washington, New Mexico, Arizona, Utah, Oregon, and Idaho. We compared milkweed records with milkweed distributions published in The Biota of North America Program (BONAP; Kartesz 2015) for record accuracy. After removing duplicate records, incorrect coordinates such as occurrences over oceans or inaccurate coordinates such as uncertainty over 1000 meters, and observations, the final databases were established included: 7,916 milkweed records for 51 species.