DOI: 10.7280/d1sq5k
title: Milkweeds (Asclepias genus) records in the western U.S.

description: 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.
description:  Milkweed (<em>Asclepias</em> 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.