Planning for Monarch Butterfly Conservation on Roadsides: Development of a Statewide Milkweed Species Distribution Model for Arizona

Monarch butterfly (Danaus plexippus) populations are in decline across North America. Loss of habitat as a result of land use conversion to agriculture, development and infrastructure uses as well as vegetation management activities pose key threats to the species. Arizona is an important movement corridor for monarchs during their migration period, with nearly equal numbers of butterflies tagged in Arizona being recovered in overwintering areas in Mexico and California. Arizona has a high diversity of biotic communities and is home to approximately 30 species of the monarch’s sole host plant, milkweed (Asclepias spp.). In a joint project, the Arizona Department of Transportation (ADOT) and the Arizona Game and Fish Department (AZGFD) developed a statewide model of the likelihood of the presence of 7 species of milkweed with input from the Arizona Monarch Collaborative research committee. The model was further refined to quantify areas within the ADOT highway system that may be suitable habitat for monarchs and identify priority areas for enacting habitat conservation measures. ADOT is using the results to inform their application to the U.S. Fish and Wildlife Service Nationwide Candidate Conservation Agreement with Assurances (CCAA) for Monarch Butterfly on Energy and Transportation Lands. Participation in the CCAA is a proactive way to address regulatory uncertainty and promote species conservation through adjustments of ADOT’s operation and maintenance practices to create a net benefit for monarchs.

Maximum Entropy (Maxent) models were developed and run at a 90m resolution across the state of Arizona for seven milkweed species. These species were selected both based on their use as larval hosts and occurrence data availability. The occurrence records were cleaned both spatially and temporally to a 90m resolution. The ENMEval package for R statistical environment was utilized to create bias files for each species specific model, as well as to determine the appropriate parameter settings for each Maxent run. Model results were then displayed using ESRI ArcGIS Pro, and used to identify potential habitat along the ADOT highway system, ranked by high, medium, low, or no suitability. High, medium, and low suitability areas were displayed using the Jenks natural breaks classification. Areas of no suitability, such as pavement, urban areas, and open water were subtracted from the original raster output and not included in final calculations. The raster values were then extracted to the legal right of way, and acreage was calculated for each suitability type. These model outputs will not only identify areas with high suitability for milkweed, but will also quantify areas with medium rankings that could be enhanced by altering maintenance practices or actively adding milkweed and/or nectar plants. The model design also allows for future refinement based on field truthing results and additional data inputs.