Identifying priority survey sites for early-season milkweed conservation

Monarch butterfly (Danaus plexippus) populations are experiencing decline due to habitat degradation and climate threats. In 2024, this iconic species became protected under the Endangered Species Act. Critical to the persistence of this species of cultural and ecological importance is milkweed (Asclepias spp.), which monarchs rely on as an essential resource for food and reproduction. Motivated by concerns over the loss of monarch habitat, the Santa Barbara Botanic Garden (SBBG) and local collaborators are working to identify potential restoration areas in the Los Padres National Forest (LPNF). However, the vast size and complex terrain of the LPNF pose a logistical challenge to surveying. Therefore, there is a need for a tool to aid in selecting sites to prioritize. Our team created a priority index for surveying milkweed within the LPNF by predicting the habitat suitability for four early-season milkweed species using Maximum Entropy (MaxEnt) species distribution modeling and creating a novel survey site accessibility index. We identified high priority sites for the team to survey based on high predicted suitability for each species of early-season milkweed and high physical accessibility. To communicate these results, we created an interactive web dashboard, which the SBBG team will use for field planning to support the ongoing monitoring of milkweed populations for monarch habitat restoration efforts within the LPNF.

Methods
Our team collected data from publicly available data sources between 2024-01-10 and 2024-03-22. More details on these sources can be found in our README.md. All data cleaning, processing, and development of modeling and indices for our project have been recorded in notebooks and repositories in our team's GitHub organization, which is also linked as "Related Works" to this archival. We developed a species distribution model to predict habitat suitability for four species of early-season milkweed (Asclepias californica, Asclepias eriocarpa, Asclepias erosa, and Asclepias vestita) in the southern region of the Los Padres National Forest (LPNF), using publicly available environmental data, along with milkweed survey data collected in 2023 by the Santa Barbara Botanic Garden (SBBG). We also created a novel survey site accessibility index to assess how physically accessible locations in the LPNF are for the SBBG team. Since the forest has a vast extent (1.75 million acres) and the garden's survey team has limited resources, we used the data in this archive to identify high-priority locations for the team to visit in upcoming field surveys by determining locations that were both highly suitable for each species of milkweed and relatively physically accessible for the SBBG team.