PS 22-51 - CANCELLED - The importance of shade in breeding habitat for migratory western monarchs

Abstract: The migratory population of western monarchs has rapidly declined over the last several decades, with the overwintering population dropping to as low as < 1% of its historic numbers at one point in 2020. Western monarchs differ from eastern population in that they overwinter in coastal California and migrate throughout neighboring states, breeding west of the Rockies. Research on monarch breeding habitat has primarily been focused on availability of milkweed. There is minimal research on other aspects of monarch habitat that are important to population growth and survival. The goal of this study was to gain knowledge on the structural elements of habitat that are important to the migratory population of western monarchs. Our work was primarily on Department of Defense (DoD) lands in the west because DoD is keenly interested in learning mechanisms to manage habitat for species-of-concern while balancing installations needs for use of their lands. Our work aims to identify efficient and effective methods to maximize monarch breeding performance on DoD lands in the west. We surveyed for immature monarchs on milkweeds in shaded and exposed habitats on DoD and nearby land in Southern California, Northern California, Nevada, Idaho, Oregon, and Washington. Monthly throughout the summer breeding season, we recorded the number of milkweed stems in different shade categories and the number of monarch egg, larva, and pupa on each stem. We also tracked hourly temperature in a subset of plots. We found that western monarchs prefer shaded habitat when it is available. We observed a higher density of immatures per milkweed stem in shady areas, but the relative density depends also on region and time of year. We also found that shaded sites stayed cooler during the day and warmer at night compared to sites in full sun, meaning less temperature variability in each 24 hour cycle. Proximity to shade likely offers refuge for immatures in late summer, as drought and extreme heat can be detrimental to monarch survival and cause decreases in abundance. Our findings suggest that recovery efforts that increase the availability of shade habitat may be more effective for increasing monarch productivity in regions experiencing extreme summer heat. This work will inform conservation actions and next steps for management of western monarch breeding habitat across the West.