PS 21-33 - Got Milkweed? Information gaps for west-coast Danaus plexippus populations limits habitat conservation

Abstract: The iconic pollinator, Danaus plexippus (monarch butterfly) has the ability to connect people from across the country through the nature of their unique migration patterns and by bringing ecology to people of all and experiences, making a citizen scientist out of anyone. However, decreasing breeding habitats contribute to rapid population decline. These issues may be compounded for the western US populations, which are far less numerous, understudied, and where recent trends in wildfire activity may exacerbate habitat losses compared to eastern US populations. Though they are genetically indistinguishable, they differ phenologically in their migration and overwintering behavior. D. plexippus relies exclusively on milkweeds from the genus Asclepias for oviposition and larval consumption; thus, increased data collection and information on the composition of adequate habitat and D. plexippus migration is imperative for conservation. We performed a systematic literature review of the current research being conducted on D. plexippus and compared citizen records (via iNaturalist) of butterfly sightings and milkweed patch presence across different databases. We aimed to assess what we know about their habitat on the west vs. east coast and the role of recent fires and droughts exacerbated by climate change in the habitat availability throughout the western migration route.

In general, we found that habitat information and recent impacts of climate change on west coast D. plexippus populations is incredibly limited (two established programs for reporting individual monarch observations and one established program to report milkweed stations, not including iNaturalist reports). The more expansive literature base from east coast populations provides a general understanding of the species life cycle, habitat preference, and basic knowledge of their migration patterns. However, this information is insufficient to support efforts to track and quantify milkweed habitat or D. plexippus breeding sites for west-coast populations. Citizen science monitoring programs via platforms such as Monarch Watch, or iNaturalist provide important opportunities to improve our knowledge of west-coast D. plexippus populations through reporting of tagged D. plexippus individuals and logging of milkweed habitat observations . Although the ability to identify milkweed habitat is not complete, there is emerging evidence that the habitat is decreasing in size due to the use of herbicides and technological advancements in agriculture. Conservation of west coast D. plexippus populations requires improved identification, quantification, and development of milkweed habitat across the summer breeding areas in order to provide necessary oviposition area and larval sustenance through increased citizen science and uniform monitoring efforts.