COS 90-2 - Monarch butterfly survival in the Anthropocene: how variation in abiotic conditions impacts monarch performance from a tritrophic perspective

Background/Question/Methods

The western monarch butterfly (Danaus plexippus) population has suffered a ~95% decline in the last 40 years. This precipitous drop is driven by a combination of changing climatic conditions, increased pesticide-use, and habitat loss. Despite the crisis condition of monarchs in the arid West, few studies have examined the impact of global change on monarch survival through a tritrophic lens. Monarch developmental timing and predator communities vary with climate and by food-plant species (within the genus Asclepias). To investigate how water availability and ambient temperatures affect survival dynamics, we used four common gardens of two of the most widespread species of milkweed (A. fascicularis and A. speciosa) across a 580-km environmental gradient in the West. Specifically, we asked: 1) how does monarch survival differ along this gradient?; and 2) how is survival related to changes in the abundance and diversity of predators along this same gradient? To answer these questions, second-instar monarch larvae were assigned to factorial combinations of the two milkweed species and either predator exclusion cages or mock controls. Larvae were monitored daily until they reached the fifth instar, at which point all larvae were enclosed to complete development. Predator communities were assessed visually and with pitfall traps.

Results/Conclusions

Survival varied among common garden environments, with the highest rates of survival occurring in eastern Nevada, at a relatively wet, cool site. Predation had a strong impact on larval success: un-caged monarch larvae were less likely to survive than caged larvae at all sites. The experiment spanned a period of extremely poor air quality owing to a series of western wildfires. Perhaps due to these conditions, even caged larvae suffered reduced survival compared to that in previous survival studies. Larvae reared in the driest site, also characterized by sandy soil, took the longest to develop and emerged as smaller adults. Predator communities varied in composition by garden. Across all gardens, larvae on A. speciosa were more likely to survive but gained less weight. Larvae reared on A. speciosa may thus be less suited to return to overwintering grounds in California in the fall. Taken together, these findings suggest that monarch performance will likely decline under global climate change. Moreover, monarch-predator interactions depend on local climate, yet are also influenced by milkweed species. Climatic considerations should influence the choice of milkweed species and locations to target for monarch habitat restoration across the West.