COS 138-8
The fate of western monarchs
The effects of shifting climatic conditions on populations of plants and animals have received increasing attention in the past decade. These studies have often focused on populations that are located in single locations, and little is known about how climate change affects migratory species that exist across broad regions encompassing heterogenous local climates. Monarchs, Danaus plexippus, have two primary populations in North America that migrate across large geographic ranges; eastern populations migrate south to Mexico and western populations migrate to California. Recent research has cast doubt on the population stability of the eastern monarchs, but the western monarchs have not been as intensively examined. Using a Bayesian hierarchical model, sightings of the western monarchs over 38 years were investigated using records from 10 sites along an altitudinal transect in Northern California. Multiple weather variables were examined, including local and regional temperature and precipitation. Finally, population data from western overwintering sites and eastern sites were examined.
Results/Conclusions
Monarch sightings across the western sites show a clear negative trend, with substantially fewer sightings per year in recent years (e.g. across all sites the regression coefficient for years is -0.59). Warmer winter, spring, and fall temperature had positive effects on western monarchs, while warmer summer temperatures had a negative association. While both positive and negative effects of climatic trends on the monarchs are observed, we find considerable unexplained variance associated with declining sightings (presumably attributable to non-climatic drivers not quantified in our study). Population fluctuations in our focal sites are positively correlated with fluctuations in western overwintering sites, but not strongly correlated with eastern dynamics.