OOS 29-3
Human activities can shift animal migration behaviors and increase pathogen spread: citizen science insights from monarch butterflies
Long-distance migrations are fascinating and can have profound consequences for infectious diseases, in some cases lowering pathogen transmission by (i) allowing animals to periodically escape from contaminated habitats, and (ii) removing infected animals from the population during strenuous journeys. In response to environmental change, migrations are now shifting or disappearing for many animal species, with some migratory species now adopting more sedentary lifestyles. This talk focuses on interactions between monarchs and a specialist pathogen to ask how human impacts on migratory species can alter pathogen risk. Monarchs occur worldwide and are best known for their long-distance migrations in North America. Milder winters and human planting of exotic milkweeds have allowed a growing number of monarchs to abandon fall migrations and breed throughout the winter months in the southern U.S. Here, we collaborated with citizen scientists through the program Monarch Health to collect parasite samples from wild monarchs to address two interrelated questions: (1) Do shifts from migratory to sedentary behavior affect pathogen prevalence in local monarch populations? (2) What are the broader consequences of the loss of migration for North American monarchs?
Results/Conclusions
Analyses of multi-year citizen science data on pathogen infection showed that prevalence increased over time within a given breeding season, and further increased with measures of larval density. These findings indicate that pathogen infectious stages build up over time in the environment in a density-dependent fashion. Results from citizen science monitoring over 35 sites characterized by winter-breeding in the southern U.S. showed that sedentary monarchs are, on average, much more likely to be infected than migratory monarchs. Within these same sedentary monarchs, infection prevalence increased over time and with greater larval density. An exception to this pattern occurred in Jan 2014, coincident with a polar vortex across the southern U.S. that killed off tropical milkweeds, removing monarchs (and parasites) from most monitoring sites. More broadly, high pathogen prevalence in winter-breeding sites could increase transmission across the larger migratory population, especially if mixing between sedentary monarchs and spring migrants occurs. This hypothesis is consistent with a rise in infection levels across North American migratory monarchs observed since 2002. Given that many other animal migrations are imperiled by human activities, studies of pathogen dynamics in migratory species are urgently needed to predict future disease risks for wildlife. To this end, citizen science methods could facilitate monitoring efforts across broad geographic scales.