Each year, billions of animals migrate long distances to track seasonal resources. In some cases, these migrations promote healthy populations by reducing levels of infectious disease. This occurs if infected animals are weeded out of the population during strenuous journeys or if migration allows animals to escape contaminated habitats for part of the year. In recent decades, some animals have stopped migrating, instead forming sedentary populations that breed year-round and leading to greater opportunities for harmful pathogens to take hold of populations. For example, all monarch butterflies in eastern North America once migrated to Mexico every fall, but some monarchs now remain year-round in the southern U.S., where they breed on exotic species of milkweed. These sedentary (non-migratory) monarchs face greater infection risk from a tiny parasite. This project examines whether these sedentary monarchs pose risks for migratory monarchs by acting as sources of infection or by causing more of them to abandon migration.
This research is important because changes in animal migration can alter infectious disease levels in wildlife and may influence the transmission of pathogens between wildlife and humans. As diverse species around the globe undergo dramatic changes in migrations, understanding the ecological consequences of these changes will be valuable for wildlife and human health. This project will engage the public to help investigate this problem. Since 2006, hundreds of volunteers across North America have sampled monarchs for parasite infection through the citizen science program Monarch Health. With the support of this grant, Monarch Health will recruit new volunteers to conduct capture-mark-recapture surveys and will produce an online video to highlight how citizen scientists enhance ecological knowledge. Finally, this project has the potential to contribute to monarch conservation by elucidating how the planting of exotic milkweeds affects monarchs.
Specifically, this research has two goals: (1) To determine whether sedentary populations of monarchs somehow cause migrating individuals -- particularly infected ones -- to stop migrating, and (2) to determine whether sedentary populations act as sources of infection for migratory populations. Earlier work showed that sedentary monarch populations in the southern U.S. suffer extremely high prevalence of protozoan infection. To accomplish the first goal, wild monarchs will be collected in the southern U.S. during fall migration and chemically analyzed to distinguish migratory from sedentary individuals. These data will be used to test whether infected migrants are more likely than healthy migrants to stop migrating. To accomplish the second goal, field sampling of monarchs will be done in the spring to distinguish sedentary monarchs from migratory monarchs re-colonizing the U.S. from their Mexican wintering grounds, and to quantify the extent of spatial overlap between these groups. Infections in larval monarchs at these same sites will be monitored to evaluate the potential for sedentary individuals to transmit pathogens to migratory individuals.
