Dara Satterfield

I am fascinated with animal migration -- “the story of life on the move." The billions of birds, fish, and other animals that migrate each year play ecological roles. These animals are pollinators, predators, prey, and transport vessels for seeds, nutrients, and parasites. For many migratory species, migration is now changing. In the face of habitat alteration and climate change, some migratory populations are shifting the timing or extent of their journeys. Other populations have stopped migrating altogether and remain year-round in the same location. Will changes in migration have ecological impacts? And in particular, will the loss of migration alter how parasites and pathogens spread, persist, and evolve?

 

To examine these questions, I am focusing on monarch butterflies and a specialist protozoan parasite Ophryocystis elektroscirrha (OE) for my doctoral research. This work involves a combination of approaches, including field monitoring, lab and field experiments, modeling, and citizen science.

 

Study system: Monarch butterflies and a protozoan parasite

Each fall in eastern North America, monarchs migrate from the northern U.S. and Canada to central Mexico, where they spend the winter clustering by the millions in high-altitude forests. Previous work showed that this long-distance migration, spanning up to 3000 km, helps to reduce protozoan OE parasites in the monarch population. Migration can reduce parasitism by weeding out infected monarchs during the strenuous migratory journey, or by allowing monarchs to temporarily leave behind parasite-contaminated milkweed when they embark on their migration.

 

The monarch migration is threatened. In the last decade, the number of migratory monarchs in Mexico has severely declined. The decline has been attributed in part to the loss of milkweed plants in the Midwest. Milkweeds (including over 100 species) are the only plants that monarchs can eat as caterpillars, and native milkweed abundance has plummeted due to more intensive herbicide use in agriculture in recent years. While there are fewer migratory monarchs in Mexico each year, there appear to be more reports of monarchs skipping migration. Small populations of monarchs have been found to breed year-round in the southern U.S. These non-migratory monarchs are surviving on a single species of milkweed, the non-native Asclepias curassavica, also called tropical milkweed. Tropical milkweed does not die back during the winter as native milkweeds do, so it can provide food for monarch caterpillars all year in parts of the southern U.S. and California.

 

For my dissertation research, I am comparing migratory populations and non-migratory populations of monarchs (in tropical milkweed patches), to ask whether the loss of monarch migration affects protozoan parasite transmission dynamics and leads to greater infection prevalence and virulence. We are also examining whether non-migratory monarchs transmit parasites to migratory monarchs. We frequently work with citizen scientists to conduct this research.

 

Collaborators: Sonia Altizer, John C. Maerz, Andrew Park, Jaap de Roode, Rich Shefferson, Mark Hunter, Lincoln Brower, Eduardo Rendón-Salinas, Andy Davis, Alexa Fritzsche, Kelly Nail, Wendy Caldwell, Karen Oberhauser, Billy McCord