What’s Really Happening to the Monarchs?


Assistant Professor of Biology Leslie Ries’s research challenges the current understanding of the decline of Monarch butterflies. Photo by Kuna Malik Hamad.

September 29, 2015—Popular media will tell you that Monarch butterflies are disappearing. But Assistant Professor of Biology Leslie Ries recently found that decades of data show a more complex story.

Trained as a field ecologist, Ries is interested in understanding conservation and large-scale landscape issues. “Butterflies turn out to be a great model system for understanding different areas of biology,” she explained. Ries calls Monarchs an “indicator species.” Monarchs respond quickly to changes in climate and landscape. “So you can look at how they are responding as a barometer for how other communities might respond,” she continued.

Monarch butterfly populations live all over the world, but recent research has highlighted the decline of the Monarchs that migrate to Mexico each winter. These Monarchs have a multigenerational migration cycle. Each spring, the first generation of butterflies lays eggs in the southern part of the United States. The next generation spreads out over the northern half of the United States and Canada. At the end of the summer, the last generation stops breeding and migrates to a few mountaintops in central Mexico. “As far as I know, there’s no other species that ends up in such a small, concentrated overwintering ground,” Ries explained.

Researchers have seen a decline in the winter populations in Mexico, but there has been little research on the butterflies during the summer months. To fill this gap, Ries searched for the best data source to understand the “large-scale patterns” of these butterflies. Ries could have used a traditional research route, collecting her own data for two to three years and then providing an analysis of those data. But limiting data collection to a few years would have meant limiting the scale of her research questions. So she turned to decades of data collected by citizen-science programs.  

A natural affinity for butterflies, and Monarchs in particular, has led individuals to identify, collect, and study butterflies with programs such as the North American Butterfly Association (NABA). Bird researchers have drawn on citizen-science data from similar organizations, like the Audubon Society, for years. “I knew there were a lot of citizen-science programs focused on butterflies, but I hadn’t seen a lot of research out of them,” Ries said. “I found it was a really underused data source. We don’t think there’s any other species that’s so meticulously monitored.”

Ries started by helping citizen-science programs transfer data to useable formats for researchers. She often found data scribbled in notebooks and stashed away in boxes. But with these data she was able to conduct the first large-scale analysis of Monarch data from the United States and Canada.

If Monarch populations were truly disappearing, Ries thought, the populations should also be declining in the United States and Canada. But that’s not what Ries found. “When you look at the summer data, you don’t see the declines, which is not what you would expect,” she said. This research led her to two possible conclusions: “the population in the summering grounds are declining and we just aren’t detecting it or they are not making it down to Mexico,” she explained.

According to Ries, past research has produced the dominant story that modern agricultural practices are killing milkweed—the Monarchs’ main source of food—leading to a decline in Monarch populations. Ries believes that explanation may be too simple, although she encourages butterfly enthusiasts to plant milkweed. “Conservation decisions are being made on this simple story rather than the complex one. We don’t even know the complex story.”

In her own work, Ries focuses on understanding community responses through particular traits. She looks at population responses, species interactions, and landscape changes. Monarchs need milkweed for food, but they also need nectar plants. Global climate change, Ries says, has caused changes in the timing of certain plants, which may not align with the migratory cycle of Monarchs. By studying which traits lead to a species doing well or poorly, Ries hopes to learn “how to buffer the impacts of these global changes.”

In her next research projects, Ries plans to explore what is really happening to the Monarch populations. Citizen-science programs, Ries says, help her by providing valuable scientific data that she wouldn’t be able to collect on her own. “With the Monarch butterfly, on a very large scale, we are able to look at mechanisms and dynamics that we would never be able to look at for other species.”

—Elizabeth Wilson

Related Information

Ries’s recent research, The Disconnect Between Summer and Winter Monarch Trends for the Eastern Migratory Population, was recently published in the Annals of the Entomological Society of America.

Read more about Ries’s research in the Washington Post and Science Magazine.