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Climate Change contributing to Eastern Monarch decline

Michigan State University ecologists Elise Zipkin and Erin Zylstra led an international research partnership of professional and volunteer scientists to reveal new insights into what's driving the already-dwindling population of eastern monarch butterflies even lower. Erin, the study's lead author, said, "What we do is develop models to understand why monarchs are declining and what's happening to biodiversity in general. A lot of it is not good news. But in understanding the reasons why a species is declining, there is also a message of hope: there's something we can do about it," she said. "We did this study not just to say what's causing changes in the monarch butterfly population, but also learn how we can make it better."

Eastern Monarchs migrate between Mexico and the eastern half of the United States and southern Canada every year '€” with summer layovers in Michigan and other U.S. states. Since the mid-1990s, though, there has been a dramatic decline in their population, with worst-case estimates projecting that the current population is a mere 20 percent of what it was just a few decades ago.

About a decade ago, Leslie Ries of Georgetown University and Elise Zipkin, now an associate professor of integrative biology at MSU, came to a realization. Researchers and volunteers were collecting an increasing amount of data that could help make a more definitive determination of what's driving the monarch population decline.

With support from the National Science Foundation, the team analyzed data from more than 18,000 surveys of monarchs in different locations across the midwestern U.S., central Mexico and southern Canada between 1994 and 2018. Most of these surveys were performed by local volunteers who helped count adult butterflies.

"Almost all of those data were not collected by professional scientists and that is really, really cool," Elise said. "There is no group of scientists out there that could collect all the data that we needed. But these volunteers go out every year and record data in a very structured way. That's the only way we could do this analysis."

"The level of expertise among the volunteers is really incredible," Erin added. She led the effort to develop a model based on these observations and draw meaningful conclusions. In particular, the team was interested in what the data said about the three leading theories behind the eastern monarch's population decline: milkweed habitat loss, mortality during the autumn migration and resettlement on the overwintering grounds, and climate change's detrimental impact on monarch breeding success.

Looking at the problem holistically '€” across many years and multiple countries '€” makes it clear that climate change has been the dominant disruptive force since 2004. Unfortunately, there isn't enough data in agricultural regions to definitively determine what happened between the mid-1990s and the mid-2000s, the period of the most pronounced decline.

"This study gives us information on where to spend our limited dollars on restoration," Erin said. "Although we can't simply turn off climate change, we can, for example, focus on restoring milkweed in the regions that remain most conducive to monarch reproduction despite warming temperatures and shifting precipitation patterns. That said, anything we can do to curb climate change will also improve the outlook for both monarchs and humanity," she added.

And although curbing climate change is a huge lift, Elise pointed out that this study reminds us of the power of partnerships to confront large challenges.

"We're talking about three countries that this is directly affecting: the U.S., Canada and Mexico. It's not something that we have to do alone," she said. "Partnerships do matter."

Working out what's behind the population decline proved that. Between the professional scientists and volunteer data collectors, residents of all three countries made this study possible.

"You need those kinds of partnerships. You need people with different expertise. We showed that's how we can figure out what's going on. Now, what can we do with conservation?" Elise asked. "We can work together."