Why is the eastern monarch butterfly disappearing?

Environmentalists at Michigan State University have led an international research partnership of professional scientists and volunteers to reveal new information about what is further driving down the already declining population of Eastern Monarchs.

Between 2004 and 2018, climate change in the monarch’s spring and summer breeding grounds had the greatest impact on this declining population. In fact, the effects of climate change have been nearly seven times greater than other factors, such as habitat loss. The team published its report on July 19 in the journal Ecology and evolution of nature.

“What we are doing is developing models to understand why monarchs are in decline and what is happening to biodiversity in general,” said Erin Zylstra, lead author of the study. Zylstra is a postdoctoral fellow in the Department of Integrative Biology and the Ecology, Evolution and Behavior Program, both of the MSU College of Natural Sciences.

“This isn’t good news for a lot. But understanding the reasons why a species is in decline, there is also a message of hope: there is something we can do about it,” Zylstra said. . “We did this study not only to tell what is causing changes in the monarch butterfly population, but also to learn how we can improve it.”

Understanding the monarch’s decline and doing what we can to reverse it is important not only for preserving biodiversity, but also because insects are prolific pollinators. The eastern population of monarchs migrates annually between Mexico and the eastern half of the United States and southern Canada, with summer stops in Michigan and other US states. Since the mid-1990s, however, there has been a dramatic decline in their population, with worst estimates projecting the current population to be just 20% of what it was just a few decades ago.

The mid-1990s to mid-2000s saw the most dramatic decline, coinciding with a period when glyphosate-based weedkillers became extremely popular in the agricultural industry. Farmers have grown crops designed to resist glyphosate, which allows them to apply the chemical on a large scale, decimating the milkweed plants that are the sole host and food source for the monarch caterpillars.

The dominant theory during this period was that the loss of milkweed from agricultural areas was responsible for the severe declines. Since then, monarch populations have steadily declined. Although the loss of milkweed caused by glyphosate remained a possible explanation, other theories have emerged over time. Today, researchers are divided over what is delaying the monarch’s population.

About a decade ago, however, Georgetown University’s Leslie Ries and Elise Zipkin, now an associate professor of integrative biology at MSU, became aware. Researchers and volunteers were collecting an increasing amount of data that could help determine more definitively what is causing the decline in the monarch population.

“People have different assumptions,” said Zipkin, lead author of the new study and director of the Ecology, Evolution and Behavior program. “So we tried to come as an impartial team, take the time and put all of these pieces together to really analyze the contributions of various stressors.”

Part of what makes the decline so difficult to understand is the Eastern Monarch’s complicated life cycle. These monarchs spend their winters, from November to February, in central Mexico. When the weather starts to warm up, they move north to the southeastern United States, especially eastern Texas.

Once there, the adults reproduce, lay eggs and then die. It is the next generation that continues the migration, starting around May, flying to the Midwest and parts of Canada, where it produces two to three additional generations. Butterflies that develop in late August shut down their reproductive systems and expend their energy migrating south to Mexico, where the cycle begins again.

With support from the National Science Foundation, the team analyzed data from more than 18,000 monarch surveys in locations across the west-central United States, central Mexico, and southern Canada between 1994 and 2018. Most of these readings were done by local volunteers who helped count the adults. butterflies.

“Almost all of this data wasn’t collected by professional scientists and it’s really, really cool,” Zipkin said. “There is no group of scientists that could collect all the data that we needed. But these volunteers come out every year and record the data in a very structured way. That’s the only way we could do that analysis.”

“The level of expertise among the volunteers is truly amazing,” said Zylstra.

Zylstra led the effort to develop a model based on these observations and draw meaningful conclusions. In particular, the team looked at what the data said about the three main theories behind the Eastern Monarch’s population decline: loss of milkweed habitat, mortality during fall migration, and resettlement on wintering grounds, and the adverse impact of climate change on the monarch’s reproductive success.

“I think everyone is partially right. All of those things play a role. With monarchs, everything is nuanced and everything is delicate,” Zylstra said. “But in recent years, while glyphosate applications have remained more stable, although still very high, there is strong evidence that population changes are due to climate on the spring and summer breeding grounds.”

Either of these assumptions can contribute to butterfly loss on a smaller scale, Zylstra explained. But if we look at the problem holistically, over many years and across countries, it is clear that climate change has been the dominant disruptive force since 2004. Unfortunately, there is not enough data in the regions. to determine with certainty what happened between the mid-1990s and the mid-2000s, the period of the sharpest decline.

To get a full picture of the population decline, the team needed to understand the dynamics of many generations in many places. Hence the need for thousands and thousands of inquiries. The Herculean effort to collect and understand this data also reaped two great rewards.

First, by proving the model’s potential to unravel population dynamics for something as complicated as the Eastern Monarch, the team is optimistic they can adapt the model to understand what is driving population changes in d. other species as well.

Second, this understanding should help determine where conservation efforts can bring the most benefit to the number of Eastern Monarchs.

“This study gives us information on where to spend our limited restaurant dollars,” Zylstra said.

While we can’t just turn off climate change, we can, for example, focus on restoring milkweed in areas that remain most conducive to monarch breeding despite warming temperatures and changing conditions. rainfall patterns, she said. That said, anything we can do to curb climate change will also improve the outlook for monarchs and humanity, she added.

And while the fight against climate change is a huge step forward, Zipkin said this study reminds us of the power of partnerships to tackle big challenges.

“We’re talking about three countries that this directly affects: the United States, Canada, and Mexico. It’s not something we have to do alone,” Zipkin said. “Partnerships matter. “

Finding out what lies behind the population decline has proven it. Between professional scientists and volunteer data collectors, residents of the three countries made this study possible.

“You need those kinds of partnerships. You need people with different expertise. We have shown that this is how we can understand what is going on. Now what can we do with conservation? Zipkin asked. “We can work together.”