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Issues Of The Environment: Monarch Butterfly Population Declines Because Of Human Actions

Erin Zylstra
Erin Zylstra

For many, Monarch butterflies symbolize some of nature's beauty. Yet, you may have noticed fewer of them around in recent years. Erin Zylstra is a postdoctoral researcher in Michigan State University's department of integrative biology. She joined WEMU's David Fair to discuss the human impact that has put the monarch on a path toward extinction. 


  • A 2020 meta-analysis found that, globally, terrestrial insects appear to be declining at a rate of about 9% per decade. Population declines for certain species are even steeper. Overall eastern monarchs have declined by more than 80% over the past two decades. The 2021 count on the wintering grounds in Mexicowas down 26% over 2020, despite much of the continent being shut down by the COVID-19 pandemic.

  •  A team of researchers from Michigan State University published a study in 2021 that zeroed in on climate change as the most significant reason monarchs are declining. They analyzed data from more than 18,000 surveys of monarchs (most collected by local volunteers) in different locations across the midwestern U.S., central Mexico and southern Canada between 1994 and 2018. 

  • Pinning down why the eastern monarch butterfly is disappearing has proven difficult. Theories include:
    • widespread use of herbicides that destroy the milkweed host plant primarily glyphosate since the 1990s)
    • mortality during migration and deforestation of the wintering habitat in Mexico
    • climate change’s detrimental impact on monarch breeding success.

  • The abstract of the MSU study explains, “Between 2004 and 2018, breeding-season weather was nearly seven times more important than other factors in explaining variation in summer population size, which was positively associated with the size of the subsequent overwintering population. Although data limitations prevent definitive evaluation of the factors governing population size between 1994 and 2003 (the period of the steepest monarch decline coinciding with a widespread increase in herbicide use), breeding-season weather was similarly identified as an important driver of monarch population size.”

  • Ann Arbor and Washtenaw County have become more pollinator-friendly in recent years. Both regions are certified NWF Community Wildlife Habitats. The Ann Arbor Environmental Commission formed The Ann Arbor Pollinator Subcommittee in 2017 and meets monthly to improve pollinator health. The University of Michigan and Washtenaw County Natural Areas Preservation has eliminated the use of neonicotinoids pesticides in their grounds management practices. U of M is completing an inventory of pollinator friendly plants in their planting beds, and identifying pollinator corridors for increased plantings and expansion of "natural" areas.

  • Currently, there is no federal protection for monarchs or their habitat. During the Trump administration, monarchs were put on the waiting list for Endangered Species Act, even though The U.S. Fish and Wildlife Service has estimated up to an 80% probability of population collapse for eastern monarchs within 50 years and a 96-100% probability for the western population.

  • Erin Zylstra is a Post-doctoral researcher in the department of Integrative Biology at Michigan State University. She says this finding is particularly concerning because if climate change continues to progress, more areas of the monarchs' breeding habitat may become unsuitable, and there may be a tipping point were monarch populations cannot rebound. 

2021 MSU Study - Abstract

Declines in the abundance and diversity of insects pose a substantial threat to terrestrial ecosystems worldwide. Yet, identifying the causes of these declines has proved difficult, even for well-studied species like monarch butterflies, whose eastern North American population has decreased markedly over the last three decades. Three hypotheses have been proposed to explain the changes observed in the eastern monarch population: loss of milkweed host plants from increased herbicide use, mortality during autumn migration and/or early-winter resettlement and changes in breeding-season climate. Here, we use a hierarchical modelling approach, combining data from >18,000 systematic surveys to evaluate support for each of these hypotheses over a 25-yr period. Between 2004 and 2018, breeding-season weather was nearly seven times more important than other factors in explaining variation in summer population size, which was positively associated with the size of the subsequent overwintering population. Although data limitations prevent definitive evaluation of the factors governing population size between 1994 and 2003 (the period of the steepest monarch decline coinciding with a widespread increase in herbicide use), breeding-season weather was similarly identified as an important driver of monarch population size. If observed changes in spring and summer climate continue, portions of the current breeding range may become inhospitable for monarchs. Our results highlight the increasingly important contribution of a changing climate to insect declines. (Source: *directly quoted* htps://www.nature.com/articles/s41559-021-01504-1)

Monarchs are disappearing fast

Michigan State University ecologists 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.

Between 2004 and 2018, changing climate at the monarch’s spring and summer breeding grounds has had the most significant impact on this declining population. In fact, the effects of climate change have been nearly seven times more significant than other contributors, such as habitat loss. The team published its report July 19 in the journal Nature Ecology & Evolution.

“What we do is develop models to understand why monarchs are declining and what’s happening to biodiversity in general,” said Erin Zylstra, the study’s lead author. Zylstra is a postdoctoral researcher in the Department of Integrative Biology and the Ecology, Evolution and Behavior Program, both in MSU’s College of Natural Science.

“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,” said Zylstra. “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.”

Understanding the monarch decline and doing what we can to reverse it is important not just for preserving biodiversity, but also because insects are prolific pollinators. The eastern population of monarchs migrates between Mexico and the eastern half of the U.S. 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% of what it was just a few decades ago.

The mid-1990s through the mid-2000s saw the most dramatic decline, coinciding with a period when glyphosate weed killers became hugely popular in the agricultural industry. Farmers grew crops that were engineered to be resistant to glyphosate, allowing them to apply the chemical widely, decimating milkweed plants that are the sole host and food source for monarch caterpillars.

The prevailing theory during that period has been that the loss of milkweed from agricultural areas was responsible for the severe declines. Since then, monarch populations have continued to fall. Although glyphosate-driven milkweed loss remained one possible explanation, other theories emerged over time. Today, researchers are divided on what’s stunting the monarch’s population.

About a decade ago, however, 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.

“People have different hypotheses,” said Zipkin, the senior author on the new study and director of the Ecology, Evolution and Behavior Program. “So we tried to come in as an impartial team, take the time and put all these pieces together to really parse out the contributions of various stressors.”

Part of what makes it so difficult to understand the decline is the eastern monarch’s complicated life cycle. These monarchs spend their winters, November through February, in central Mexico. When the weather starts to warm, they head north to the southeastern U.S., particularly eastern Texas.

Once there, the adults breed, lay eggs and then die. It’s the next generation that continues the migration, starting in about May, flying to the Midwest and parts of Canada, where they produce two to three more generations. The butterflies that develop in late August shut down their reproductive systems and spend their energy migrating south back to Mexico, where the cycle begins anew.

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,” Zipkin 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,” said Zylstra.

Zylstra 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.

“I think that everyone is partially right. All of these things do play some role. With monarchs, everything is nuanced, and everything is tricky,” said Zylstra. “But in recent years, as glyphosate applications have remained more stable, although still very high, there is strong evidence that population changes are driven by climate on the spring and summer breeding grounds.”

Each of these hypotheses can contribute to lost butterflies at smaller scales, Zylstra explained. But 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.

To get the full picture of the population decline, the team needed to understand the dynamics of many generations in many locations. Hence the need for thousands upon thousands of surveys. The herculean effort of collecting and making sense of this data has also reaped two large rewards.

First, by proving the model’s potential to tease out population dynamics for something as complicated as the eastern monarch, the team is optimistic it can adapt the model to understand what’s driving population changes in other species, too.

Secondly, this understanding should help inform where conservation efforts can provide the greatest benefit for the eastern monarch’s numbers.

“This study gives us information on where to spend our limited dollars on restoration,” Zylstra 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, she said. 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, Zipkin 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,” Zipkin 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 with monarchs. Now, what can we do with conservation?” Zipkin asked. “We can work together.”

Note for media: Please include a link to the original paper in online coverage: https://www.nature.com/articles/s41559-021-01504-1

Washtenaw County Pollinator Programs

  • A new subcommittee of the Ann Arbor Environmental Commission, The Ann Arbor Pollinator Subcommittee, was established in 2017 with the aim of improving pollinator health. The Pollinator Subcommittee meets monthly and is working to identify concrete actions that can be taken throughout the city, including engaging Ann Arbor residents to help.
  • Natural Area Preservation does not use neonicotinoids to maintain City natural areas and parks.  
  • Our City Council designated Ann Arbor as a BEE CITY USA and is committed to working towards a pollinator-friendly community.
  • Mayor Christopher Taylor has signed on to the National Wildlife Federation's Mayors' Monarch Pledge and is taking action to protect monarch butterflies.
  • Ann Arbor and Washtenaw County are both certified as National Wildlife Federation Community Wildlife Habitats.
  • The University of Michigan is eliminating the use of neonicotinoids in their grounds management practices, completing an inventory of pollinator friendly plants in their planting beds, and identifying pollinator corridors for increased plantings and expansion of "natural" areas.

Local Monarch Resources

(Source: *directly quoted* https://www.a2gov.org/departments/Parks-Recreation/NAP/Pages/Pollinators.aspx)


David Fair: This is 89 one WEMU, and I have a question for you. Did it seem like you saw this monarch butterflies over the summer than you usually do? Chances are that's the case, and scientists say there are reasons for it. I'm David Fairs and welcome to this week's edition of Issues of the Environment. Butterflies are pollinators and an important food source for birds, small animals, and other insects. Our guest today is here to help us understand why population numbers are on the decline and what we can do about it. Erin Zylstra is a postdoctoral researcher of integrative biology at Michigan State University. Thank you so much for the time today, Erin. 

Erin Zylstra: Yes, glad to join you. 

David Fair: Just how important are butterflies, and in particular, monarch butterflies, to our ecology? 

Erin Zylstra: Monarch butterflies are actually quite important. As you mentioned, they are pollinators, and they also form an important part of the ecological community around us, and that means pollinating some of the plants. It also means serving as a food source for some things that are higher up in the food chain. And then, they play an important cultural role and scientific role in our world. A lot of us, as children in school, learn about butterflies and metamorphosis by actually interacting with monarch butterflies. And then, we have this cultural component that's important here in the U.S., but also in Mexico, where they, over winter, they play an important role that kind of connects us to the environment and the world that we live in. 

David Fair: I was going to ask. Michigan is, of course, part of the pathway of the monarch migration between Mexico and Canada. What impact do monarchs traditionally have on our state? 

Erin Zylstra: Yes, Michigan is pretty central in their summer breeding grounds for the eastern population of monarch butterflies. And so, here, we do see quite a few monarch butterflies. And so, here during the summer, we'll actually get several generations of monarchs that are produced here. So, the first monarchs we'll see arrive in May or so--earlier summer. Those adults have come up from the southern U.S., and then they are busy. They create a couple new generations. Those adults actually die after they reproduce. And so, the next generation grows up, and then they produce yet another generation. And then, finally, those last individuals that are produced later in the summer--in August and September--those individuals then actually migrate all the way south, back down to Mexico where they spend the winter.

David Fair: Now, we keep talking about generations upon generations over the course of a season, but we have significant declines in monarch populations. How significant is it?

Erin Zylstra: Yes. So, we have seen declines in the population since the mid 1990s, basically, when we started systematically monitoring the monarch population. So, most of the information we have about declines actually comes from Mexico. And that's because that's where the same population that we see here in Michigan during the summer spends the winter in central Mexico. And the reason that we use those numbers to help us identify declines is because, on a summer breeding grounds, monarchs are spread over this huge area. But, during that overwintering period, all of the monarchs are gathered in very small pockets of forest in central Mexico. So, it becomes easier to assess from year to year how big or small the population is. So, we tend to use the Mexico numbers to assess decline. 

David Fair: And, of course, there, deforestation is a huge issue.

Erin Zylstra: Yes, yes. The good news is there is there's actually been improvement. So, when we first started monitoring monarchs down there, we did see declines, and there were large problems with both legal and illegal logging of some of those forests. But, actually, since we started monitoring in the mid 1990s, there's been the establishment of this preserve in Mexico, and there, the government and non-governmental organizations have really worked hard to preserve those environments for monarchs. So, they've actually seen declines in the amount of logging and deforestation that's happened there. So, that is actually a bright spot for the population since over the last few decades. 

David Fair: This is Issues of the Environment on Eighty-Nine one WEMU. And we're talking with Erin Zylstra. She is a post-doctoral researcher in the Department of Integrative Biology at Michigan State University. Now, monarchs are dependent on milkweed. And so, as we try and care for these pollinators here in Michigan, what role does development and, in some cases, overdevelopment play in the reduction in its population? 

Erin Zylstra: Yeah, so the milkweed plays a critical role in our monarch populations and how well they've done over time. So, just to illustrate that, for people who aren't as familiar with monarchs, monarchs only lay their eggs on milkweed plants, and then the caterpillars only eat milkweed. So, they're highly dependent on those plants. Milkweed generally tends to prefer they grow well in disturbed environments and agricultural environments. They like these relatively open areas. 

David Fair: And we as humans like to kill it. 

Erin Zylstra: Yes. So the problem--much of the problem, and we think some of the earlier declines of the population, are related to the fact that milkweed was really abundant in these agricultural areas. But, obviously that's a problem. Farmers don't like to have their fields inundated with these milkweed, which are weeds for them. And so, there's been the development of these crops, corn and soybean crops, that are tolerant of certain herbicides. And what happens then is that the farmers are able to apply large amounts of these herbicides, particularly glyphosate, to those crops. And that has really knocked back the population of milkweed in agricultural areas. And then that has consequences for monarchs. So, the amount of milkweed and habitat for monarchs in agricultural regions has really declined since the mid and early 1990s because of the development of these herbicide-tolerant crops. 

David Fair: And continuing down the pathway of consequences and human cause, like almost every other aspect of our lives, I suspect climate change is proving detrimental to monarchs as well. How significant is that issue in the future well-being of monarchs? 

Erin Zylstra: Yeah, so actually this is what's kind of interesting is that the milkweed problem we just discussed really took the largest toll on the monarch population earlier on. So, as the amount of of herbicide application increased a lot, and that happened in the mid 1990s to mid 2000s. Since that time, the milkweed population has stayed relatively low. So, we haven't had a lot more losses there. But what we have seen is evidence that climate change is having an impact on monarch populations, especially more recently. And so, what we're seeing here is in the spring, when the monarchs stop in the Texas region and breed there, and then in the summer in places like Michigan and their breeding range, what we see is that the population has still continued to decline, even though, you know, we're still applying the same amount of herbicide that's out there. And that seems to be linked to climate change. So, when we have extremely hot springs or extremely dry summers, some of these extreme conditions in the spring and summer breeding grounds, that has a negative impact on the monarch population. And, obviously, that has some implications with climate change and what changes we expect in these areas over the long term. 

David Fair: We're talking monarch butterflies with Erin Zylstra from MSU on Eighty-Nine one WEMU's Issues of the Environment. Now, during the Trump administration, there was discussion of placing monarchs on the endangered species list. In fact, they were on the waiting list. But the U.S. Fish and Wildlife Department projected that, within 50 years, there's an 80 percent chance there will be a population collapse for the eastern monarch and a 96 to 100 percent chance for the western population. Do you advocate an endangered species designation? 

Erin Zylstra: I think that's a tough question. Actually, you know, the U.S. Fish and Wildlife Service did determine that it was warranted to list the species under that act. But just they have higher priority species, right? There are some species that are in imminent danger of going extinct, and therefore, the monarchs are on what we call the candidate list. So, essentially, the service will review their status every few years and determine whether then they should be elevated to be listed as threatened or endangered. So, I think there's good evidence that both the eastern and especially the western population of monarchs are in some trouble over the longer term. All the evidence we have points to the fact that they are declining, and they will likely continue to decline over time. So, I think there's good reason to say that that we should be doing a lot to advocate for their conservation and whatnot. But I think it's a tough call for federal agencies who have so many species that are threatened and endangered. And so, you know, it's a matter of limited resources, I think. 

David Fair: As we look to the immediate and longer term future, what measures in your estimation need to be taken to ensure the survival of the species? 

Erin Zylstra: So, I think there's a few things we can think about from the big picture down to our choices we make in our in our everyday life. I think one of the biggest things, when we think about climate change and having this more important impact on monarchs currently, I think that we can really think about what we can do to advocate for, you know, reducing our carbon emissions and trying to stem some of these negative impacts of climate change. So, that means thinking about who we elect and what sorts of policies they put into place. So, that's sort of a big picture item, but it's really important. And then, I think, as individuals, we have a lot of choices to make in terms of the environment around us and what we can do. So, I think leaving places, like our yards or patches of environment, our neighborhood, leaving those places wild, allowing natural, native plants to grow things, like milkweed. You know, this doesn't result in really manicured landscape, but sort of leaving things wild benefits monarchs and lots of other species. And then, I think a lot of people have been interested in planting more milkweed. As we talked about, the amount of milkweed has really declined over time. And so, I think that's something that we can do. We want to focus on native species of milkweed, but planting those in our yards or on the roadsides or in some fields in our environment. That can only benefit monarchs and lots of other species that use those plants. 

David Fair: Erin, thank you for the time today. Erin Zylstra is a post-doctoral researcher in the Department of Integrative Biology at Michigan State University and has been our guest on Issues of the Environment. This weekly series is produced in partnership with the office of the Washtenaw County Water Resources Commissioner and is heard every Wednesday. I'm David Fair, and this is your community NPR Station, Eighty-Nine one WEMU FM and WEMU HD one Ypsilanti.

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Contact David: dfair@emich.edu
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