Is There Something We Can Do About It?
By MICHIGAN STATE UNIVERSITY
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| A monarch butterfly sits atop flowering swamp milkweed in a Michigan garden. Credit: Jim Hudgins/USFWS |
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 today (July
19, 2021) 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.”
Reference: “Changes in climate drive
recent monarch butterfly dynamics” by Erin R. Zylstra, Leslie Ries, Naresh
Neupane, Sarah P. Saunders, M. Isabel Ramírez, Eduardo Rendón-Salinas, Karen S.
Oberhauser, Matthew T. Farr and Elise F. Zipkin, 19 July 2021, Nature Ecology & Evolution.