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Monarch butterfly numbers plummet despite recovery last winter, but 1 year never tells the whole story

The number of monarchs that have survived the migration to Mexico is estimated to have fallen to near-record lows this winter. But experts say these numbers don't mean much on their own, as reporting on year-to-year changes doesn't give an accurate representation of the health of the population.

Population spikes generate headlines, but experts say it's critical to monitor long-term trends

Though monarch butterflies can see their populations spike in a given year, the overall trend is the species is in decline. Researchers say that's the more important figure to watch. (Nic Coury/The Associated Press)

CBC's Great Lakes Climate Change Project is a joint initiative between CBC's Ontario stations to explore climate change from a provincial lens. Darius Mahdavi, a scientist with a degree in conservation biology and immunology and a minor in environmental biology from the University of Toronto, explains how issues related to climate change affect people across the province and explores solutions, especially in smaller cities and communities.

The number of monarch butterflies that have survived the migration to Mexico this fall appears to have plummeted, with early estimates suggesting they covered just one hectare or less of their overwintering grounds — well under half the area they covered last year. 

The figure is based on preliminary estimates from Monarch Watch, an education and research organization operating out of the University of Kansas. Researchers there say this could be the lowest total in a decade and "probably one of the all-time low numbers" for monarchs overwintering in Mexico.

It's a startling figure, bound to generate headlines — much like last year's report that the population was up 35 per cent compared to 2020-2021

But experts have long been adamant these numbers don't mean much on their own. They say reporting on year-to-year changes does not give an accurate representation of the health of the population. They also say this type of reporting distracts from the real issue — that from 1996 to 2014, the population declined 86 per cent, a trend that has continued. 

"Watch the trend, not the annual numbers," said Jeremy Kerr, a professor and research chair of macroecology and conservation at the University of Ottawa who has studied pollinators extensively. 

"Spiky trends [like monarch populations], half the time they're increasing and half the time they're decreasing, but the increases are small and the decreases are larger."   

Overwintering monarch population sizes are measured in hectares of land covered due to the impracticality of precise counts when they roost in high densities. (Jasmine Hohenstein)

This is emblematic of a larger problem. Monarchs are pollinators that depend on native plant species, and are often considered a "canary in the coal mine" for other pollinators essential to ecosystems across North America, and especially in Ontario. 

They are also a fragile species — small insects that travel thousands of kilometres each fall, from areas as far north as Thunder Bay, Ont., down to the oyamel fir forests in Mexico, and then back a few months later. Just one extreme storm or drought, or lack of habitat on the way down south could decimate the overwintering numbers that year, Kerr explained. 

Likewise, if conditions are good, the population will rebound. You can see that happening in this graph showing the numbers over the past 30 years: 

The number of hectares that monarchs cover when overwintering in Mexico is a good proxy for their population size, which is impractical to measure due to the density of monarchs when roosting. 

Residing in and passing through so many different areas each year means there are many opportunities for the population to be driven to declines by suboptimal conditions, but it also gives them many chances to recover. 

If one of their habitats or migratory conditions is not ideal, doing well in another habitat gives them a chance to build their numbers back up, since they go through at least four generations each year. At least half start in Ontario. 

"What we have to do is stop focusing on the numbers for any particular year and think about the trend over five years, or 10 years or 20 years," Kerr said. "And what we see from last year is that although it was better than two years ago, it was still historically pretty awful."

It is only from Ontario that a sufficient number of monarchs get to Mexico to continue the population.- Chip Taylor, Monarch Watch

That long-term trend is the reason the monarch was listed as endangered by the International Union for Conservation of Nature (IUCN) in 2022. In Canada, monarchs were recommended for endangered status in 2016, but are currently still listed as a special concern. 

Extreme weather caused by climate change seems to be exacerbating these fluctuations. Until recently, climate change wasn't considered a major driver of declines, but that no longer seems to be the case. 

Threats changing, but solutions remain the same

There are two populations of monarchs in Canada. The eastern migratory population spends the summer in Ontario (and, to a lesser extent, neighbouring provinces) and overwinters in Mexico. The western migratory population moves primarily between B.C. and California. 

The eastern migratory population of the monarch — the ones we see in Ontario — declined by over 85 per cent between 1996 and 2014, according to the IUCN, and has continued to shrink since. The western population has declined 99.9 per cent since the 1980s. 

By 2036, the risk of Ontario's monarchs disappearing entirely — an event known as "quasi-extinction" — has been estimated to be between 11 per cent and 57 per cent, according to a 2016 paper published in the journal Nature

The authors explain this is primarily due to the small population size coupled with the monarch's high sensitivity to environmental variability, particularly weather. They estimate the population would need to be five times larger to reduce the quasi-extinction risk by half. 

Previous declines can be attributed primarily to loss of habitat as the plants the monarch requires, particularly milkweed and native wildflowers, were eliminated. Another contributor was the use of pesticides, which harmed the monarch directly and killed the plants it relies upon. 

Now climate change can be added to the mix of threats, Kerr said. 

"Climate change creates extreme weather. That extreme weather is a real problem for monarch butterflies everywhere they go," he said. 

WATCH | See monarch butterflies in their winter sanctuary in Mexico:

Sanctuary welcomes endangered monarch butterflies

6 months ago
Duration 1:22
Visitors watch in awe as millions of endangered monarch butterflies blanket trees in Mexico's Sierra Chincua Sanctuary. Every year, migratory monarchs travel up to 3,000 km from eastern Canada and the U.S. to spend the winter in Mexico's central and western forests.

Because monarchs move over such large distances, they will be exposed to that extreme weather and its varied impacts across North America. 

Chip Taylor is a professor of ecology and evolutionary biology at the University of Kansas and the founder and director of Monarch Watch.

Taylor has also identified climate change as the biggest emerging threat to monarchs, primarily due to the more long-term effects it will have on storms and temperatures in critical monarch habitats. .

He also made a key distinction between weather and climate.

Weather refers to immediate and short-term conditions, and has always been the cause of monarchs' year-to-year population variation; climate focuses on decades-long trends, like rising temperatures. While climate change may bring more extreme weather and drive down monarch populations in that way as well, it's important to disentangle the two phenomena.

"Over short intervals, monarchs go up and down with the weather. In the long term, the outcome will be determined by available habitat and increasing temperatures at critical stages in the annual cycle," Taylor added in an email. "The direction of changes in the weather in March and September, if they continue, will have a significant negative impact on the development of the population each year.

"In other words, in the long term, declining habitat and climate change will drive down monarch numbers."

Fortunately, research has suggested that if the monarch's summer habitat recovers, the population could stabilize at a healthy size despite year-to-year changes caused by weather. It will also be the best buffer against the more long-term threat of a changing climate, Taylor said. 

Saving monarchs in your own backyard

When it comes to saving migratory monarchs, Ontario is the place to do it, Taylor said. 

"It is only from Ontario that a sufficient number of monarchs get to Mexico to continue the population," he said, adding the most important goal is to "sustain and increase monarch habitat" in the province. 

This is because besides reducing emissions, there is little we can do to prevent the effects of climate chaos on the monarch, Taylor said. The best way forward is to build up the population during the summer breeding season here in Ontario. 

"It's important to remember that monarch butterflies began to decline not because of climate change, but because of widespread habitat loss," said Kerr.  "What this means is that we can buy time to get climate change a little bit more under control if we start to try to reverse some of the habitat loss."

Fortunately, individuals, community groups and municipalities across Ontario are doing just that. 

In Thunder Bay, the community has rallied to protect the monarch with a variety of individual and community initiatives, including wildflower and milkweed seed giveaways, the creation of a volunteer-maintained butterfly garden, and a successful campaign to change the city bylaws around lawn naturalization.

Community members also convinced the city to take the Mayor's Monarch Pledge, joining a handful of other Ontario cities like Toronto in a commitment to protect and enrich monarch habitat. Applications for cities to take the pledge are currently open until March 31. 

A monarch butterfly feeds on a large flowering bush near the Beaches area of Toronto on Oct. 14, 2021. (Evan Buhler/The Canadian Press)

"There's so much that every individual can do," said Kyla Moore, the Thunder Bay resident who spearheaded the bylaw effort. "When you look at how many people can even give a small portion of their front yard over to native habitat, you add all that together and you can make a huge difference." 

For those hoping to naturalize their own properties, Moore said you should check your city's bylaws first. For cities looking to update their rules, Toronto's naturalization bylaw is the gold standard, she said. 

The Waterloo Region District School Board is also working to provide monarch habitat under an effort led by Sean McCammon, an outdoor educator and board member. 

"Some schools offered up to us 100 square metres, 500 square metres. And so we had students out there, we had a mix of seeds with like 20 native wildflowers and we got them planting," said McCammon. "We probably have had 80 [out of 125 schools] sign up now, and I think it's probably just going to be a systematic thing where every school has a monarch waystation."

Picture of a garden with native plants. Sign says "Adelaide Butterfly Garden".
Thunder Bay, Ont., has designated natural areas to help the monarch population rebound. Naturalizing your own property or unused patches of land can support monarch populations, building them up for the long migration to Mexico. (Submitted by Kyla Moore)

He added that anyone can have a naturalized area at their home, school or business certified as a monarch waystation by Monarch Watch for a small fee, provided they meet certain requirements about size, light availability and plant species. 

When it comes to community science, there are few people with more experience than Don Davis. He will be the first to tell you he is not a scientist by trade — he spent 40 years working as a child welfare advocate in Toronto. 

But as a lifelong wildlife enthusiast, he tagged his first monarch in 1967. Last year, he consulted with the federal government on the listing status of the species, which is currently up for federal protection under the endangered status in Canada. 

When asked about how we can save the monarch, he quoted his friend (and monarch expert) Karen Oberhauser: "All hands on deck."

"All hands on deck, that's a good approach," Davis said. "You really have to work together. The citizen, scientist, scientist, governments, land owners, the public. All hands on deck."


Darius Mahdavi

Science communicator

Darius Mahdavi is a CBC science specialist covering the impacts of climate change on the people and ecosystems of Ontario. He's worked as a researcher and graduated from the University of Toronto, where he earned a degree in conservation biology and immunology with a minor in environmental biology. If you have a science or climate question, reach out at