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

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 based at 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."   

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."