INSIDE Toronto’s rodent population is booming, study finds
This article was written by Amy Dempsey Raven and Kate Allen, and was published in the Toronto Star on February 1, 2025.
Toronto’s rat population is growing faster than the rodents of New York City, Chicago or Amsterdam, according to a new study, illuminating how climate change and urbanization are turbocharging rat growth in the absence of effective control strategies.
Residents who have complained for years about a perceived rodent surge now have peerreviewed evidence, published in the journal Science Advances on Friday, that lays bare the scale of Toronto’s problem. Of the 16 cities analyzed, Toronto had the thirdfastest growing rat population.
The scientists who led the research say their findings should compel cities to invest in coordinated mitigation strategies — or in Toronto’s case, create one in the first place. City officials have been tasked to produce a rat strategy by later this year, after two previous efforts were shelved.
Currently, Toronto does not have a dedicated department or officer in charge of tackling its booming rat population.
“For a city of that size, that’s a glaring omission,” said Jonathan Richardson, a professor of biology at the University of Richmond in Virginia and the new study’s lead author. Richardson’s research was prompted by news reports from different cities complaining of rising rat problems. As an urban ecologist, he wanted to put real numbers to those concerns.
“Naively, I was like, OK, let’s just dive into this and get all the data that’s available, and we’ll be able to do this for dozens, if not a hundred cities, and see what’s happening,” he said. “But it turned out, data was really hard to come by.”
Because so few cities had reliable longterm rat data, the scientists were “pretty discouraged” to only get information from 13 U. S. cities and three internationally.
The study relies on public complaints, which in Toronto come from the 311 reporting system. While Richardson and others said this type of data is imperfect — it can’t reliably demonstrate differences among neighbourhoods, for example, because people with insecure citizenship or rental status may be less likely to report — it can still illustrate longterm trends.
“It’s the best we have, at the moment, to answer a question like this,” said Kaylee Byers, a senior scientist with the Pacific Institute on Pathogens, Pandemics and Society at Simon Fraser University, who was not involved in the study, but spent years researching urban rats and disease ecology as part of the Vancouver Rat Project.
To learn how rats are establishing themselves in cities and responding to climate change, “we really want to invest in municipal strategies that track actual numbers of rats across cities and over time,” Byers said.
Cities aren’t doing that because tracking rats is expensive, timeconsuming and often not a top priority, she added.
Of the 16 cities the study analyzed, 11 had significant increases in rat populations, led by Washington, San Francisco and Toronto. Only three experienced declines: Tokyo, New Orleans and Louisville, Ky.
When the scientists analyzed the relationship between these trends and the urban environment, they found that the biggest influence on rat growth was climate warming: the cities that saw the biggest temperature increases over their historical baseline average also saw larger booms in rat numbers.
Toronto is 1.9 degrees warmer than its historical average, the fastest warming city in the study after New York, according to the study’s data.
Experiments in lab rats have shown that they are very responsive to temperature, Richardson explained. Cold temperatures act like a brake on rat reproductive cycles, and warming releases that brake. Foraging gets easier too. Fewer rats die in deep winter if those periods get warmer. It’s unclear which of these is happening, or if it’s all three.
After climate warming, urbanization and population density also had a big impact, likely because they create more habitat and food sources for rats to exploit.
Study coauthor Bobby Corrigan, a rodentologist who leads New York City’s Rat Academy, said that when green space is replaced by buildings, it creates more “subterranean infrastructure” — underground pipes, sewer lines and crevices — where rats thrive.
“There is a rat world right below our feet,” Corrigan said, but it’s difficult to examine those spaces .
City of Toronto staff declined to provide more details about the inprogress rat action plan, or say if it would move up its completion due to residents’ concerns.
“Like most major cities, rodents are common in Toronto,” said spokesperson Russell Baker. “While the City of Toronto is unable to track the total rodent population in Toronto,” the city is “always open” to trying to improve its rodent control, Baker said, citing last year’s direction from council to explore an “interdivisional action plan,” due later this year.
This is not Toronto’s first attempt at creating a rat strategy.
In 2018, after what became known as “the summer of rats,” city council asked staff to come up with a plan. A policy team worked for 18 months, but it was never finished. City staff said it was shelved due to pandemicrelated resource constraints.
Since then, rodentrelated property standards complaints have shot up 70 per cent, prompting the latest push for a strategy.
The study authors said their findings make a case for cities to commit more resources to rat mitigation and start collecting reliable and systematic data.
Urban rat strategies should focus on integrated pest management, which emphasizes monitoring, removing food sources ( namely, trash) and shoring up infrastructure weaknesses as first steps, with chemical tools such as rodenticide baits meant to be a last resort, said Corrigan.
Corrigan, who works for the New York City Department of Health, said we should beware rats’ potential as carriers of disease pathogens.
“The big takeaway is this is not going to get better,” Corrigan said, as climate change creates a more favourable environment for urban wildlife. “And the question is, are we ready?”
“ The big takeaway is this is not going to get better. BOBBY CORRIGAN RODENTOLOGIST WHO LEADS NEW YORK CITY’S RAT ACADEMY
My Climate Change Website 