Washington DC, San Francisco, Toronto, New York and Amsterdam have seen the biggest increases in rat populations, according to a new study that collected data from 16 cities around the world. Eleven of these cities have seen “significant increases in rat populations,” according to the study published in the journal Science Advances, and these growth trends are likely to continue due to rising temperatures, The Guardian reports. Over the past decade, rat populations have increased by 11 percent in Washington DC, 390 percent in San Francisco, 300 percent in Toronto and 186 percent in New York, according to researchers who analyzed public spaces and rat distribution reports.
Some major cities were not included, such as London and Paris, because they did not have the necessary data. In Toronto, one of the places with the highest rat population, residents of Canada’s largest city have experienced problems.
“When you walk down the streets of Toronto, under your feet, deep in the sewer system, it’s a place full of rats,” said Alice Sinia, chief entomologist at the country’s largest pest control company, Orkin. “We’re increasingly forcing them out into the open, either through construction or flooding, and they have to go somewhere.” The city of Toronto’s rat hotline has received 1600 calls about rats, up from about 940 in 2019. Orkin has also seen an increase in calls. Rising temperatures are linked to an increase in rat numbers because small mammals like mice have a harder time breeding in the winter, but in warmer temperatures they can breed and feed longer.
“In Toronto, the cold winters have acted as natural rat controllers,” Sinia said, killing off large groups of rats. But the mild temperatures have helped all types of rats in the city to thrive.
The fact that rat populations increased the most in cities where temperatures rose the fastest was “the study’s darkest finding.” Last year was the hottest year on record, with average temperatures 1.6 degrees Celsius above pre-industrial levels. Rats cause billions of dollars in damage by infiltrating buildings every year, and can transmit about 60 diseases to humans, as well as impacting the ecology of other animals in the city.