You do smell a rat.
Climate change emerged as a driving factor behind urban rat swarms, the researchers report today in Science Advances. As temperatures rise, they conclude, and people flock to urban areas and convert formerly “green” spaces into neighborhoods and shopping centers, they created a perfect storm for rat populations to explode. And the city that’s fared the worst over the past decade? Washington, D.C.
“This study provides an impressive bird’s-eye view of global trends in rat populations,” says Inbal Ben-Ami Bartal, a neuroscientist at Tel Aviv University who was not involved with the work. And the news isn’t good, adds Jason Munshi-South, an urban ecologist and evolutionary biologist at Drexel University. “There are likely dozens if not hundreds of cities around the world that have increasing urban rat populations.”
Smart, cooperative, and resilient, rats have coevolved with humans for millennia and have fine-tuned their ability to take advantage of garbage, debris piles, sewers, and small postage stamp–size plots of soil along sidewalks for food and nesting. The animals can transmit disease, spoil food and animal feed supplies—costing the United States $27 billion per year—and cause mental anguish in city dwellers. “Like the proverbial ‘canary in the mine’ our ‘rats in the city’ provide an indication of human welfare,” Bartal says.
To learn more about this threat, Richardson and colleagues reached out to city governments around the U.S. to collect data on rat populations, as well as average temperature, human population, and property development trends. And because so few places keep or share rat data, they expanded the study to cities outside the U.S. and eventually ended up with 16 where there were inspection, trapping, and rat sighting records across an average of 12 years that had been compiled by these municipalities.
“It is a lot of work to build these databases,” says Miriam Maas, who studies animal-borne infectious at the Centre for Infectious Disease Control and was not involved with the work. “[But] when done on 16 cities, it is possible to see trends.”
Cities that experienced greater rates of temperature increase and more people moving in were more likely to have bigger rat problems, Richardson and colleagues report. That makes sense, Maas notes, as cold weather slows reproduction and foraging. Moreover, denser populations mean more dumpsters, more restaurants, and more opportunities for rats to eat their fill.
Washington Post:
Over the past decade, rat sightings in D.C. jumped by more than 300 percent, while in New York they went up by 162 percent, according to Richardson.
“We live in an infinite sea of rats,” said Niamh Quinn, a human-wildlife interactions adviser at the University of California Division of Agriculture and Natural Resources who was not involved in the study.
About 40 percent of the overall increase in rat sightings is linked to rising temperatures in cities, according to the analysis. Researchers also found that the more densely populated a city is, and the less green space it has, the more urban rats thrived.
The research is the first to find a link between climate change and rat populations, said Kaylee Byers, an assistant professor and urban rat expert at Simon Fraser University in Canada.
“It’s a question we’ve had for a while,” said Byers, who was not involved in the research. The study is “the first one of its kind,” she said, adding that she would like to see more research in labs on the physiology of rodents to see how they respond to temperature changes.
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With temperatures poised to continue to rise and make rat infestations worse, the findings underscore how municipal governments need to move away from simply trying to poison rats and instead removing the food waste and debris that give them sustenance and shelter, Richardson said.
“Understanding that climate warming may lead to a general increase in rats isn’t good news,” he said, “but it’s really important to know the challenges you’re facing ahead of time so that you can dedicate more resources to try to slow that trend.”