Climate change is amazing—if you’re a rat
In the first study of its kind, scientists found a correlation between climate change and a rise in urban rat populations.
Scientists have already linked climate change to more severe weather, rising sea levels, and melting polar ice caps. And now, a new study offers more bad news.
Urban rat populations appear to be rising with the mercury.
Rats already cost the United States approximately $27 billion each year in damage to infrastructure, crops, and contaminated food supplies. At the same time, media reports tend to suggest that rat populations are skyrocketing. And yet, when scientists started looking into the supposed rodent boom, they hit a wall.
“It seemed like that data would be pretty easy to get,” says Jonathan Richardson, an urban ecologist at the University of Richmond. “But we were wrong about that.”
Richardson says the pest control departments in most cities are under-funded and under-staffed, which means what resources they do have tend to go toward trying to reduce populations. Often, nothing is left over for research or even simple data collection.
To fill this gap, Richardson and his team tapped into a rich data set provided by complaint calls made to 311 numbers. Using data from 16 cities that had at least seven years of calls—and some with up to 17-years’ worth of calls—the scientists then used that information as a proxy for rodent populations.
“What we found is that, yes, rat numbers are generally increasing in cities around the world,” says Richardson. “And cities that are warming faster have larger increases in rats over time.”
(Learn more about how rats became an inescapable part of city living.)
While the new study, published today in the journal Science Advances, did not assess why climate change may be linked to a surge in rat numbers, Richardson notes that if these cities are warmer going into and coming out of winter, it may allow rodents to forage longer. And this in turn may allow them squeeze in another breeding event or two—leading to larger population sizes.
Five cities have become rat paradises
In 11 of the 16 cities, rat numbers rose significantly over the study period. Among those, Washington D.C. had the highest increase, with San Francisco, Toronto, New York City, and Amsterdam showing the next-strongest positive trends.
The complaint calls reveal that Oakland, Buffalo, Chicago, Boston, Kansas City, and Cincinnati also have increasing rat populations, though less significantly than the top five.
In addition to climate change, the scientists learned that rat numbers also tend to grow with more urbanization and a growing human population. The study also looked at whether an area’s wealth or minimum temperature were related to rat populations but could not find statistical significance.
Interestingly, three cities with enough data to be included in the study managed to reverse the trend, with these urban areas showing a decrease in rat numbers over the years. These include New Orleans; Louisville, Kentucky; and Tokyo.
Notably, New Orleans is also the only city on the list with a sub-tropical climate, but Richardson says temperature was likely not behind the decrease in rodents. Rather, he credits the city’s pro-active approach to pest management.
“They have a fairly large rodent control team, and they focus on education campaigns to try to get the word out to residents about how to make your property less likely to be infested by rats,” he says.
As for Tokyo, the scientists note that there may be a cultural component at play.
“Their expectations for sanitation are very high, and from what our co-author, Yasushi Kiyokawa, indicates, a person there is very likely to report a food service business that has a rat sighting or a mouse sighting right away via social media,” says Richardson.
“Coexistence with rats”
While the new study sheds light on the forces that may influence urban rat populations, cities remain highly complex environments.
For instance, while the scientists found a connection between increased urbanization, which they used lack of vegetation to indicate, other research has found that urban green spaces are a bit of a mixed bag when it comes to rodent numbers.
One study found that complaints about rats went down the nearer you got to green spaces. But two other studies actually suggested the opposite—that increases in rats were linked with proximity to public open spaces and vacant lots.
The disparity might be a matter of perspective, says Tali Caspi, an urban ecologist at the University of California, Davis who was not involved in the new study.
In other words, a city may appear to be more urbanized as a whole, but when you zoom in you might find green spaces within the city serving as cover for burgeoning rat populations.
In a study also published this month in the journal Ecosphere, Caspi and her team found similar discrepancies about the kinds of foods coyotes eat around the city of San Francisco.
(Learn more about how wild animals are adapting to city life in surprisingly savvy ways.)
“Coyotes were eating more rats where there were more restaurants in their territory,” she says. “We tend to see more rats where there are more restaurants, because there’s a lot of waste in those areas.”
Simultaneously, coyotes just a short walking distance away subsisted mainly on human food sources, revealing that urban ecosystems are diverse, and making broad conclusions about wildlife difficult to draw.
Similarly, while Caspi applauded the scientists for using call data where more direct observations of rodents didn’t exist, she notes that many factors, including socioeconomic disparities and public trust in government, can affect who reports a rat and how often.
That caveat aside, Caspi says it’s important that scientists keep trying to disentangle the complexities of urban environments, as well as the predators and prey that live within them, if we want to safely coexist with urban animals.
“I think it’s always worth mentioning that rats are a remarkably impressive species, in terms of their ability to move across the world and…succeed really well amongst humans,” says Richardson. “We’re trying to combat a species that is a worthy adversary.”
