These US cities face a growing rat problem, and scientists say they know why – USA TODAY

Scientists have some good news for rats and some bad news for city-dwellers.
Rat populations are rising in cities including Washington D.C. and San Francisco, reports a study released last Friday that includes data from 16 global cities. Primary among the reasons found for the growing problem: warmer average temperatures due to climate change.
Warmer winters, denser living conditions, less vegetation and limited rat control resources also contributed to the growth in urban rat populations. The trend was most notable in Washington D.C., San Francisco, and Toronto, among the cities that were included in the study.
“The most concerning of these connections we found is the link between climate warming and rat trends, as global temperatures are beyond the control of individual cities,” said study lead author Jonathan Richardson of the University of Richmond,
According to the research, rats’ tendency to infiltrate buildings and spoil supplies can cause billions of dollars of damage to cities, where they live in the highest numbers.
According to Richardson, “while there aren’t good studies on how wild rats respond to temperature changes, we do expect that the warmer temperatures – particularly during the colder times of the year – will increase the time rats spend above ground foraging for food.”
An extra week or two of foraging can result in one or two more reproductive cycles, accelerating population growth, he said.
Overall, the study found that increases in average temperature, winter temperature, and living density, alongside decreases in vegetation and funding for rat control, correlates with faster urban rat expansion. Of these, the variable most tightly tied to rat increase was change in average ambient temperature over time.
“Cities need to be prepared for the potential for [climate] warming to exacerbate current rodent pest infestation levels. More financial and personnel resources will need to be dedicated to municipal rodent control efforts,” the study said.
Washington, D.C., has the fastest growth in rats of all the cities in the study, Richardson said.
After Washington, the next four cities with the fastest growth rates include San Francisco, Toronto, New York City, and Amsterdam.
Within these top five, the researchers saw large differences in the magnitude of that growth. For example, Washington, D.C. had 1.5 times greater growth in rat populations than New York City.
“Unfortunately, we don’t have the data necessary to estimate raw abundance of rats in these cities, just trends in the changes,” he told USA TODAY.
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Researchers analyzed an average of about 12 years of rat sightings, trapping reports, and inspection records from 16 international cities and compared the data to patterns in human density, annual minimum temperatures, and average ambient temperatures over time.
They also compared rat population changes to trends in vegetation abundance and accessibility to rat control resources.
The study found that 11 of 16 cities (69%) had significant increasing trends in rat numbers.
“Only by confronting the environmental factors that allow rats to thrive, and by giving municipal rodent managers the resources and tools they need, can we hope to rein in our growing rat problem,” Richardson said.
The study was published Friday in the peer-reviewed journal Science Advances, a publication of the American Association for the Advancement of Science.

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