What’s new: Cities experiencing warmer temperatures, fewer green spaces and denser human populations are seeing a rise in rat numbers, a recent study shows.
What the study says:
- Jonathan Richardson, a biology professor at the University of Richmond, U.S., and his colleagues wanted to check if anecdotal media claims about increasing urban rat (Rattus spp.) populations are true. Since there’s very little long-term scientifically collected data on city rats, the researchers used a proxy: publicly available data on public rat sightings and complaints, and pest inspection reports. They picked 16 cities — 13 in the U.S., as well as Tokyo, Toronto and Amsterdam — which had seven to 17 years’ worth of such data. In these cities, the data collection methods and sources were mostly consistent throughout the years.
- Only Tokyo, Louisville and New Orleans, saw declining rat numbers, the study found. Meanwhile, 11 cities, or 69%, saw a considerable surge in rat numbers over the years. Washington, D.C., San Francisco, Toronto, New York City and Amsterdam had the largest increases, followed by Oakland, Buffalo, Chicago, Boston, Kansas City and Cincinnati. The researchers noted wide variations in rat trends. For example, the increase in Washington, D.C., was three times larger than that in Boston.
- On exploring various factors that could explain the rat trends, the study found the strongest association with a city’s average temperature increase. “Cities that had a greater rise in temperature over time had larger increases in rat sightings,” the authors write. They speculate that since rat activity is constrained by colder temperatures, warmer weather could be making them more active, where they seek out food or mate.
- The analysis also revealed that cities with fewer green spaces, or increased urbanization, saw greater increases in rats. The researchers, however, caution that previous studies have found both positive and negative association of rats with green areas.
- The third big factor linked to increasing rat population was high human population density. “As more people inhabit a city, more food waste becomes available as a resource for rats,” the authors write, suggesting a possible explanation.
What this means:
Increased rat numbers may cause shifts in “urban food webs,” the researchers write, adding that the effects still need to be further understood.
“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,” Richardson said in a press release. “Those working to manage rat populations will need to factor this climate-accelerated growth into their rat management planning.”
Richardson said cities will need to “confront the environmental factors that allow rats to thrive” while ensuring that sufficient resources and tools are allotted for rodent management.
In another interview with Livescience, Richardson said he hopes this study nudges city officials and scientists to gather more data about rats.
Banner image of a roof rat (Rattus rattus) by Rhett A. Butler/Mongabay.
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