Rat populations in cities are booming as the planet warms up

It has long been predicted that many pest species will thrive as the planet warms – and now a study of 16 major cities has found that rat populations are growing fastest in areas where average temperatures are rising quickest.

It is extremely difficult to estimate the number of rats in a city, so Jonathan Richardson at the University of Richmond in Virginia and his colleagues didn’t attempt this. Instead, they got a sense of how populations are changing by looking at the number of complaints about rats recorded by cities.

In the US, this information is often publicly available and the team was also able to get data for a few places outside the US by contacting city officials. The researchers only included cities in their study if at least seven years of data was available and the methods for collecting it hadn’t changed. That left them with data for 13 US cities, as well as Tokyo, Amsterdam and Toronto.

Their analysis suggests rat numbers are declining in New Orleans, Louisville in Kentucky and Tokyo, are stable in Dallas and St Louis, and are rising in the other 11 cities, with the fastest growth in Washington DC, San Francisco, Toronto, New York and Amsterdam.

Richardson and his colleagues then looked at several factors that might explain the trends. They found the strongest link was with the average temperature increase over the past century. The next strongest link was with urbanisation, assessed from satellite photos, followed by human population density. The city’s GDP did not show a link with rat trends.

It is known that in colder cities, rat numbers fall during the winter and peak in summer, so it makes sense that rising temperatures are leading to rising populations, the researchers say. More rats mean a greater risk of people getting rat-borne diseases, such as leptospirosis, also known as Weil’s disease.

The findings show that cities need to do more to control rat populations as the planet warms, and cutting off their food supply is the single most important measure, says Richardson.

“Securing food waste and making it inaccessible to rats is the approach that will have the biggest impact on controlling rats,” he says. “We’re seeing New York City pilot that in certain neighbourhoods – finally – and it’s putting a measurable dent in the rat numbers.”

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