Only cities with at least seven years of reliable data were included in the study: 13 American cities, plus Tokyo, Amsterdam and Toronto.
The results show that rat populations are declining in New Orleans, Louisville and Tokyo, while they are stable in Dallas and St Louis.
Interestingly, rat populations are increasing in the 11 other cities analysed. Washington DC, San Francisco, Toronto, New York and Amsterdam are among those with the most marked increases.
To understand these trends, the researchers examined a number of factors.
They identified a particularly strong correlation between the rise in average temperatures over the last century and the increase in the number of rats in urban areas.
Next came the rate of urbanisation, determined from satellite images, and human population density.
In contrast, the level of wealth in cities, as measured by GDP, appears to have no influence on the resurgence of these small mammals.
The researchers explain that, in colder cities, rat populations decline in winter and multiply in summer. However, with milder temperatures throughout the year, these rodents enjoy more favourable conditions for survival and reproduction, which encourages their proliferation.
This phenomenon worries experts, as it increases the risk of disease transmission, notably leptospirosis, a serious bacterial infection also known as Weil’s disease.
An animal with a bad reputation
To counter this phenomenon, public authorities must step up their efforts to limit the proliferation of rats.
Worldwide, rat control in cities costs some US$500 million a year. Municipalities are setting up rat control units to reduce the number of rats in public spaces.
But, for Jonathan Richardson, the most effective solution lies above all in better management of food waste.
“Securing food waste and making it inaccessible to rats is the approach that will have the biggest impact on controlling rats. We’re seeing New York City pilot that in certain neighbourhoods – finally – and it’s putting a measurable dent in the rat numbers,” he told New Scientist magazine.
Although city rats are often associated with dirt and disease in the collective imagination, their image has evolved since the success of the animated movie “Ratatouille” (2007). So much so, in fact, that some tourists are now tempted to take guided tours of New York neighbourhoods where these rodents abound.
While the presence of rats in the city raises a number of concerns, it also reveals how our urban environments are being transformed by climate change.
Rather than seeing them solely as a nuisance, these rodents highlight the need to adapt our cities to new ecological challenges.
Better waste management and more sustainable urban policies will not only limit their proliferation, but also enable the authorities to rethink how people cohabitate with urban wildlife in a more responsible way.