Rat populations are increasing in cities as temperatures rise

A new study reveals that rat infestations in many cities worldwide are on the rise, with Washington, D.C. emerging as the biggest hotspot.

The study, which was focused on rat sighting reports from 16 global cities, attributes the increasing rat populations to warming temperatures, urbanization, and other human activities.

Rat sightings on the rise

The researchers conducted a first-of-its-kind analysis using publicly submitted rat complaint data. They discovered that in 11 of the 16 cities analyzed, rat sightings have increased in recent years. 

Based on individual city trends, Washington D.C. led the pack, followed by San Francisco, Toronto, New York City, and Amsterdam. According to the study, the rat reporting trend in Washington D.C. was three times greater than that of Boston.

Only three cities – New Orleans, Louisville, and Tokyo – experienced significant decreases in rat reports, with New Orleans showing the largest drop. The experts suggest that New Orleans’ strategies could serve as a model for other cities seeking to control rat populations.

Key drivers of rat population growth

The research team performed statistical analyses of rat sighting reports and concluded that a little over 40% of the rising trend is due to warming temperatures.

This warming is primarily a result of burning coal, oil, and natural gas, which leads to milder winters and longer foraging periods for rats. 

Study lead author Jonathan Richardson, a biologist at the University of Richmond, explained the mechanism.

“We’re seeing these increasing trends in rats in cities that are warming the fastest, probably because this is a small mammal that has physiological challenges in the cold weather months,” said Richardson. 

“If we’re warming the climate and winter starts a week or two later and spring arrives a week or two earlier, that’s one, two, maybe even three or four weeks across the entire year where those rats can be above ground foraging, acquiring more food and maybe squeezing out one or two more reproductive cycles.”

Urbanization and higher population density

Even a few extra weeks of activity can have a dramatic effect, given that female rats can produce a litter every month. With each litter consisting of eight to 16 pups, this additional time significantly accelerates population growth.

In addition to the effects of warming, the study identified increased urbanization and higher population densities as major factors. Rats thrive in built environments where food waste is abundant and human activity creates numerous shelters. 

The experts note that these conditions allow rats to flourish in areas where people and their refuse provide a constant source of sustenance.

Rats: Nature’s ultimate urban survivors

Rats have evolved to be remarkably adaptive in urban settings. New York City rat czar Kathleen Corradi stated during a conference that the rat is the third most successful mammal behind humans and house mice. So it evolved and engineered to live alongside us. 

“They followed humans, Homo sapiens, across the continents and are in every single continent except Antarctica. So it’s considered a wicked problem.”

Their ability to adapt and survive in close proximity to humans explains why rat populations are expanding even as cities implement various control measures.

Consequences of rising rat populations

While rats are undeniably adaptable, their increasing numbers pose serious risks. When rodent populations soar, public health and urban infrastructure can suffer.

“When rodent populations are high, people get sick, motor vehicles become disabled, mental health declines, fires are started and foods fouled,” said Houston rat expert Michael Parsons, who was not involved in the study.

This statement captures the multifaceted impact of rat infestations. High rat populations can lead to increased transmission of diseases, disruption of transportation systems, and other societal challenges.

People are understandably disturbed by rats, primarily due to an instinctive fear of creatures that can transmit disease.

Quantifying city rats

Rats are notoriously difficult to count accurately due to their secretive and elusive nature. The study did not count rats directly; instead, it analyzed rat sighting complaints submitted by city residents.

Although these reports may not reflect the actual number of rats, they provide valuable insight into trends over time. Richardson emphasized that the long-term consistency of the reporting methods used in the study lends credibility to the observed trends.

Even though Washington D.C. exhibited the highest rate of increase, Richardson clarified that this does not necessarily mean the city has the most rats overall – it simply indicates that the number of rat complaints is rising faster than in other cities.

Prevention over poison

The study’s findings suggest that traditional methods of rat control, such as poisoning and trapping, may not be sufficient to combat the escalating problem.

Instead, prevention measures appear to be more effective. In cities like New Orleans, public authorities are taking proactive steps.

For instance, educational workshops and community campaigns focus on reducing conditions that attract rats, such as improper waste management and excess food availability.

“In New Orleans they make a big effort to get out into neighborhoods and do educational workshops and campaigns to talk to residents about what makes a property less likely to have rats,” Richardson said.

Similarly, New York City has initiated innovative strategies, including replacing street garbage bags with rat-resistant containers and forming an elite volunteer group known as the “NYC rat pack.”

Early results from these initiatives are promising, although comprehensive data have yet to fully reflect their impact.

A complex urban challenge

While it is possible to better manage rat populations through improved urban planning and public education, complete eradication is unrealistic.

As University of Michigan conservation scientist Neil Carter pointed out, we can fight them better, but in the end “people need to coexist with wildlife in urban environments, even with rats.”

“Zero rats is impossible,” noted Richardson. “But I think an expectation that we need to live with the number of rats that we’re seeing in many of these cities is also an unhealthy perspective on this problem.”

In conclusion, this study offers a critical, data-driven look at the complex factors driving rat population growth in urban environments.

As cities continue to warm, expand, and densify, the challenge of managing rat populations will require innovative and sustainable approaches that focus on prevention and public awareness.

The study is published in the journal Science Advances.

Image Credit: AP Photo/Mary Altaffer

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