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Climate change could boost Finland’s rat population

There could be as many as half a million rats in Helsinki, and climate change could feed their growing numbers according to a university rat-tracking project.

Rotat lisääntyvät ilmastonmuutoksen myötä Suomessa kesät talvet, mutta kukaan ei tiedä, kuinka paljon rottia kaupungeissa liikkuu. Pelkästään Helsingissä niitä arvellaan olevan sadasta tuhannesta puoleen miljoonaan.
Image: Matias Väänänen

The milder winters, brought by climate change, will help Finland’s rat population to grow, researchers say.

According to Tuomas Aivelo from the University of Helsinki, a rat can have as many as 70 descendants in a year, and warmer winters will enable the rodent to reproduce the year around.

Aivelo currently works on the Helsinki Urban Rat Project, which aims to map the population and dynamics of urban rats in Helsinki. In addition, the project looks at what parasites and pathogens rats carry.

Extermination is a relatively big business in Helsinki, but the size of Helsinki’s rat population is a mystery, Aivelo says. "The number of rats varies a lot depending on the season. Right now, in late autumn, the population is at its biggest."

The usual ballpark figure given about the number of rats in Helsinki is 100,000, but according to some estimates, there could be as many as half a million of them. The rat project aims to calculate the number of rats by using track plates and live-trapping.

While rats rarely spread diseases now, they cause a great deal of material damage, Aivelo says. Rats eat through electricity lines and destroy structures, and thereby create more living space for themselves.

"If a rat decides to chew through a concrete wall, it will do it as long as it takes."

Rat poison ban

Since February this year, rat poison has no longer available for purchase to the general public as it has been deemed harmful to reproduction. Pest controllers are still able to use the poison though.

However, all city residents can help reduce the number of rats by making sure that the animals have no access to garbage rooms or bird-feeding areas, Aivelo says.

Apples and rosehips should be collected off the ground as they provide nourishment to rats, and composts should be covered, adds Jammu Rantanen from pest control company Anticimex.

Rantanen says he has found rats everywhere: "I've seen a rat sleeping under a shower drain cover, eating dog food under the kitchen sink and munching on potato chips on the living room table."

The war against rats will not be won ever, Rantanen concedes.

On the other hand, rats are an important part of a city’s ecosystem and make up an essential part of the diet for foxes, hawks and owls. "People have many opinions about rats. Rats get exterminated, but it does not feel good to kill an animal. Then again, rats cause damage," Aivelo says.

"Rat is a controversial animal."

A documentary on a rat hunt in Helsinki is available on Yle Areena (in Finnish).

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