IE 11 is not supported. For an optimal experience visit our site on another browser.

Why cats are an invasive alien species, and what to do about it

The designation, while unwelcome by some feline lovers, is needed to address the problems caused by cats.
A Cat Sits
Jope, a 1-year-old cat who is waiting to be adopted at the Animal Rescue League of Boston, on July 8.Matt Stone / MediaNews Group/Boston Herald via Getty Images file

Conservationists often have to educate the public about how to identify invasive species like the spotted lanternfly and Asian carp. But the Polish Academy of Sciences just added another animal to its list of invasive alien species, and it might be in the same room with you right now: the domestic cat

To be designated an “invasive alien species,” cats needed to meet two criteria: They had to be non-native, and they had to “cause economic or environmental harm or harm to human health.” Cats actually meet both of these criteria, and not just in Poland. Here in the United States, too, felines can have a huge effect on the environment, and not in a good way. 

To claim that cats are not invasive is a sign of myopic love for cats and ignorance of the many species they harm.

This designation of the Polish Academy of Sciences is meant mostly for educational purposes and to inform policymaking. It in no way means that people in Poland can’t have pet cats, but it is an important step in growing awareness about the significant ecological harm cats can cause. 

The U.S. Department of Agriculture doesn’t name cats in its partial listing of invasive species in the U.S., but it did say they are invasive in a paper published last year. These moves by the Polish Academy of Sciences and the USDA, while unwelcome by some cat lovers, are needed steps in addressing the problems caused by cats.

Scientific research shows that cats were likely domesticated in Middle Eastern farming villages, and from there people spread the species around the world. So the Polish Academy of Sciences considers them to be non-native — unlike the wildcat and lynx, which are both native to Poland.

So cats are “alien” in Poland, and the U.S. But cats are famous for helping people by eating pests like rats and mice (as well as being welcome companions to frazzled pet owners). Cats have even been used in American cities specifically for pest control. Some Chicago residents claim that a group of cats was the only thing that could keep their backyards rat-free. 

But while cats seem to have some success in keeping rats out of specific spots, studies have found that cats are better at making us think there are fewer rats than at actually eliminating them. When scientists observed cats and rats in a recycling plant in Brooklyn, they found that cats would make rats run for cover, but that they only managed to kill a grand total of two. 

A study that observed cats and rats in Baltimore alleys found more cases of cats hunting rats but no connection between the number of cats in an alley and the number of rats. While humans would like the cats to hunt the rats, cats are generalists that eat a wide range of food, and sometimes cats even eat the same food as rats — at the same time. City rats may also simply be too large for cats to bother to hunt, preferring smaller prey. 

But while rats may be too big, lots of other animals are just the right size — and not all of them are mice. In Poland, cats eat an estimated 136 million birds every year, a number that researchers called “alarming,” as the number of birds in farming areas decreased by 20% between 2000 and 2018.

In the U.S., cats kill an estimated 2.4 billion birds every year, more than double the number of birds killed by cars, windows, wind turbines, power lines and communication towers combined. And this is all on top of the estimated 478 million reptiles, 173 million amphibians and 12.3 billion mammals that cats in the U.S. kill every year. Some of those mammals are undoubtedly mice. Many are not. An Australian study found that feral, or free-ranging unowned cats, prey on 400 different species there, including 28 threatened and endangered species.

A single cat and her offspring, brought to New Zealand’s remote Stephens Island in 1894, wiped the Stephens Island wren off the face of the earth. That’s just one of the more than 60 vertebrate extinctions around the world that domestic cats have played a hand in, making up a quarter of all modern reptile, bird and mammal extinctions.

So yes, cats are an “invasive alien species.” To claim that cats are not invasive is a sign of myopic love for cats and ignorance of the many species they harm. For the record, I like cats. There are two cats lazing around the house as I write this. But I also like ecological diversity and the joy of seeing frogs peek out of the water and hearing birdsongs in my backyard.

That’s why my family keeps our cats inside. Indoor cats, it should be noted, aren’t part of the problem. The hunting that wreaks environmental havoc is done by outdoor cats, both pet and feral, so the destruction is limited if they’re kept in (as a bonus, feline friends will also live longer if they remain indoors). If you really want your cats to go outside, make sure to accompany them and consider putting them on a leash.

It’s also important to prevent pet cats from becoming stray cats. Releasing pets into the wild, regardless of the species, is always a bad idea (and can even be illegal). If you can’t take care of your pets, find a responsible owner for them or give them to a shelter. And because there are more than enough cats for anyone who wants one, get your cats spayed or neutered. 

Want more articles like this? Follow THINK on Instagram to get updates on the week’s most important political analysis 

The problems with feral cats are harder to control. In Australia, where invasive cats are likely an even bigger problem than in the U.S., government programs cull feral cat populations

Culling cats is a hard sell in America, however. Most people don’t know how bad cats are for the environment, and the emotional reaction last month to cats being designated an invasive species by a foreign scientific institute most Americans had never heard of suggests that any killing of cats is likely to spark outrage. 

Studies have found that cats are better at making us think there are fewer rats than at actually eliminating them.

An often-touted alternative is trap-neuter-return, in which feral cats are caught, sterilized and then released. But like using cats to hunt rats, it sounds a lot better than it actually is. Studies have shown the approach has very limited success. But even if it succeeds, it still leaves cats alive in the wild where they can continue to hunt native wildlife.

At the very least, we should avoid feeding feral cats. Garbage should be secured to keep cats and rats (and, sometimes, bears) from hanging out for easy meals. And food shouldn’t be left out for them — that’s only likely to increase the feral cat population long term. You also might just be attracting more cats and other animals like rats.

So please keep your cats indoors, get them spayed or neutered, don’t abandon them and don’t feed feral cats (and rats). It’s far better for cats to paw at birds on the TV screen than claw at birds in real life.