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  • Writer's pictureSusan Houser

Are Cats an Invasive Species as Defined by the U.S. Government?

When cat advocates try to establish programs to help and manage outdoor cats, they frequently meet resistance from wildlife conservationists who argue that cats should not be outdoors at all because they are an invasive species. For example, a wildlife conservationist argued in a 2016 book called Cat Wars that cats are an invasive species in the U.S. and should be removed from the environment, even if that meant killing them.[i] The definition of “invasive species” established by the federal National Invasive Species Council (NISC)[ii] may shed some light on this issue and help support advocacy for outdoor cats.

The NISC defines an “invasive species” as: (1) one that is non-native (or alien) to the ecosystem under consideration and (2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health. An NISC whitepaper adds the caveat that a non-native species can be classified as either invasive or non-invasive, depending on the impact of the species on its local environment. Thus, “non-native” by itself does not equal “invasive.”

The fact that the NISC definition requires looking at the local effect of a species is important. As the NISC whitepaper says on page 4: “An invasive species may be invasive in one part of the country, but not in another. A biogeographical context must be included when assessing whether a non-native species should be considered an invasive species.” For example: “Kentucky bluegrass would be considered an invasive species in Rocky Mountain National Park in Colorado, but considered non-invasive a mere 60 miles away at a golf course in Denver.”

Part 2 of the NISC definition offers a framework for evaluating the biogeographical context. At least one of three types of harm must be shown — economic, environmental, or public health. The guidance in the NISC whitepaper suggests that determining whether a species is invasive in a particular biogeographical context may require balancing benefits versus harms. I’ll look at each of the three types of harm listed in the NISC definition in turn.

Economic Harm

Pet cats are a big business. Estimated revenue for the total pet industry in the U.S. for 2018 is over $70 billion, and cats make up roughly half of owned pets. Less money is spent on cats than dogs, but even so, the cat industry in the U.S. probably produces $20-$30 billion in transactions per year, if not more. Virtually every city and town will have several local businesses that serve pet cats – including pet stores, veterinarians, and pet sitters. Local economies benefit directly from tax revenue from those businesses, and also from purchases made and taxes paid by their employees.

But what do pet cats have to do with outdoor cats, many of whom are homeless? First, a lot of outdoor cats are looked after by people who provide food, shelter, and veterinary care for them. Second, the outdoor-cat population is a critical source of pet cats. Fewer than 10% of owned cats are acquired as commercial purchases, meaning that some 90% of cat acquisitions are from non-commercial sources such as shelters, rescues, and litters of kittens had by pet cats. Therefore, a high percentage of the multi-billion-dollar pet-cat market depends on homeless cats.

Some people might argue that if outdoor cats were exterminated, commercial breeders would move in to supply the market demand. What seems more likely, however, is that the percentage of pet cats that are spayed or neutered would fall as people realized that they could sell kittens. Those unsterilized pets would then continue to seed the outdoor cat population. There is no easy way for commercial breeders to capture the pet-cat market – if there were, commercial breeders would have captured that market long ago.

Cats may have a positive economic impact in other ways too, such as by protecting food supplies from rodents. That type of economic impact is hard to quantify, though, whereas it’s easy to understand the importance of the pet economy to a city or town.

Environmental Harm

The issue of whether cats cause environmental harm is very complex. A full discussion of this issue would take a book-length work and is obviously beyond the scope of this blog post. However, I want to discuss one very important fact about cats which bears on the issue of environmental harm but receives little attention.

Cats are a commensal species. “Commensal” refers to a species that thrives by living in close association with humans.[iii] We humans have created an ecological niche for commensal animals by building structures that they can appropriate for shelter and discarding food that they can eat. In fact, outdoor cats in urban areas outnumber outdoor cats in rural areas by more than 20 to 1, per unit of ground.[iv] As Gary Patronek has noted: “The population of cats that do not depend in any fashion on humans for subsistence may be small.”[v]

The fact that cats live where people live would seem to have enormous implications for evaluating the effect of outdoor cats on the environment. Cats occupy an ecological space that is primarily provided by humans. If cats are removed from that space, other commensal animals will simply increase their numbers to take advantage of the food and shelter that are no longer being used by cats. There are many commensal animals, including possums, raccoons, and squirrels. Some of the most abundant and ubiquitous commensal animals are rats and mice. We generally think of cats as deterring rats and mice through predation, but cats also deter rats and mice by eating food and occupying shelter that would otherwise support rodents.

If we were to remove all outdoor cats, it would be a giant leap into the unknown, because our modern cities have literally never been without cats. Cats did not arrive in the U.S. by accident. They were brought here because early European settlers knew cats could protect food stores from rodents. Today’s cities and towns have much better sanitation than 18th and 19th century cities, but cities still have problems with rodents, and urban policymakers are well aware of that fact.

Cats in urban areas might be the type of situation where the NISC recognizes that a non-native species should not be considered “invasive.” Cats may be non-native, but they are an important cog in the ecosystems of our modern cities and towns. The NISC uses the example of Kentucky bluegrass, which may be invasive in a wildlife area but is a highly desirable non-invasive species on a golf course or horse farm. Similarly, outdoor cats can be very valuable in urban areas in displacing rats and mice.

What about the suburbs that surround cities and towns? In the book Welcome to Subirdia, wildlife professor John Marzluff of the University of Washington reported that suburbs that have at least 30% of their land area as green space (including parks, trees, gardens, ponds, and natural lawns), are excellent habitats for diverse species of birds.[vi] Marzluff believes that cats harm native species in the suburbs, but a research project he supervised indicated that cat predation on fledglings was not common in the area he studied. Of 122 fledglings who were radio-tracked for several weeks after leaving their nests (the time of life when healthy birds are most vulnerable to predation), there was only one death attributed to a cat, and that attribution was tentative.

Green suburbs have lots of birds, and they also tend to have lots of cats, since many people have cats as pets. Marzluff’s findings that bird diversity is good in green suburbs raises the question of whether the presence of cats might encourage bird diversity in green suburbs (although it should be noted that Marzluff does not interpret his findings that way). Perhaps cats, by warding off rats and other commensal animals, improve the environment for diverse species of birds in some manner. This is a question that warrants further study.

The NISC whitepaper suggests that balancing all harms and benefits is the appropriate way to determine whether a species should be considered “invasive” in a particular location. In the case of environmental harm and benefits, that balancing is very difficult because we just don’t have enough information to know how cats are affecting or will affect every biogeographical context. I think the uncertainty over these issues, especially in the urban and suburban context, should weigh against cities and towns taking any dramatic actions to exterminate cats on environmental grounds. They might well find that environmental problems are worse without cats.

Harm to Human Health

Toxoplasma gondii, the infectious agent for toxoplasmosis, is the most-cited public health issue associated with cats. Toxoplasmosis is a serious public health problem, but the experts on public health at the Centers for Disease Control (CDC) do not recommend trying to exterminate outdoor cats to control the disease. Instead, they recommend developing a vaccine to protect cats from T. gondii infection. Furthermore, the CDC states that even pregnant women and people with compromised immune systems, who are at higher risk of ill effects from T. gondii infection, do not need to give up their cats.

The CDC probably has more credibility in speaking on public health issues than any other organization in the world, so the fact that it does not support eradication of outdoor cats on public health grounds should put an end to that notion.

There has been speculation that T. gondii infection is associated with mental illness, but that idea appears to be a myth. Early studies suggesting such a link had methodological flaws. More recent studies (published in 2016 and 2017) that have better methodology have concluded that T. gondii infection is not associated with mental illness.

Even though the population of the U.S. is increasing and the number of pet cats is growing, rates of T. gondii infection have been declining. The decline has been substantial, and shows that we can fight toxoplasmosis without exterminating outdoor cats.

On the benefits side for public health, as mentioned above, outdoor cats are filling a specific environmental niche in cities and towns. We do not know what would happen if cats were exterminated from a city or town, but it seems likely that other species would move into that niche. If we have more rats, mice, coyotes, racoons, and possums in our cities and neighborhoods, will we be healthier than we were with cats? Other species can carry diseases. One example is hantavirus, which is carried by rats and mice and can kill a person within hours of symptoms becoming apparent. Rats can also carry a variety of drug-resistant, disease-causing bacteria. This article, which appeared on CNN in 2016, discusses the increasing phenomenon of deploying working cats as a first-line defense against rat infestations in cities.

An important point in favor of outdoor cats is that, unlike populations of other commensal animals, outdoor cat populations can be managed with Trap-Neuter-Return (TNR). TNR generally includes vaccinating cats for rabies, and in many cases “colony caregivers” monitor cats for disease and take sick ones to veterinarians. This work is largely done by volunteers and supported by private donations. It is a great bargain for public health.


The issues surrounding outdoor cats are very complex. This blog addresses the “invasive species” definition, but there are many other strong arguments in favor of managing outdoor cats with TNR, including the fact that cat extermination programs are a non-starter with the public. In recent years we have seen more and more cooperation between wildlife biologists and cat advocates. Hopefully this is a trend that will continue.


[i] Peter Marra and Chris Santella, Cat Wars: The Devastating Consequences of a Cuddly Killer (Princeton University Press, 2016), 19, 164.

[ii] The NISC was established in 1999 by Executive Order 13112. For links to additional executive orders regarding invasive species, see this page. The National Invasive Species Information Center is the public-information arm of the NISC.

[iii] Terry O’Connor, Animals as Neighbors: The Past and Present of Commensal Animals (East Lansing, MI: Michigan State University Press, 2013), 57–66.

[iv] Olof Liberg et al., “Density, Spatial Organization and Reproductive Tactics in the Domestic Cat and other Felids,” in The Domestic Cat: The Biology of its Behavior, 2nd ed., ed. Dennis Turner and Patrick Bateson (Cambridge, UK: Cambridge University Press, 2000) 124, table 7.2; cited in John Bradshaw, Rachel Casey, and Sarah Brown, The Behavior of the Domestic Cat, 2nd ed. (Wallingford, UK: CAB International, 2012), 140.

[v] Gary Patronek, “Special Report: Free-Roaming and Feral Cats—Their Impact on Wildlife and Human Beings,” Journal of the American Veterinary Medical Association 212, no. 2 (January 15, 1998): 218.

[vi] John M. Marzluff, Welcome to Subirdia: Sharing Our Neighborhoods with Wrens, Robins, Woodpeckers, and Other Wildlife (New Haven, CT: Yale University Press, 2014). Marzluff emphasizes bird diversity, which is a common theme for wildlife conservationists. Wildlife conservationists regard many common birds like pigeons and starlings the same way they regard cats – as harmful, non-native intruders that decrease the diversity of bird species.


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