A couple of weeks ago I received an email from my friend and fellow environmental enthusiast Patrick Kopnicky asking me for my opinion about feral cats and their impacts on their environments. Patrick and his wife Mardelle, as I have mentioned in several previous postings, head up the Environmental Learning Center and the Friends of Harrison Hills organization at Harrison Hills Park (Allegheny County). The feral cat discussion that he was involved in centered on not only the maintenance of some area feral cat colonies but the proposed establishment (and support) of new colonies in some of the county’s other public parks.
So let’s get some facts lined up about feral cats. A feral cat is defined as a domesticated cat (Felis catus (or Felis sylvestris catus , if you prefer)) that has returned to the wild. A feral cat is not the same as a “stray” cat. Stray cats are domesticated cats that are lost or abandoned although the kittens of stray cats can, indeed, grow up to be feral cats. Feral cats typically live in colonies of three to twenty-five individuals. They are wild animals that live by consuming human refuse or by hunting small mammals, reptiles and birds (and it is the magnitude of this hunting that is the crux of the problem concerning feral cat populations!). It is estimated that there may be up to ninety million feral cats in the United States alone!
Some feral cat colonies are tended by human caretakers. These volunteers provide food, shelter and a degree of protection for the colony. Attention to disease and parasite prevention is also a part of this colony maintenance. These tended feral cat colonies are made up of animals that have very similar levels of health, vigor, and expected life spans that are seen in populations of domesticated (“house pet”) cats. A notable exception to this concerns rates of disease and mortality in kittens which are much higher in feral cat colonies than in “housecat” cohorts.
Cats are not native to North America. Feral cats, then, by definition, are alien, exotic species in our ecosystems, and they can have impacts on small mammal, reptile and bird populations that are, potentially, quite significant. The Smithsonian Institution and U.S. Fish and Wildlife Service released a study recently in which they estimated that up to 3.7 billion birds and 20.7 billion mammals are killed by cats in the United States each year. “Un-owned” cats (strays, feral cats, and barn cats) kill three quarters of these small animals while “owned” housecats are responsible for the remainder of kills.
Cats of all definitions are marvelously efficient hunters. It is this ability to hunt and kill that led the World Conservation Union to list the cat as one of the world’s one hundred worst invasive species. I have found a number of specific studies that correlate the presence of cats to the decline in the populations of many types of birds. One of the most compelling papers that I read described a seabird chick survival study set on two of the smaller islands in the Hawaiian chain. The island that had established populations of cats had a sea bird chick survival rate of 13%, while the island that had no cats had a sea bird chick survival rate of 83%. Cats are very active hunters!
An observation closer to home concerning the impacts of feral cats comes from a conversation I had a few years ago with John and Marilou McNavage. They told me (and recently confirmed that this is still the situation) that no longer had any chipmunks around their house. They correlated the lack of chipmunks to the presence of a near-by feral cat colony.
Now I have written about my two cats in this blog on a number of occasions. Two years ago I proposed “Housecat Day” as a logical (and ecologically sound) February alternative to Groundhog Day, and, as I write this, one of my cats is laying half across my computer key board and is purring so loudly that is hard to keep a train of thought going.
In short, I love cats very much! I also acknowledge, though, that they are incredibly efficient predators!
The Audubon Society endorses the American Bird Conservancy’s “cats indoors” campaign. The Audubon web site states that “worldwide cats may have been involved in the extinction of more bird species than any other cause except habitat destruction.” The American Association for the Prevention of Cruelty to Animals (“ASPCA”) states on their web site that feral cats mostly hunt and kill rodents not birds. The Smithsonian data listed above agrees with that: a cat will on average kill 5.6 small mammals for every bird. But, although the kill ratio seems to favor rodent control, the numbers of birds killed each year is still staggeringly high!
The ASPCA estimates that a feral cat without human intervention has an expected life span of about two years if it survives its time as a kitten. With human intervention and management of a feral cat colony the expected life span of the cats is ten years. The ASPCA also states that if a feral cat colony is eradicated (i.e. the cats are killed) a “vacuum effect” occurs and cats from outside the colony come into the area and rapidly re-establish the colony. Killing feral cats, then, only opens up resources for other feral cats.
The ASPCA and many other animal welfare groups advocate programs of Trapping…Neutering…and Releasing (“TNR”) as the most effective way to deal with feral cat populations (a variation on the TNR is “TVHR” (Trap-Vasectomy-Hysterectomy-Release) and there are studies that compare the relative effectiveness of each type of population control program). These types of programs have been used extensively in the United States and also in Europe. How effective are they? Studies in North Carolina and Florida showed 36% declines in colony populations in TNR treated feral cat colonies after two years and an 85% decline after eleven years. In Rome (the feral cats of the Coliseum!) there was a 32% decline in treated feral cat populations after six years.
TNR (or TVHR) works. The treated feral cat colony slowly declines in numbers and should, if there are no additional cats added to its population via stray or abandoned animals, result in the eventual extinction of the colony.
So what do I think about feral cats? Having some of these small predators in our ecosystems actually might be a benefit (we have far too many white-footed mice out there for, example, and these mice are important intermediate hosts for the bacterium that causes Lyme disease. Increasing predator control of these mice might be a way to get our local Lyme epidemic under control!), but having ninety million of these small predators out in our ecosystems is excessive. I personally could not go out and trap and kill a cat, but we need to control their numbers (via TNR and TVHR?) and also deal with the source of the feral animals, irresponsible pet owners who do not spay or neuter their cats and allow them to reproduce so excessively that the numbers of abandoned pet cats that head off into feral existences explosively increases each year.