Earlier this week, I published an article remembering a cat I once owned and loved named Lucky. She was my last outdoor cat, partly because her own death was a bloody reminder of just how dangerous the outdoor life can be for the cats themselves: She died one night, torn to pieces by a bobcat, after 10 years of wandering freely around the neighborhood.
But she was also my last outdoor cat because I realized, after the fact, how deadly outdoor cats can be for wildlife: By letting Lucky roam at will, I had made it possible for her to kill hundreds of birds, mammals, reptiles, and amphibians over the years. While writing about wildlife and maintaining my own yard with wildlife in mind, I had unintentionally been stripping wildlife from the entire neighborhood.
Other cat owners are increasingly coming to the same grim realization—in part because of a federal study last year that added up the billions of animals killed by cats every year in the lower 48 states. I argued in my article that outdoor cats will soon be as socially unacceptable as smoking in the office or leaving dog poop on the sidewalk. The editors headlined it “The Evil of Outdoor Cats,” and it attracted widespread attention on the Web, some of it angry. One reader commented that a better headline would have been “The Evil of Humans.”
A lot of readers misunderstood a central point I was trying to make about the dramatic decline in bird populations in this country, and about the loss of habitat. Readers commented correctly that the real menace to wildlife comes from suburban sprawl, agricultural intensification, logging, mining, industrial pollution, and climate change. But a lot of cat owners seemed to think that that was an argument for continuing to let their cats go outside to kill. “Whatever damage cats are doing,” a reader in Seattle commented, “it can only be a small fraction of the many human-caused threats to wildlife, in particular habitat destruction.” It was sort of like arguing that because there are wars going on out there, my little murders shouldn’t count.
In fact, killings by outdoor cats matter far more than they might in a healthier world. My basic argument was that there’s hardly any place left for birds and other wildlife to live, and what habitat remains is often heavily fragmented. That makes animals far more vulnerable to predation by cats. Several readers who are scientists pointed out factors that make the effect of cat predation far worse than I had suspected.
The first is what biologists call “mesopredator release.” When humans remove wolves, coyotes, and other large predators from a habitat, the midsize (or "meso-") predators they would otherwise kill or out-compete suddenly flourish. It’s boom time for skunks, raccoons, and especially house cats—which then go on to kill birds and other small animals in huge numbers.
Creating protected habitat, as that same Seattle reader suggested, doesn’t fix this problem. A 1999 study of mesopredator release looked at what happened when coyote populations declined and habitat became fragmented in southern California. The most important effect was indirect: The 46 percent of cat owners who sometimes kept their cats indoors because they didn’t want them to be eaten by coyotes no longer had to worry about that.
Around one habitat fragment in San Diego, 77 percent of neighboring cat owners let their cats wander outdoors—and almost all of those cats were killers. The math was devastating for wildlife: 35 cats from 100 nearby houses hunted in a 49-acre parcel of land. Some bird species hung on in that habitat with as few as 10 individuals. But the cats were bringing home 525 dead birds a year and probably leaving many more tattered remains in the bush. Local extinctions were inevitable.
The other factor I failed to recognize was something biologists call “hyperpredation.” In a natural habitat, the predators tend to live by boom and bust. One year the foxes get fat because there are so many chipmunks to eat. The next year, there are too many foxes and too few chipmunks, and the foxes starve. But pet cats, and colonies of feral cats, don’t live like that. We feed them, and we maintain them at densities many times higher than what’s normal for foxes or all other similar-sized natural predators combined. When the chipmunks or sparrows go bust, the cats just go home for dinner. Then they come out again next day to hunt some more. It’s a miracle anything else manages to survive.
In the light of what I have learned from readers, I am beginning to think it’s time to re-name the popular practice of trying to control feral cat populations with TNR—that is, by trapping them, neutering them, and then releasing them again into the wild. A better name might be TNRH, for “trap-neuter-release-hyperpredate.”
But let’s save that hot topic for another column.