A stray cat on April 8th, 2009. (Photo: Alexander Nemenov/AFP/Getty Images)

When we consider threats to ecological stability, cats rarely leap to mind. But recent research indicates they should. Cats just made the list of the world's top 100 invasive species, alongside nutria and Burmese pythons On islands they are especially ruthless, accounting for 14 percent of bird, mammal, and amphibian extinctions. Annually, they kill up to 3.7 billion birds (two-thirds of them native), nearly 20 billion small mammals (which harms owls and hawks), and nearly 300 billion amphibians—all on the United States mainland alone. The authors of the most comprehensive study we have on the topic of feline destruction write that the environmental impact of Felis catus "far exceeds all prior estimates."

Feral cats—of which there are 70 million in the U.S.—(and house cats allowed outside) are primarily to blame for this swath of destruction. "There is little question that free-ranging cats," according to Peter P. Marra and Chris Santella, authors of the recent Cat Wars: The Devastating Consequences of a Cuddly Killer, "pose a pending ecological and public-health disaster."

For most invasive species the answer to this ecological dilemma would be simple and sangfroid: kill them. Think about feral hogs. Although the battle against this especially aggressive invasive animal is hardly going well (pigs easily outsmart us), the hunting community has, in a collective and unmitigated froth, chased down these savvy beasts with semi-automatic weaponry fired from helicopters (see Helibacon)—all in an effort to reduce their numbers. Few object to these methods.

In terms of the hunt, feral cats would be a less formidable opponent than hogs. If we wanted to exterminate cats, we very likely could. But, of course, there'd be an uprising. Cats are our most popular pets. They purr, solicit our affection, and curl into our laps. Cat ownership can be healthy, lowering the risk of depression, stroke, and heart disease. Hardened criminals who have been allowed to care for cats have become non-violent sweethearts. There is such a thing as an "international cat video festival." These animals, ecologically destructive as they are, are deeply meaningful to tens of millions of Americans—whether they own cats or just watch them on YouTube.

The conflict over free-range cats is diffuse, occurring in virtually every community in the country. This is perhaps why it rages in relative quietude. But, as Marra and Santella confirm, passions over the cat run deep. And nowhere is the emotion more manifest than in the debate between bird watchers and cat lovers.

There are an estimated 60 million bird watchers in the U.S. It's safe to say that most of them oppose the carnage perpetuated on bird life by feral cats. As one fan of piping plovers explained to the New York Times, "The American taxpayers spend millions of dollars to protect birds ... and yet here are these cats killing the birds, and nobody's doing anything to stop it."

Advocates of feral cats—there are about 90 million cat owners—are quick to fight back. They argue that their favored animals are only doing what they're meant to do—hunt—and it's unfair to punish them for exercising that natural instinct. As the cat advocacy group Alley Cat Allies (which dubiously calls cats "citizens of the natural landscape") insists, "every cat deserves a safe community in which to live."

Cat Wars: The Devastating Consequences of a Cuddly Killer. (Photo: Princeton University Press)

Given the depth of our respective loyalties, finding an expert who thinks critically—and fairly—about this problem is no small task. But Deborah Rivel, who recently co-authored Birdwatching in New York City and on Long Island, has long pondered the battle between cat and bird lovers from both sides. More important, she had a personal experience that clarified her thoughts on what she acknowledges to be an immensely complicated problem: She managed feral cats on her mother's New Jersey property. As an avid bird lover (when we spoke she had just returned from a bird watching trip to Thailand), she is rare in that she has seen the feral cat problem from both perspectives.

Rivel's mother, now deceased, had made the innocent mistake of feeding one stray cat prowling her large yard. A year later there were a dozen stray cats prowling her yard. And they were having babies. Rivel, who knew these animals were killing an abundance of birds, also knew they were just doing what came naturally to them. She responded in a way that she knew was far from ideal, but, without other alternatives, was better than doing nothing. Following the Humane Society's recommendation, she trapped, neutered, and released (TNR) them. The work was arduous (you know, herding cats) and it took decades. It also involved careful monitoring and ongoing trapping of the population, making sure new cats did not join the colony. In other words, practically a full-time task.

Eventually, when all the cats were neutered, a "population crash" occurred. Rivel is not entirely sure why this happened. She knows that some cats were hit by cars, others felled by disease, and still others may have been poisoned. Summing up the experience, Rivel says: "Nightmare, truly, for all parties—humans, birds, and cats." But when she sold her mother's home, the population of cats was, to her surprise, back down to one.

The ultimate lesson for Rivel was twofold. For one, she better appreciated cats and openly acknowledged their appeal. While she may not agree with Alley Cat Allies that cats are "citizens of the natural landscape," she certainly understands why cat lovers would feel the way they do. They are charming, tenacious, and intriguing animals. We love them for good reasons.

But second, as she also learned, our affection may not justify their unfettered existence. Rivel's intimate and extended experience with feral cats led her to realize how destructively misplaced they were in an ecosystem where their prey completely lacked the defense mechanisms to protect themselves from this non-native introduction.

"Birds comprise many of the building blocks of the balance of nature," she says. "They each have their own purpose. Some eat insects, others pollinate, and others eat seeds and deposit them elsewhere." When domestic cats—animals that have never lived in a wild ecosystem—enter this environment, this critical ecological work is compromised. "With all the pressures placed on them—many of them human induced—including habitat destruction, the birds," she says, "really don't have a chance when feral cats are added to the mix."

Rivel knows as well as anyone that conservationists can never perfectly manage an ecosystem. Humans have irretrievably altered nature to the point that "natural" is almost a meaningless term. But she also knows that we should address the problems that a. create the most obvious ecological imbalance and b. can be addressed. After her experience managing the colony at her mother's house, Rivel now understands through direct experience that the scientific consensus on the cat problem is undoubtedly correct: These animals are fascinating. But—even with TNR—they are dangerously invasive.

What to do about the problem is an altogether thornier issue.

"The problem with cats," Rivel says, "is people." When people abandon their pets they contribute to the problem of colony proliferation. Likewise, she explains, "pet owners who let their cats run around outside must take responsibility for their pets and keep them inside, where they are neither harmed nor do harm." When I noted how difficult this task could be (I once had a cat that yowled endlessly when inside) she mentioned the "catio"—an extended screened-in-porch that allows felines to be closer to the real outdoors without the opportunity to kill (or be killed).

Birdwatching in New York City and on Long Island. (Photo: UPNE)

As for TNR, Rivel is hardly alone in doubting its effectiveness. While Rivel reluctantly returned the cats in her mother's yard after trapping and neutering them (and after trying to train and adopt some of them out), her "success" may have been an anomaly. Citing a range of studies, Marra and Santella argue that "there is the uncomfortable truth that TNR has repeatedly been shown to fail to reduce free-ranging cat populations."

Yet, even if people behave, even learn to keep cats indoors, and even if we become fanatical about spaying and neutering, the ecological devastation, although perhaps moderated, will persist. The problem with cats, in other words, is also cats. "Some cats will not come to traps," Marra and Santella write, "and will need to be removed from the landscape by other means."

And this is where we need to take a deep breath and decide exactly how proactive we want to be about addressing a recognized and well-documented environmental problem. Others have done it. On Marion Island, in the Indian Ocean, 3,500 cats killed a half million petrels—a native bird—after five of the felines were introduced in 1949. Through the use of poisoned bait, the purposeful spread of the disease panleukopenia, and human hunting, the Marion Island Cat Eradication Program was able to rid the island of feral cats between 1977 and 1991. If it we want it, in short, we can have it. Eradication works.

As a final point, it's worth noting an unlikely ideological convergence. Conservationists have an airtight case for eliminating cats. It is, however, often assumed that animal rights advocates are their direst opponents. But a powerful wing of the animal rights movement also agrees that all domesticated animals should not exist—including cats. "Continued domestication," the animal rights philosopher Gary Francione says, "is not morally acceptable." As he writes in an email: "I am opposed to continuing to produce domesticated nonhumans for any human purpose."

Seems that the last step is the most difficult and significant: We need to figure out how to get rid of the ones that remain outdoors.