Wildlife increasingly hangs on in the margins, in parks and on forgotten scraps of land, which function, as it happens, a lot like islands.

And wildlife in this country must share this land with a growing population of about 84 million owned cats, and anywhere from 30 to 80 million feral or stray cats. When all of them do “what’s natural” in a fragmented natural world, it adds up. Using deliberately conservative assumptions, federal researchers recently estimated that free-ranging cats killed about 2.4 billion birds annually in the Lower 48 states, a substantial bite out of the total bird population. Outdoor cats also kill about 12.3 billion small mammals a year — not just the proverbial rats and mice but also chipmunks, rabbits and squirrels — and about 650 million reptiles and amphibians. In some cases, they are pushing endangered species toward extinction.

But here is the number that sticks in my mind: Letting my own cat, Lucky, outdoors may have consigned as many as 33 birds and dozens of mammals to death every year. If you have ever seen a cat toy with its victim, you know these are not quick, or pretty, or painless deaths. So you might expect animal welfare groups to be ardently campaigning against outdoor cats, and particularly against the care and feeding of feral or stray cats, which do most of the killing.

Instead, these groups have mainly addressed the feral cat problem with a strategy called T.N.R., which involves trapping cats, neutering and immunizing them, and then releasing them again. Scientific studies have generally found that T.N.R. is not particularly effective at reducing feral cat colonies. The practice has also come under attack from one animal welfare group: PETA has described T.N.R. as a way for shelters to put a better spin on their image, because they don’t have to euthanize as many unwanted cats. But given the number of birds and small mammals the released cats go on to kill, I question whether the Humane Society and other T.N.R. backers should call themselves “animal welfare” groups anymore.

None of this may sound as if outdoor cats are on the way to becoming socially unacceptable, although when birders and cat lovers start shouting at each other about outdoor cats, it can seem as if we are en route to open warfare. But the change in attitudes toward smoking didn’t come easy, either. The smoking analogy is also more apt than may at first appear because outdoor cats, like secondhand smoke, also threaten the health of innocent human bystanders.

Cats are three to four times more likely than dogs to carry rabies, according to the Centers for Disease Control and Prevention. They also share many other parasites or infectious microbes with humans, including roundworms, hookworms, giardia and campylobacter. When cats live outdoors it is almost impossible to predict what they will bring home next. In Massachusetts and New York, for instance, cats recently turned up infected with a worm normally found in raccoons. One owner pulled four of them, about six inches long, through her cat’s skin, “which isn’t the best idea,” says one of the Cornell University scientists who reported the cases.

Most insidiously, outdoor cats are the primary hosts of toxoplasmosis, which is estimated to infect almost 30 percent of all humans worldwide. Toxoplasmosis produces lifelong parasitic cysts in the brain, and though it is generally asymptomatic it has been linked to neurological impairments, depression, blindness and birth defects. Even in asymptomatic individuals, the infection is associated with significant loss of memory in later life, according to a study last month in the journal Brain, Behavior, and Immunity. But I’m not really arguing that outdoor cats will become socially unacceptable because they are bad for humans. Rather, I think ardent cat lovers will eventually see that the multiple hazards of outdoor living are also terrible for cats.