There has been a lot of publicity given recently to a couple studies in Canada and the U.S. which allege that cats are responsible for millions of wildlife deaths yearly. It's probably true, but what the media treatments don't address is what the statistics mean. The implication in the commentary is that a vast campaign to eradicate stray and feral cats is needed. Once that task is accomplished, then we will see a flourishing of song birds and other small cuddly animals. It is a refrain that often appears in the columns of bird watcher magazines and newsletters.
It seems to me that some crucial aspects of the issue are being neglected in the general media speculation. For instance, it should be asked how many of the billion or so animals killed by cats over the past year would still be alive today if all the cats in North America had suddenly disappeared a year ago. I'm going to go out on a limb and guess that the mortality total would not look very different. Most non-predators have low survival rates and short lives and are going to end up dead in a relatively short time regardless of the specifics of predation in their environment.
Introduced predators can certainly have a devastating effect on prey populations in island environments, but in general predators do not wipe out prey species over wide areas. When the numbers of specific prey species are significantly reduced, hunting them becomes uneconomical and the predators tend to move on to other more numerous prey, or the predators themselves become scarcer if they are not sufficiently adaptable. Eliminating the top predator in any given environment generally just makes way for a new top dog (or top pathogen).
The real issue, I maintain, is habitat. Stray and feral cats are seldom a problem in rural and wilderness areas. You will find some strays around farms where they can find shelter and an occasional hand-out, but their chances of survival outside of the barn are very precarious. Life for cats in such places is short and brutal because they are prey as often as predator. In cities, of course, it is a somewhat different picture. There we have let cats become the top predator by making our urban environments hostile to any other predators. The urban environment also distorts prey populations, discouraging many song birds, but allowing others like pigeons and starlings to thrive regardless of the best efforts of the cat population.
I would be happy to see some greatly enhanced efforts to control urban cat populations. It seems like a useful and humane thing to do. However, I'm very skeptical that putting many more times the current expenditures into rounding up stray cats would result in a corresponding enhancement of urban wildlife diversity. For that to be a likely outcome, we really need to pay attention first to habitat diversity. There needs to be green spaces set aside within and close to cities which are large enough to encourage a proliferation of desirable species including healthy populations of top predators like cougars, wolves, coyotes, owls, hawks and snakes.