Should urban housecats be considered an invasive species? Maybe! Natalie Angier has a terrific story in the Science section of the Times today:
In the view of many wildlife researchers, a pet cat on a lap may be a piece of self-cleaning perfection, but a pet cat on the loose is like a snakefish or English ivy: an invasive species. Although domestic cats have been in this country since the colonial era, they are thought to be the descendants of a Middle Eastern species of wild cat, and there is nothing quite like them native to North America. As a result, many local prey species are poorly equipped to parry a domestic cat’s stealth approach. “People fool themselves into believing that by simply putting a bell on a cat they could prevent mortality to birds,” Mr. Schroeder said. “But a bell ringing means nothing to a bird.”
Moreover, free-ranging domestic cats are considered subsidized predators. They eat cat food at home, and then hunt just for sport, a strategy that allows them to exist at densities far greater than carnivores achieve in nature. “It’s estimated that there are 117 million to 150 million free-ranging cats” in the United States, Dr. Marra said. “They’re the most abundant carnivore in North America today.”
In particular, housecats that are allowed to prowl freely outside can inflict heavy damage on local bird populations—baby birds crouching in the bushes don't stand much chance—but alas, contrary to myth, they don't seem to have much of a taste for rats.