As a family, cats have successfully colonized the Americas, Europe, Africa and Asia, with only Australia among the larger landmasses lacking native species. They range in size from the immense Siberian tiger to the diminutive black-footed cat of southern Africa, and take an equally diverse range of prey. Much of their success is perhaps due to their nocturnal or crepuscular habits, as well as them having little to fear from natural predators -- other than members of their own family. But Man is their nemesis.

This said, one of the family's members, the domestic cat (Felis cattus), has benefited so considerably from human attention that it has now spread to almost every part of the globe. The only places in which I have not encountered them are the high Himalayas and Antarctica.

From the cliffs of the Scottish Shetland Islands to the forests of New Zealand to downtown Tokyo, and countless places in between -- cats are everywhere. At any awards ceremony for global success, the domesticated moggy would stand up, take a bow and be hailed a winner.

Although comprising only one species, Felis cattus comes in three rather different forms. The familiar, house-friendly cat we should dub the pet cat, for it is a stay-at-home feline fond of creature comforts. Then there is the semiferal type, willing to allow its "owners" to believe they really do possess it, yet taking what they offer, using them as a central base and wandering the neigborhood at will. Finally, there is the truly feral form, the "domestic" cat that has turned its back on human society and gone to live a more honestly catlike existence, living entirely off its hunting skills.

I have encountered these elusive feral beasts repeatedly on my travels, but perhaps most often around seabird colonies, such as along the cliffs of Hokkaido's Teuri Island and on the Scottish island of Yell. Introduced to islands that lack indigenous terrestrial predators, these canny hunters wreak havoc among species that have no instinctive response to them.

In short, the domesticated cat, a secretive alien predator, is more successful than any of its larger cousins. After all, neither the wide-ranging leopard nor the cougar has succeeded in crossing the Pacific; Felis cattus has.

Tell a domestic cat lover that they are harboring an alien killer and you are unlikely to score any Brownie points, but there's no point beating about the bush -- especially if that bush is hiding a hunter. The statistics attesting to Felis cattus' success, in those countries where they have been studied, are staggering.

In a 1997 survey, the Mammal Society of Britain found that the nearly 1,000 cats they studied killed more than 14,000 mammals, birds, reptiles and amphibians during spring and summer alone -- including several rare and even protected species such as dormice and water voles. On that basis, Britain's estimated 9 million pet cats are likely to be killing some 250 million creatures a year.

As horrific as these figures are, however, they exclude innumerable prey items not brought home by the cats, and the victims of an estimated 800,000 stray and feral cats. Free-ranging domestic cats in the United States are similarly implicated in the deaths each year of an estimated 1 billion small mammals and hundreds of millions of birds.

Numbers like these suggest that domestic cats are significant small-mammal predators. Of course, the more common garden birds are high on their hit list, but an astonishing variety of species was reported, from woodpeckers to swallows, while some moggies even take bats by waiting at their roost sites. However, warmblooded creatures are not the only victims, with frogs the most common prey among a range of reptiles and amphibians killed by cats.

The British survey also revealed the crux of the problem. A well-fed cat is fit, has time on its paws and still retains the highly honed killing instincts of its wild cousins.

Alas, many of these highly effective killing machines are now kept in countries or regions lacking other native predators. There, they are not only an additional predation pressure -- they may be the only one, in an island region that once lacked them. So, for example, the islands of New Zealand, and the Japanese archipelago (except Iriomote-jima and Tsushima) lacked indigenous cats, thus giving the introduced moggy carte blanche to eat their way through species genetically unaccustomed to their habits.

Meanwhile, the same survey also showed that the amount of food supplied to a cat made no difference to its kill-rate, although age and obesity did, with fat cats and older cats catching fewer prey than lithe youngsters. Whereas the populations of wild predators will fluctuate in relation to the availability of prey, domestic cats are buffered from such food-supply effects. Even if a preferred species of prey becomes scarce, a cat receiving its main meal "at home" will survive to hunt the dwindling prey. A wild predator, under similar circumstances, would not. The presence of hunting cats not only has an impact on prey species, but also creates competition with native predators such as hawks, owls, weasels, martens and polecats. Wandering cats have also been implicated in the transmission of disease into wildlife populations.

The domestic cat's propensity for murdering mammals can be curbed by a nocturnal curfew, as most kills are made at night. However, birds are readily taken during daytime. In this respect, though, some relief is in sight, as sonic cat collars have been shown to reduce bird predation by as much as 65 percent.

The range that cats wander from their home base varies considerably. In Australia, for example, suburban cats have small daytime ranges, but much wider nocturnal ranges of up to 1 km from home. Clearly the wide-ranging cat has a free rein to impact native creatures. In Britain, the significance of domestic cats' predation pressure is evident from the population figures; they outnumber native red foxes by 38 to one, and there are six times more cats than all other native terrestrial predators together.

In an island country such as Japan, where actual figures are not available, we can take heed from the alarming figures from New Zealand, Australia and Britain. Nonetheless, for some odd reason they are the only pets that are legally allowed to roam free. For the sake of millions of native creatures, surely it is time for a change in the law to control these alien predators.

The jury is still out on questions of domestic cat predation. Particularly moot is whether feline hunting leads to equilibrium with the prey base, or whether cats may selectively predate some species to extinction. There are probably as many answers as there are types of cats and environments they live in.

In the meantime, a gram of prevention is worth a kilo of cure, and if you are a cat "owner," but wish to play a responsible part in protecting wildlife, there are measures you can take. Enforce a cat curfew; lock the catflap at night and provide a litter tray. Fit your cat with a collar and a bell or, better still, a CatAlert sonic collar, as these will reduce its hunting success. Keep your cat stimulated and provide it with toys to reduce its boredom and so curb its recreational killing -- also show it plenty of affection. If you are a bird-loving catowner who also feeds birds, make sure your bird table is catproof and sited where cats cannot make surprise attacks.

Finally, to find out more about preventing your pet from eating wildlife, check the following Web addresses for some useful tips: American Bird Conservancy's, Cats Indoors Campaign, and the Canadian Cats in Kennels Program. Finally, ensure that your cute pet is neutered so that it cannot go bush and add further to the ferocious ferals.