This is a summary of a speech given by Merritt Clifton, editor and founder of US based advocacy group, ANIMAL PEOPLE.
Killing cats, dogs, and other mammals in a futile attempt to achieve permanent population reduction is an approach repeatedly attempted by just about every government of every nation on every continent, sometimes on a continuous basis since the Middle Ages, when cat pogroms helped to accelerate the spread of the black rats whose fleas carried bubonic plague. Even after the Black Death killed a third of the human population of Europe, the fallacy of attempting to exterminate cats was not understood, and the civic officials of London repeated the same mistake about 300 years later”.
No extermination programme directed at any fast-breeding mammal species such as dogs, cats, coyotes, deer, rabbits, pigs, rats or mice has ever achieved more than short-term results in a mainland habitat.
This is because two ecological laws work against successful extermination:
1) Nature abhors a void. Open a habitat niche by exterminating the occupants, and something will promptly fill it.
2) Mammals raise litters of size varying according to food availability. This was one of our major evolutionary advantages over the dinosaurs and birds, whose egg clutch size was and is more-or-less fixed at a relatively low number. Among mammals, lowering food competition accelerates the fecundity of the surviving population. Larger litters are born; more of each litter survive. Birds which might compete with some of the mammals for the habitat simply cannot reproduce as rapidly to fill a void -so what happens is that exterminating the mammals usually just results in proliferation of their major prey species, such as mice and rats, followed by reoccupation of the habitat by more of the same species of mammalian predators who were just exterminated, moving in from other areas.
The New York City animal control statistics offer an excellent long-term illustration. From 1895, when records first were kept, until 1962, no U.S. city more vigorously exterminated stray dogs and cats. Yet the number of dogs and cats killed rose every year, topping 100,000 for the first time in 1908 (after approximately 75 years of killing strays and 13 years of record-keeping). The New York City numbers continued to rise each and every year, peaking at 250,000 in 1962 and remaining at that level until 1966.
Every year, no matter how many animals were killed the year before, more were found at large to kill. San Francisco also began keeping records of animal control killing in 1895, and saw the same trend. Record-keeping started much later in most other cities, but not one U.S. city of any size ever achieved a lasting downward trend in dog and cat killing or in stray dog and cat pickups until more than 30 years after the last major U.S. outbreak of canine rabies, which occurred in the late 1940s and early 1950s.
In 1957, Friends of Animals started the first low-cost dog and cat sterilization project in the U.S. in the New York City area. After 10 years of effort, it was fixing enough animals per year to stop the growth of the stray population, and started branch programs in other parts of the country.
The most important lesson here is that despite the obvious fact that it is less expensive to kill any one animal than to vaccinate and sterilize the animal, you can kill animals to infinity and not get rid of large free-roaming populations.
Street dog and feral cat populations can be eliminated – by sterilizing them, and allowing them to hold their habitat with diminished reproductive capacity while addressing the conditions that permit them to proliferate. In the long run, the only really effective way to eliminate street dogs is to eliminate their food sources by improving public sanitation, introducing refrigeration, and getting rid of uncovered trash dumps.
As long as you have rats, open-air disposal of either animal or human feces, decomposing animal carcasses in the streets, and large amounts of easily accessible food waste, you will have street dogs, because you will be maintaining the conditions which are conducive to their reproduction.
Almost the same observations pertain to cats. In regions with abundant street dogs, feral cats tend to be few. They live on rooftops and are mostly nocturnal, because dogs outcompete them for the ground-level daytime food sources–and dogs also control the feral cat population by killing cats, especially kittens.
When you eliminate street dogs, however, cats claim the habitat. In warm climates, cats have approximately twice the reproductive capacity of dogs. If you think you have a lot of dogs to deal with now, just wait and see what happens should you manage to reduce the dog numbers substantially without doing anything to eliminate the food sources and slow the fecundity of cats.
Perhaps the best-known study of cat predation, and the study most often cited out of context by people who want to blame cats for vanishing birds, was published by the British-based Mammal Society in February 1998. To produce that survey, 800 British cat owners recorded their cats’ kills for six months–for roughly 144,000 cat-days of activity. Among all those cats, the most active killer was Missy, with 125 kills in 180 days, including 28 birds. Almost all the rest were mice, voles, and other small rodents. The runner-up was Kipper, with 82 kills in 180 days, including six birds. That’s 34 birds in 360 cat-days, by the most predatory cats (by far) among the entire sample base. Those most skilled of feline killers managed to kill birds at a rate amounting to just 16% of their total prey, and succeeded in killing a bird on only 9.4% of the days they hunted. Even at that, cats are rarely the primary cause of the death of the birds they catch. Instead, they pick off the sick, the injured, and the elderly; sometimes the young of ground-nesting species.
The importance of disease as a causal factor in “cat kills” of birds has only just begun to be recognized. A landmark in that regard was published in the June 3, 2000 edition of The Economist by researchers Anders Moller and Johannes Erritzoe of the Universite Pierre et Marie Curie in Paris.
After examining the spleens of 500 birds who had been killed by cats, were killed in collisions with windows, or were hit by cars, they reported in that the spleens of the birds killed by cats were a third smaller on average, in 16 of 18 species, than in the birds killed in accidents. In part this was because 70% of the cat-killed birds were juveniles; only half of the others were. But a more important factor, they suggested, was that “Birds succumbing to lots of infections, or inundated with energy-sapping parasites, have smaller spleens than healthy birds.”
In short, the interactions of cats and birds are very, very complex, and deserve much more serious study, not least because the outdoor free-roaming cat population–contrary to what many bird-lovers believe–is now declining even faster than neotropical migratory songbirds are, and has been for approximately a decade. When free-roaming cat populations decline, coyotes take over some of their prey sources. Hawks and owls tend to take the rest. But hawks and owls breed relatively slowly, producing a maximum of two young per pair per year and usually fewer. If you simply kill cats, instead of making more prey available to avian predators, you create a habitat void which lures in more cats. If instead you sterilize cats, their numbers decline over time more-or-less in step with the reproductive capacity of hawks and owls to prevent a void, and then the replacement of cats as a non-native predator with native bird species can be successful.