About a decade ago I had a student in my Conservation Biology class named Scott Boomer. We were discussing the problem of non-native organisms that week, and Scott told me he had kept some interesting records on the behavior of three cats that he and his wife had at that time. The cats (one male, two females, and all neutered) had access to food and water in Scott’s apartment around the clock. The three natural predators had the habit of capturing prey outside and bringing it back to Scott, often dropping it at his feet or putting it in their bathtub. Scott is a biologist and he was able to identify all the prey items returned to his apartment over a 2-year period.
The list included:
Mammals: 5 deer mice, 2 woodland jumping mice, 5 Eastern chipmunks, 4 meadow voles, 1 gray squirrel, 6 star-nosed moles, 4 short-tailed shrews, 1 cinereus shrew, 2 little brown bats, 1 Eastern cottontail
Amphibians: 2 green frogs
Reptiles: 1 Eastern painted turtle, 3 Eastern garter snakes
Birds: 3 common yellowthroats, 2 black-capped chickadees, 1 house wren
Total: 43 animals
Now, there are about 90 million cats in the U.S., according to the 2005-2006 National Pet Owners Survey. A certain percentage of those cats never go outside. But anyway you run the numbers, the collective mortality on native wildlife by U.S. cats must total millions of individuals of dozens of species. In some places in the world, feral cats, those that have gone completely wild, are responsible for the demise of rare species of birds. The Stephens Island wren (a flightless species) in New Zealand went extinct in the late 1800’s due to the island’s cats, or so that story goes. The wedge-tailed shearwater in Hawaii is also impacted by cats. Conservation biologists actively control cats (as well as non-native rats, mongoose, etc.) in such places today, especially on oceanic islands.
We hear a lot these days about our “ecological footprint”, or the impact that a human has on the earth’s natural resources and ecosystems. I doubt that our pet ownership is included in these calculations. Remember that I tend to think in terms of quantity and quality of habitat for biodiversity. I usually think of our “habitat footprint” as defined by the boundaries of our house and the lawn surrounding it. But the effect of that living space penetrates further depending on the chemicals we use on the property, how far away we or our children trounce on the environment, and the influence of our pets, of which cats are probably the worst offenders. There are zones of concentric circles beginning with the epicenter of the house itself, which include areas of decreasing influence on the fauna and flora that is there now, as one moves respectively outward. Cats probably have an effect in each of those zones, but they may represent the only threat in the outermost circle, which is perhaps several hundred meters from the edge of the house.
In fact, last year someone built a new house about 100 meters from the edge of my woodlot. For the past few months, I have had two cats roaming my property that I am sure live at that house. I have not had cats on my property in 20 years. And so it goes. We increase our collective ecological footprint, we chip away at the quantity and quality of wildlife habitat and, in my opinion, the quality of life is diminished just a little bit more---again.
In these few paragraphs I wanted to increase your awareness of an idea that perhaps you have not thought much about--- how that lovable pet cat of yours is possibly reducing the biological diversity in your neighborhood. I do not intend to explore a detailed solution to this problem, although attaching a simple bell to your outdoor cat would probably reduce its kill rate. You might be thinking that cats kill organisms that people do not like very much anyway, so what the heck. But I can assure you that every one of those species killed by Scott’s cats represents a unique and interesting biological story. Remember that not so long ago, nearly everyone thought it was fine to shoot, trap, or poison wolves, mountain lions, and eagles.