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13.7: Cosmos And Culture
Sun February 3, 2013
Do We Really Know That Cats Kill By The Billions? Not So Fast
Those headlines reported a startling result from a new study in the journal Nature Communications: free-ranging domestic cats in the United States kill many billions of birds and small mammals per year, far more than previously thought. Many cat owners, Williams reported, took umbrage. He featured a photograph of a cat from Rhode Island named Magoo who had sent in to the NBC studios a protest note ("I am not a bird murderer; don't judge me"). Williams commented that the bird community has so far been silent, possibly because of its "decimated" numbers.
In fact, the situation is no laughing matter. Cats are hunters and other creatures do fall prey to them in significant numbers.
And yet there are serious reasons to suspect the reliability of the new, extreme cat-killer statistics.
The study at issue is a meta-analysis, an overarching review that aggregates data from previously published sources. The accuracy of meta-studies in health and medicine raises some concern, and it's easy to see why: for a meta-analysis to be solid, wise choices must be made among the available sources of information, and results that may vary wildly must be weighed fairly.
In the Nature Communications study, authors Scott R. Loss, Tom Will, and Peter P. Marra needed to incorporate into their model the number of "un-owned cats" (such as stray, feral, and barn cats) in the U.S. As they note in an appendix to the article, "no empirically driven estimate of un-owned cat abundance exists for the contiguous U.S." Estimates that are available range from 20-120 million, with 60-100 million being the most commonly cited. In response to this huge uncertainty in the numbers, they performed mathematical calculations using what they feel to be a conservative figure (specifically, they "defined a uniform distribution with minimum and maximum of 30 and 80 million, respectively.")
At this juncture, the authors note that local analyses of cat numbers are "often conducted in areas with above average density." That is an obvious problem, yet when they estimated the proportion of owned cats with access to the outdoors (and thus to hunting), of eight sources of information, "three [were] based on nationwide pet-owner surveys and five based on research in individual study areas." Are the local studies representative of the national situation? For that matter, are the different owner surveys administered in a consistent enough manner to allow them to be aggregated?
Of course, the authors make statistical perturbations designed to increase the reliability of their conclusions, but it seems to me there's an unsettling degree of uncertainty in the study's key numbers.
It seems this way to others also.
Wayne Pacelle, president of the Humane Society of the United States, had this to say in response to the study: "It's virtually impossible to determine how many cats live outside, or how many spend some portion of the day outside. Loss, Will, and Marra have thrown out a provocative number for cat predation totals, and their piece has been published in a highly credible publication, but they admit the study has many deficiencies. We don't quarrel with the conclusion that the impact is big, but the numbers are informed guesswork."
If even animal advocates admit "the impact is big," why do the specific numbers matter so much? Because when people start thinking of cats primarily as murderers, it then becomes the cats' lives that may be seriously endangered. Of concern are not only extremists like the man in New Zealand who recently suggested a ban on pet cats; cat advocate organization Alley Cat Allies says that the study is so "biased" that it amounts to an invitation to "ramp up the mass killings of outdoor cats."
As a cat rescuer, I know such threats to outdoor cats are real. I've heard them. And as a cat person, I also care very much about the lives of birds and small mammals, taking steps in my own life to reduce our cats' predation upon them. The truth is that we do need to better understand the relationship between cats and the greater natural world.
Demonizing cats with shaky statistics, however, won't help us build the pillar of understanding required to strike a satisfying balance between the needs of cats and their supporters with the needs of wildlife facing a feline threat.
You can keep up with more of what Barbara is thinking on Twitter: @bjkingape