Recent research from Australia finds that lethal methods might actually backfire, increasing an area’s population of free-roaming cats.
While evidence of TNR’s effectiveness continues to mount, the case for the “traditional” approach to community cat management (i.e., complaint-driven impoundment typically resulting in death) grows increasingly indefensible. Of course, the very fact that the debate over “the feral cat problem” persists illustrates the point: if trap-and-kill worked, the evidence would be plentiful by now, and the debate would have ended.
Nevertheless, there are those who cling desperately and inexplicably to the perverse hope that we might be able to kill our way to a day when there are simply no more outdoor cats (including pets). A recently published Australian study, however, challenges such wishful thinking with unusually compelling findings.
Indeed, the researchers involved found that the “low-level culling of feral cats”  led not to a population decrease, but an increase in their numbers. And, because the number of cats being trapped decreased over time, it appeared the lethal efforts were actually effective.
Don’t expect a press release from the American Bird Conservancy, U.S. Fish and Wildlife Service, PETA, or any of the other organizations that continue to promote the senseless killing of outdoor cats.
Using remote trail cameras, Lazenby et al. estimated the number of cats at two southern Tasmania study sites before and after “a 13-month pulse of low-level culling” intended to “simulate the resource-effort that typically might be available to and expended by natural resource managers.” 
In fact, the resource-effort expended was anything but low-level.
Over the course of 13 months, the researchers managed to squeeze in 2,764 trap-nights*—an average of seven traps set for every day of the culling period. Each of the 26 cats trapped was, after being left in the trap for up to 12 hours or more, “euthanased by a single shot to the head from a 0.22 rifle using hollow point ammunition.”  (Lazenby et al. explain that this fate awaited only those cats lacking a microchip “or signs of lactation,” but fail to explain what alternative treatment awaited the others—or if there were any others.)
And the results of their culling efforts?
“Contrary to our prior expectations,” explain the researchers, the “minimum number of feral cats known to be alive” increased—an average of 75 percent at one site and 211 percent at the other. Moreover, “cat numbers fell, and were comparable with those in the pre-culling period, when culling ceased.”  Put a little less scientifically: the whole thing backfired.
All of which might be easily dismissed (and it’s safe to assume some TNR opponents will do exactly that) were it not for the rigorous methods these researchers employed. They developed, for example, a system “for identifying individuals with low, medium or high confidence” from their collection of trail camera photos. On the high-end were instances in which many “good-quality” photos were obtained “from several angles” and showing “distinct marks and/or other identifying features.” 
By contrast, identification derived from “poor-quality photos and/or few photos and/or poor camera angle” of cats with “no distinct marks and/or other identifying features” were considered “low-confidence,” and “not used in any analyses requiring identification of individuals.”  (It’s worth noting that just 18 of the 353 photos fell into this category.)
So how to explain the unexpected results?
Lazenby et al. offer two possibilities. First, culling removed dominant individuals, which “allowed greater access to resources by remaining cats, thus promoting an increase in juvenile survival.” At most, though, this “could have provided only a marginal boost.”
“This is because the reproductive potential of female feral cats within and around the study sites is unlikely to have been large enough over a 13-month period to produce the rapid changes in numbers that we observed.” 
The more likely explanation, write the researchers, is that “the culling sites experienced influxes of new [adult] individuals after dominant resident cats were removed.” 
Implications for Animal Control
It’s a little difficult to tell how the “low-intensity culling” described by Lazenby et al. compares with traditional trap-and-kill efforts (typically triggered by nuisance complaints) practiced by animal control agencies in this country. The density of cats, for example, is much smaller than what’s typically found in urban and suburban areas (and probably many rural areas, too).
Still, there are undeniable similarities. As Lazenby et al. acknowledge, “the low-level culling effort we used did not constitute a sustained, multi-faceted, long-term downward pressure on our study populations, which may be required if culling is to be used in programs of feral-cat control.”  Surely, the same can be said of the sort of trap-and-kill efforts that have proven ineffective for generations now.
Indeed, in a 2008 interview with Animal Sheltering, Mark Kumpf, then president of the National Animal Care and Control Association, compared the traditional approach to “bailing the ocean with a thimble.”  Imagine, in light of this recent study, a slightly revised analogy: as each thimbleful is removed from the ocean, a cup of water is added.
* A trap-night is, as the name suggests, defined as one trap set for one night. Setting 10 traps every night for one week, then, would constitute 70 trap-nights.
1. Lazenby, B.T., N.J. Mooney, and C.R. Dickman, Effects of low-level culling of feral cats in open populations: a case study from the forests of southern Tasmania. Wildlife Research, 2015. 41: p. 407–420.
2. Hettinger, J., Taking a Broader View of Cats in the Community, in Animal Sheltering. 2008. p. 8–9.