It’s difficult to imagine now but when I started this blog—10 years ago Monday—I worried that I’d soon run out of material to write about. Not only is there no shortage of material, but these days—when facts are up for grabs and media accounts are prone to false equivalencies cultivated in the name of “balance”—Vox Felina’s mission seems more urgent than ever.

Something else that hasn’t changed over the past 10 years: the protectionist practices of editors green-lighting the publication of badly flawed research.

Perhaps it’s oddly appropriate, then, that the impetus for this post is the same one that prompted the blog’s inaugural post—and, indeed, the blog itself. Once again, my response to an article was quickly rejected by a journal.

Authors of the article, which was published recently in Animal Conservation, boldly claim that “pet cats around the world have an ecological impact greater than native predators but concentrated within ~100 m of their homes.” 

Perhaps “bold” isn’t even the right word here. After all, the authors didn’t even document predation in any real sense. Instead, they asked survey participants to recall what sort of prey their cats brought home in the past and how often.

This was only the beginning. Through a series of analytical blunders, the authors quickly inflated their predation estimates by a factor of 10 or more.

The worst of it, though, was their conclusions about “ecological impact.” One would imagine that to arrive at such a conclusion, researchers would need to have a reasonably good understanding of the animals (or plants) being affected. Here, the authors didn’t even mention which species were affected—never mind the extent to which that was case.

Lacking empirical evidence, they attempt to make their case via proclamation. And it worked, at least as far as the journal’s reviewers and editorial staff were concerned.

My letter in response (see below), on the other hand, was promptly batted away. As the journal’s editorial office explained, “the key references are out of date and many of the arguments are purely speculative rather than the result of re-analysis.”

Purely speculative?

This from some of the same individuals who published an article all about the “ecological impact”—on no species in particular. Based entirely on the recollection of pet owners. The underlying analysis of which ignores previous research findings and violates the basic tenets of descriptive statistics. All of which escaped the attention of the paper’s 13 co-authors, as well as the journal’s reviewers and editorial staff.

As I say, Vox Felina’s mission seems more urgent than ever.

My letter to Animal Conservation

In their recently published article, “The small home ranges and large local ecological impacts of pet cats,” Kays et al. [1] conclude that “pet cats around the world have an ecological impact greater than native predators but concentrated within ~100 m of their homes.” For the curious reader, there is a lot to unpack here.

The reference to “ecological impact” (a term the authors use no fewer than 18 times) alone raises several questions. These impacts are presumed to be direct, the result of predation. But which species are affected, and to what extent? In what context? Without such specifics (as a starting point anyhow, setting aside for the moment any possible indirect effects—positive or negative—complex trophic interactions, etc.), such a sweeping claim is at best conjecture.

The authors’ claim is further undermined by weaknesses in their analysis, which overlooks multiple key factors. There is, for example, overwhelming evidence that prey return data are highly skewed, with relatively few cats returning numerous prey items while most cats return relatively few [see, for example, 2–6]. Indeed, the data compiled by Kays et al. [1] confirms this, with roughly 51% of cats bringing home no prey (Fig. S5). As Barratt [3] reported, “median numbers of prey estimated or observed to be caught per year are approximately half the mean values, and are a better representation of the average predation by house cats based on these data.” By using a mean predation rate, then, Kays et al. [1] effectively inflated by a factor of two the true level of predation occurring among their study cats.

Histograms summarizing survey responses regarding prey returned by pet cats (Fig. S5 from “The small home ranges and large local ecological impacts of pet cats”).

Barratt [3,7] also observed that predation rates estimated by 143 suburban Canberra cat owners (214 cats total) were, generally speaking, more than double those based on actual prey return data. Several years later, McDonald et al. [8] observed a similar tendency in a smaller study of 38 cats owners in two UK villages (86 cats total). The predation rate reported by Kays et al. [1] is based on a survey of cat owners, not actual prey return data. This suggests that the tendencies observed by Barratt [3] and McDonald et al. [8] likely apply to their findings as well, thereby doubling again the cats’ true predation rate.

Kays et al. [1] then apply “an adjustment factor for animals killed but not eaten (multiplied by 1.2 or 3.3).” However, the higher value (i.e., 3.3) was based solely on observations of cats successfully (or not) hunting small mammals. Specifically, Kays and DeWan [9] observed 31 attempted hunts (by 12 cats), eight of which “resulted in a capture (all small mammals, 26% capture rate)” (emphasis mine). Even setting aside the shortcomings associated with such a small sample, it should be clear that applying this adjustment factor to all prey items is quite inappropriate.

Combining these factors, the resultant predation estimates reported by Kays et al. [1] are very likely inflated by an order of magnitude, thereby challenging the authors’ assertion that “the density of cats yields levels of predation 2–10 times that which would be expected due to wild carnivores” [1]. The comparison strikes me as peculiar, not least because wild carnivores are relatively scarce (certainly compared to domestic cats) within 100 m of most homes. More to the point, though, there is still the question of “ecological impact”—home range and overall predation estimates alone tell us little about the cats’ impact. Again, to make such a determination, we need to know much more about the species being hunted and various key characteristics (e.g., abundance, densities, reproductive capacity, population trends, etc.); Kays et al. [1] do not even provide a breakdown by taxa.

In a 1936 Journal of Mammalogy article, Paul Errington [10] argued that “any study of the house cat intended thoroughly to divulge its ecological status would have to answer not only the question of what the cat eats, but the actual effect its preying may have upon the species in question” (emphasis mine). The “burden of proof” is no different 84 years later. However, the conclusions drawn by Kays et al. [1] are based not on “actual effects,” only vague predation estimates— which, as I’ve pointed out, are grossly inflated. It’s difficult, therefore, to see how the publication of this paper contributes much of value to the body of literature.

On the other hand, it’s not at all difficult to imagine how “The small home ranges and large local ecological impacts of pet cats” will be used to shape policies regarding the management of free-roaming cats (owned and unowned alike), already a highly controversial topic [see, for example, 11–13]. Indeed, its accompanying media release [14] quickly gave rise to the headlines “Killer Kitties?” [15] and “GPS Study Shows Outdoor Cats Have Oversized Effect on Neighborhood Wildlife” [16]. Another related article claims that “pet cats… can cause ecological mayhem” [17]. This is a lot of hype for a study that didn’t actually document predation—never mind its impact. Just as the underlying research has done little to advance our knowledge of the subject, its promotion in the mainstream media has done little to advance fact-based public discourse.

References

1. Kays, R.; Dunn, R.R.; Parsons, A.W.; McDonald, B.; Perkins, T.; Powers, S.A.; Shell, L.; McDonald, J.L.; Cole, H.; Kikillus, H.; et al. The small home ranges and large local ecological impacts of pet cats. Animal Conservation 2020, n/a.
2. Churcher, P.B.; Lawton, J.H. Predation by domestic cats in an English village. Journal of Zoology 1987, 212, 439–455.
3. Barratt, D.G. Predation by house cats, Felis catus (L.), in Canberra, Australia. II. Factors affecting the amount of prey caught and estimates of the impact on wildlife. Wildlife Research 1998, 25, 475–487.
4. Woods, M.; McDonald, R.A.; Harris, S. Predation of wildlife by domestic cats Felis catus in Great Britain. Mammal Review 2003, 33, 174–188.
5. Tschanz, B.; Hegglin, D.; Gloor, S.; Bontadina, F. Hunters and non-hunters: skewed predation rate by domestic cats in a rural village. European Journal of Wildlife Research 2011, 57, 597–602.
6. Bruce, S.J.; Zito, S.; Gates, M.C.; Aguilar, G.; Walker, J.K.; Goldwater, N.; Dale, A. Predation and Risk Behaviors of Free-Roaming Owned Cats in Auckland, New Zealand via the Use of Animal-Borne Cameras. Frontiers in Veterinary Science 2019, 6, 205.
7. Barratt, D.G. Predation by House Cats, Felis catus (L.), in Canberra, Australia. I. Prey Composition and Preference. Wildlife Research 1997, 24, 263–277.
8. McDonald, J.L.; Maclean, M.; Evans, M.R.; Hodgson, D.J. Reconciling actual and perceived rates of predation by domestic cats. Ecology and Evolution 2015, n/a-n/a.
9. Kays, R.W.; DeWan, A.A. Ecological impact of inside/outside house cats around a suburban nature preserve. Animal Conservation 2004, 7, 273–283.
10. Errington, P.L. Notes on Food Habits of Southwestern Wisconsin House Cats. Journal of Mammalogy 1936, 17, 64–65.
11. Marra, P.P.; Santella, C. Cat Wars: The Devastating Consequences of a Cuddly Killer; Princeton University Press: Princeton, N.J., 2016;
12. Wolf, P.J.; Schaffner, J.E. The Road to TNR: Examining Trap-Neuter-Return Through the Lens of Our Evolving Ethics. Frontiers in Veterinary Science 2019, 5, 341.
13. Wolf, P.J.; Rand, J.; Swarbrick, H.; Spehar, D.D.; Norris, J. Reply to Crawford et al.: Why Trap-Neuter-Return (TNR) Is an Ethical Solution for Stray Cat Management. Animals 2019, 9.
14. Kulikowski, M. Keeping Cats Indoors Could Blunt Adverse Effects to Wildlife. North Carolina State University News 2020.
15. Sommer, L. Killer Kitties? Scientists Track What Outdoor Cats Are Doing All Day. All Things Considered 2020.
16. Machemer, T. GPS Study Shows Outdoor Cats Have Oversized Effect on Neighborhood Wildlife. SnartNews (Smithsonian Magazine) 2020.
17. Losos, J. “Cat Tracker” study reveals the secret wanderings of 900 house cats. Domesticated (National Geographic) 2020.