In late 2009, Sharon George was struggling to recruit participants for her master’s thesis. A student in the University of Cape Town’s conservation biology program at the time, George was studying the hunting habits of suburban Cape Town’s pet cats. Although she’d distributed 600 questionnaires, only 32 had been completed—a response rate George describes as “very poor” in her thesis .
“The project in general was very challenging because of the way many cat owners perceived it. The majority of cat owners were unwilling to participate in the study because they felt it was ‘against’ cats and would lead to extreme measures being taken to control cat numbers” .
It turns out, the majority of cat owners were not wrong about that.
George’s results were published online in July as part of a larger study, accompanied by a media release (PDF) warning that “the research highlights the need to address the impact of cat predation on Cape Town’s wildlife, particularly near protected areas such as the Table Mountain National Park.” This led (not surprisingly) to sensational headlines proclaiming “‘200,000-plus’ wild animals slaughtered in Table Mountain National Park by Cape Town cats each year” and “Apocalypse Miaow II: ‘Keep cats inside property’, SANParks urges Capetonians.”
With a response rate of just 5.3 percent, George had no reason to think that her sample was representative of Cape Town residents. She had clearly hit a dead-end, but, like others before her, she set aside sound sampling practices and plowed ahead.
George’s research was just the first of three projects undertaken by UCT graduate students, all intended to investigate the hunting habits of Cape Town’s suburban pet cats. In the second study, conducted by Koebraa Peters, residents were once again asked about prey returned home by their cats . Although Peters’ response rate (28.1 percent) was a considerable improvement, the sample was quite small: 18 households with a total of 27 cats.
For the third study in the series, Frances Morling added an important element: collar-born video cameras “to determine the proportion of prey caught that is brought back to the home.” From this, a correction factor could be calculated and then “applied to predation studies that have focused only on prey brought back to the home, allowing us to improve our understanding of the total impact of domestic cat predation” .
This is not uncharted territory, of course [4,5]. Nor is it unwarranted. What’s problematic is the constant misuse of such correction factors to inflate predation rates [see, for example, 6–8].
In pursuit of a correction factor
Morling was determined to avoid the low response rate George’s survey elicited:
“Domestic cat owners were given a brief summary of the goals of the study and asked if they were willing to volunteer their domestic cats for participation. Similar to Loyd et al. , when recruiting volunteers I emphasised that I was researching ‘what cats do and where they go,’ rather than ‘how their hunting behaviour may impact other fauna.’ This is potentially important when attempting to obtain an adequate sample size as domestic cat owners may be wary of how research can impact policy and legislation that may curb ownership and freedom of movement of their domestic cats (George 2010)” .
Again, cat owners have good reason to be wary of how such research might be used.
Morling’s survey sample included 13 cats whose prey returns were recorded for two weeks; 10 of these cats were also monitored for three weeks via “KittyCams.” Video footage revealed that only 22 percent of prey captured were brought home, while “the majority (78 percent) were never seen by the cat owner.”
“We can therefore use these estimates to correct estimates in previous studies that relied on questionnaires only. A study that has used prey brought home to estimate total prey caught and predation rates, may be corrected by multiplying the value by a correction factor of 1/0.22 or 4.55 in order to produce a true estimate of total prey caught” .
Results from Peters, George, and Morling—along with those from some additional research by Morling—were compiled and subsequently published in the journal Global Ecology and Conservation, where Colleen Seymour and her co-authors (including George, Peters, and Morling) reported an updated correction factor of 5.56 .
Where things go sideways
It all seems quite straightforward until one realizes that the cats recruited for the KittyCam studies “were all confirmed by their owners as being active hunters” . Are these cats more likely to bring prey home to their owners? Less likely? Who knows. What is clear, though, is that applying such a correction factor to all pet cats with outdoor access is questionable at best.
Unfortunately, readers of the 2020 paper have no way of knowing that this was done. Neither does anybody who learns of the study from the media accounts.
More problematic is the authors’ application of their correction factor across all prey species—in contrast to their own findings.
“Of the mammals recorded (n=15), 66 percent were brought home, 27 percent were eaten and 7 percent were abandoned. For reptiles, 94 percent (n=31) were eaten, the remaining 6% were abandoned. For invertebrates, 46 percent (n= 11) were eaten, 54 percent were abandoned. The only bird caught was eaten in situ. The only amphibian caught was brought home.”
Ten of the 15 mammals predated by cats were brought home; the other five were not. Applying the author’s correction factor (5.56) would lead us to believe that 56 mammals had been predated—nearly four times what was documented. A similar situation arises if one “corrects” for the number of amphibians brought home.
For reptiles and birds, on the other hand, none was returned home—at least not as part of the KittyCams research (both Peters and George documented bird predation, which raises questions about the KittyCams possibly interfering with the hunting of birds). Obviously, the true predation rates were greater than those estimated via prey return surveys, but multiplying by 5.56 (or any other number) is of no use as it suggests a result of zero.
Faced with this conundrum, Seymour et al. apply their correction factor across all prey categories and to all of Cape Town’s pet cats, resulting in an annual predation estimate of 90 “animals” per cat per year for a total of 27.5 million “animals” killed annually, roughly 203,500 of which are from nearby Table Mountain National Park .
Obviously, mammals, birds, reptiles, amphibians, and invertebrates are all animals. Still, collapsing them all into this one general category is curious. Would readers feel differently about the predation of birds as compared to the predation of mice and insects, for example? Probably.
Perhaps this explains the heavy emphasis on “animals” in the media release (PDF) that accompanied the study’s publication online in July. Indeed, there’s no mention of invertebrates at all, though they accounted for 21 percent of documented prey.
Also highlighted was the apparent risk to “a number of threatened or endemic species, including Western Leopard Toads, Cape Rain Frogs, and Orange breasted sunbirds.” This, too, is misleading.
Of the 423 prey items documented during this series of studies, just one Western leopard toad was reported. Although the species is endangered, the 2016 International Union for Conservation of Nature and Natural Resources (IUCN) report notes that “it is locally common.”
Similarly, only three of the 423 prey items recorded were Cape rain frogs, a species classified as Near Threatened (second only to Least Concern on the spectrum of seven official categories) and one that “can be relatively common in parts of its range” according to the 2016 IUCN report. It’s possible that George’s study included one orange-breasted Sunbird, a species of Least Concern, but the bird that was collected couldn’t be identified to species.
A campaign ‘against’ cats?
To be clear: this series of studies, spanning more than 10 years, involved some carefully considered research directed at a topic of considerable interest. On the other hand, there a number of serious shortcomings—most of which are not acknowledged in the Global Ecology and Conservation paper—that undermine the authors’ conclusions.
Nevertheless, their conclusions are both amplified and further distorted in the corresponding media release. Not surprisingly, the media took the bait—sounding the alarm about the “‘200,000-plus’ wild animals slaughtered” in Table Mountain National Park, for example, noting that “most casualties butchered by this army of feline felons are native animals” .
Even if the predation estimates are to be believed, there remains the (obvious) question of impact. If this level of predation is truly occurring, wouldn’t we expect to see some population-level impact on the species in question? Although others have pointed out that cats generally catch prey species that are plentiful [5,11,12] and that predation rates “are only meaningful in the context of prey availability” , Seymour and her co-authors don’t even seem to consider such factors.
And the media accounts certainly don’t. Again, no surprise there. Instead, they’ve already moved on to talk of restrictions on pet cats and their owners. No doubt the targeting of unowned cats—which will almost certainly be less diplomatic—is soon to follow.
All of which brings to mind the concern articulated by Cape Town’s cat owners that George’s research “was ‘against’ cats and would lead to extreme measures being taken to control cat numbers” . In light of the subsequent work, media release, news stories, and general reaction—it’s difficult to see this as anything but a carefully planned effort to do exactly that (using tax dollars, courtesy of the National Research Foundation).
Note: Robert Simmons, corresponding author for the Global Ecology and Conservation paper, kindly provided me some additional information about the research. Responding to my follow-up questions, though, he recommended that I “write a piece for the journal and [he and his co-authors] can reply… there.” Experience tells me that is likely to be an exercise in futility, though; I’m just going to leave my thoughts here. (Others are, of course, free to comment.)
- George, S. Cape Town’s domestic cats: prey and movement patterns in deep-urban and urban-edge areas. Masters degree in Geography, University of Cape Town, 2010.
- Peters, K. Tracking domestic cats: Movement patterns and prey catches of cats in Glencairn, Cape Town. Bachelor of Science (Zoology) Honours, University of Cape Town, 2011.
- Morling, F. Cape Town’s cats: Reassessing predation through kitty-cams. Masters degree in Conservation Biology, University of Cape Town, 2014.
- Loyd, K.A.T.; Hernandez, S.M.; Carroll, J.P.; Abernathy, K.J.; Marshall, G.J. Quantifying free-roaming domestic cat predation using animal-borne video cameras. Biological Conservation 2013, 160, 183–189.
- 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.
- Loss, S.R.; Will, T.; Marra, P.P. The impact of free-ranging domestic cats on wildlife of the United States. Nature Communications 2013, 4.
- ABC “KittyCam” Reveals High Levels of Wildlife Being Killed by Outdoor Cats. 2013.
- Loss, S.R.; Will, T.; Longcore, T.; Marra, P.P. Responding to misinformation and criticisms regarding United States cat predation estimates. Biological Invasions 2018, 20, 3385–3396.
- Seymour, C.L.; Simmons, R.E.; Morling, F.; George, S.T.; Peters, K.; O’Riain, M.J. Caught on camera: the impacts of urban domestic cats on wild prey in an African city and neighbouring protected areas. Global Ecology and Conservation 2020, 23, e01198.
- Walters, T. ‘200,000-plus’ wild animals slaughtered in Table Mountain National Park by Cape Town cats each year. Daily Maverick 2020.
- Kays, R.W.; DeWan, A.A. Ecological impact of inside/outside house cats around a suburban nature preserve. Animal Conservation 2004, 7, 273–283.
- van Heezik, Y.; Smyth, A.; Adams, A.; Gordon, J. Do domestic cats impose an unsustainable harvest on urban bird populations? Biological Conservation 2010, 143, 121–130.
- Thomas, R.L.; Fellowes, M.D.E.; Baker, P.J. Spatio-Temporal Variation in Predation by Urban Domestic Cats (Felis catus) and the Acceptability of Possible Management Actions in the UK. PLoS ONE 2012, 7, e49369.