Posts filed in Research Protocol

The Work Speaks—Part 6: Pain by Numbers

14 Jun 2010

In April, Conservation Biology published a comment authored by Christopher A. Lepczyk, Nico Dauphiné, David M. Bird, Sheila Conant, Robert J. Cooper, David C. Duffy, Pamela Jo Hatley, Peter P. Marra, Elizabeth Stone, and Stanley A. Temple. In it, the authors “applaud the recent essay by Longcore et al. (2009) in raising the awareness about trap-neuter-return (TNR) to the conservation community,” [1] and puzzle at the lack of TNR opposition among the larger scientific community:

“…it may be that conservation biologists and wildlife ecologists believe the issue of feral cats has already been studied enough and that the work speaks for itself, suggesting that no further research is needed.”

In fact, “the work”—taken as a whole—is neither as rigorous nor as conclusive as Lepczyk et al. suggest. And far too much of it is plagued by exaggeration, misrepresentations, errors, and obvious bias. In Part 5 of this series, I critiqued Cole Hawkins’ 1998 PhD dissertation. Here, I’m going to untangle some of Lepczyk’s own PhD work: Landowners and cat predation across rural-to-urban landscapes, published in 2003.

The Study
In this study, surveys were distributed across three southeastern Michigan landscapes (rural, suburban, and urban) corresponding to established breeding bird survey (BBS) routes. [2] Among the survey questions:

“If you or members of your household own cats that are allowed access to the outside, approximately how many dead or injured birds a week do all the cats bring in during the spring and summer months (April through August) (0, 1, 2–3, 4–5, 6–7, 8–9, 10–15, 16–20, more than 20)?”

Based on 968 surveys returned from 1654 private landowners (a decent response rate of 58.5%), Lepczyk et al. conclude:

“Across the three landscapes there were ~800 to ~3100 cats, which kill between ~16,000 and ~47,000 birds during the breeding season, resulting in a minimum of ~1 bird killed/km/day.”

Increasing Uncertainty
How do Lepczyk and his collaborators arrive at these figures? It’s not entirely clear, actually. Despite numerous attempts, I’ve been unable to follow all of their calculations. However, using their data, I developed my own estimate: 1,119 outdoor cats, 511 of which were reported to be successful hunters.

Using this figure, I then summed across all three landscapes the birds killed or injured, plus those killed or injured by non-respondents’ hunting cats (based on the ratio of hunters to outdoor cats owned by respondents, or about 50%). The resulting estimate is 15,856 birds killed over the 22-week breeding season—close to the low estimate suggested by Lepczyk et al., but just a third of their maximum.

So, why the discrepancy?

One reason is that, at least for some of their estimates, Lepczyk et al. assumed that every landowner who didn’t respond to the survey owned outdoor cats. This, despite their survey results, which indicated that only about one-third of landowners fell into this category.

But the authors go further, generating predation estimates based on pure speculation, specifically that “non-respondents have 150% the number of outdoor cats as respondents.” [2] It should be noted that Lepczyk et al. also ran another scenario in which non-respondents had half the outdoor cats as did respondents—but, again, in both cases they assume that every non-respondent owned outdoor cats.

As a result of this approach, the authors end up in some strange territory: the estimated number of cats owned by non-respondents (based on the assumptions described above) far exceeds the number owned by respondents—by more than a two-to-one margin, in some cases. If the greatest impacts are going to be attributable to non-respondents, then what’s the point of doing the survey in the first place? There are accepted methods by which one can manage uncertainty—statistical analysis, confidence intervals, and the like. What Lepczyk et al. have done serves just one purpose: to inflate apparent predation rates.

Skewed Distributions
In addition to the flaws described above, there are some fundamental errors in the way the authors handle their data. Like so many others, Lepczyk et al. ignore the fact that their data is not normally distributed:

  1. Lepczyk et al. use the average number of birds killed/cat to calculate the total number of bids killed for each of the three landscapes. As I discussed previously), this is a highly positively skewed distribution—using a simple average, therefore, greatly overestimates the cats’ impact (by as much as a factor of two).
  2. A similar error is made when the authors use an average to describe the number of outdoor cats owned by each landowner. Again, because this is a skewed distribution, their use of a simple average exaggerates the extent of predation.
  3. The two inflated figures described in (1) and (2) are multiplied together, further inflating estimated predation rates.

Barratt has suggested that “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.” [3] Accounting for the first point alone, then, my estimate is reduced to 8,000 birds killed over the 22-week breeding season.

Accounting for the second point is somewhat trickier. For one thing, we don’t know what constitutes an outdoor cat here—the survey simply asked respondents if they owned cats “that are allowed access to the outdoors.” [2] However, we do know the results of a 2003 survey, which indicated that nearly half of the cats with outdoor access were outside for two or fewer hours a day. And 29% were outdoors for less than an hour each day. [4] Although these figures almost certainly reflect owners in urban and suburban landscapes more than those in rural landscapes, it’s clear that a simple yes-or-no question on the subject is insufficient. Indeed, such a question will invariably overestimate the number of “outdoor cats”—which in turn overestimates predation rates.

This, coupled with the error inherent in using a simple average, pushes predation estimates lower. And the third point reduces those estimates further still. Taken together, these corrections could put my estimate closer to 4,000 birds. More important, the upper estimate proposed by Lepczyk et al.—47,000 birds—could easily be 10 times too high.

The Small Print
Despite their inflated figures, Lepczyk et al. suggest—rather absurdly, in light of the substantial flaws described above—that perhaps their estimates are actually too conservative:

“One caveat to our study is that landowners may have underestimated the number of cats they allow access to the outside. Such a result was found in a similar study of landowners in Wisconsin (Coleman and Temple, 1993).” [1] (Note: After reviewing “Rural Residents’ Free-Ranging Domestic Cats: A Survey,” [5] I’ve found no evidence of such a result.)

“… we found that a very common volunteered response among landowners that had no outdoor cats was that either their neighbors owned outdoor cats or that feral cats were present in the vicinity of their land… [suggesting] that at least some landowners under reported or chose not to report the number of outdoor cats they owned.”

But what about their reports of birds brought home killed or injured—how trustworthy were those? After all, the survey (mailed during the first week of October) asked respondents to recall the number of birds their cat(s) brought home April through August. Surely, there was a lot of guesswork involved. In fact, David Barratt found this kind of guesswork to overestimate predation rates. In a study published five years prior to “Landowners and Cat Predation,” Barratt concluded, “predicted rates of predation greater than about ten prey per year generally over-estimated predation observed.” [3]

The two studies cannot be compared directly for a number of reasons, but by way of comparison, the average predation rate used by Lepczyk et al. is approximately 31 birds/cat for the 22-week breeding season. Using Barratt’s work, in which the “heaviest” six continuous months correspond to about 58% of yearly prey totals, [6] I converted this to a yearly rate of 53 birds/cat/year. Barratt has shown that the actual predation rate, at this level, is less than half the rate predicted by cat owners. In other words, predictions of 50 birds/year generally correspond to catches closer to 25 birds/year.

While Lepczyk et al. emphasize the potential for under-estimating predation levels, they never consider the risk of over-estimating these levels—or their most obvious potential source of error: landowners’ recollections of birds killed. The authors question respondents’ reports of outdoor cats, but accept without question their reports of birds injured or killed over the previous six-month period. And, as Barratt indicated, such reports can be inflated by a factor of two or more!

Something else I find troubling comes, of all places, from the Acknowledgements section. Among those thanked “for helpful and constructive reviews” are American Bird Conservancy (ABC) president George Fenwick and Linda Winter, director of ABC’s Cats Indoors! campaign. It’s not clear how Fenwick and Winter contributed to the final paper, but their involvement on any level raises questions about possible bias. Certainly, Winter has credibility issues when it comes to “research” about the impact of free-roaming cats on birds, as I’ve already described (see also pp. 18–24 of TNR Past present and future: A history of the trap-neuter-return movement [7]).

*     *     *

The same year Lepczyk’s paper was published, the American Veterinary Medicine Association held an Animal Welfare Forum “devoted to the management of abandoned and feral cats.” [8] In attendance were more than 200 veterinarians, animal control officials, wildlife conservationists, and animal advocates—each with a different perspective on feral cats in general and TNR in particular.

In welcoming this diverse group, then-President-Elect Bonnie Beaver recognized the range of contentious issues before them:

“Feral cats evoke hot debates about ecological issues, individual cat welfare, human responsibilities, intercat disease transmission, humaneness, zoonosis control, and management and dissolution of unowned cats.” [8]

Amidst the “hot debate,” though, Beaver was optimistic:

“We will not always agree, but we will come away with increased knowledge and a renewed commitment to work for the welfare of all the animals with which we share the earth” [8]

While I tend to share Beaver’s optimism, I think the debate is hurt—if not derailed entirely—by the publication of research aimed not at increasing our collective knowledge, but rather at supporting a particular position. Like Cole Hawkins’ dissertation, “Landowners and Cat Predation” is, at best, an interesting pilot study for subsequent work. And yet, it’s widely—and uncritically—cited in the feral cat/TNR literature. Longcore et al., for example, refer to it as “evidence [indicating] that cats can play an important role in fluctuations of bird populations,” [9] despite the fact that Lepczyk et al. don’t actually address the issue of bird populations at all. More recently, Dauphiné and Cooper use the inflated predation rate suggested by Lepczyk et al. (along with rates proposed by other researchers) to arrive at their “billion birds” figure. [10]

The method employed in “Landowners and Cat Predation”—asking owners of cats to recall the number and species of birds over the previous six-month period—invites overestimation from the very outset. Lepczyk et al. then inflate these numbers through both careless (e.g., using averages to describe skewed data) and deliberate (e.g., assuming all non-respondents owned cats—perhaps 50% more than respondents did) means. Rather than getting us any closer to the truth about cat predation, this study only obscured it further.

Worse, it’s been packaged and sold—and subsequently “bought”—as rigorous science, thereby giving it an undeserved legitimacy. Such efforts are impediments to knowledge and understanding—and therefore, to progress.

References
1. Lepczyk, C.A., et al., “What Conservation Biologists Can Do to Counter Trap-Neuter-Return: Response to Longcore et al.” Conservation Biology. 2010. 24(2): p. 627-629.

2. Lepczyk, C.A., Mertig, A.G., and Liu, J., “Landowners and cat predation across rural-to-urban landscapes.” Biological Conservation. 2003. 115(2): p. 191-201.

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(5): p. 475–487.

4. Clancy, E.A., Moore, A.S., and Bertone, E.R., “Evaluation of cat and owner characteristics and their relationships to outdoor access of owned cats.” Journal of the American Veterinary Medical Association. 2003. 222(11): p. 1541-1545.

5. Coleman, J.S. and Temple, S.A., “Rural Residents’ Free-Ranging Domestic Cats: A Survey.” Wildlife Society Bulletin. 1993. 21(4): p. 381–390.

6. Barratt, D.G., “Predation by House Cats, Felis catus (L.), in Canberra, Australia. I. Prey Composition and Preference.” Wildlife Research. 1997. 24(3): p. 263–277.

7. Berkeley, E.P., TNR Past present and future: A history of the trap-neuter-return movement. 2004, Bethesda, MD: Alley Cat Allies.

8. Kuehn, B.M. and Kahler, S.C. The Cat Debate. JAVMA Online 2004 November 27, 2009 [accessed 2009 December 24].  http://www.avma.org/onlnews/javma/jan04/040115a.asp.

9. Longcore, T., Rich, C., and Sullivan, L.M., “Critical Assessment of Claims Regarding Management of Feral Cats by Trap–Neuter–Return.” Conservation Biology. 2009. 23(4): p. 887–894.

10. Dauphiné, N. and Cooper, R.J., Impacts of Free-ranging Domestic Cats (Felis catus) on birds in the United States: A review of recent research with conservation and management recommendations, in Fourth International Partners in Flight Conference: Tundra to Tropics. 2010. p. 205–219.

The Work Speaks—Part 5: Jumping to Conclusions

11 Jun 2010

In April, Conservation Biology published a comment authored by Christopher A. Lepczyk, Nico Dauphiné, David M. Bird, Sheila Conant, Robert J. Cooper, David C. Duffy, Pamela Jo Hatley, Peter P. Marra, Elizabeth Stone, and Stanley A. Temple. In it, the authors “applaud the recent essay by Longcore et al. (2009) in raising the awareness about trap-neuter-return (TNR) to the conservation community,” [1] and puzzle at the lack of TNR opposition among the larger scientific community:

“…it may be that conservation biologists and wildlife ecologists believe the issue of feral cats has already been studied enough and that the work speaks for itself, suggesting that no further research is needed.”

In fact, “the work”—taken as a whole—is neither as rigorous nor as conclusive as Lepczyk et al. suggest. And far too much of it is plagued by exaggeration, misrepresentations, errors, and obvious bias. In Part 4 of this series, I discussed how feral cat/TNR researchers often misuse averages to characterize skewed distributions, and how that error overestimates the impact of free-roaming cats on wildlife.

For the next few posts, I’m going to critique three of the studies most often cited by these researchers, starting with Cole Hawkins’ 1998 PhD dissertation, Impact of a subsidized exotic predator on native biota: Effect of house cats (Felis catus) on California birds and rodents. I mentioned Hawkins’ dissertation previously, but only briefly. Here, I’ll take a closer look, paying particular attention to how he gets from his results to his rather dubious conclusions.

The Study
Hawkins’ two-year study was conducted in Alameda County, CA, spread across two adjacent parks. He started by designating a “cat area” (where, nearby, free-roaming cats were being fed) and a “no-cat area” (where no cats were being fed), and then designated “rodent grids” (nine locations used for trapping and counting rodents) and walking transects (from which bird surveys were conducted) in each area. Hawkins then compared the number of birds and rodents detected in the two areas.

Among Hawkins’ conclusions:

“The differences observed in this study were the results of the cat’s predatory behavior.” [2] (It should be noted that Hawkins tempered this assertion in his 1999 article summarizing the work: “The differences observed in this study may have been due to the cats’ predatory behavior.” [3] (italics mine))

And this:

“The presence of cats in this study area already has caused a shift in the composition of the rodent community; it is possible that a shift in the larger biotic community could follow.”

And, finally:

“It is not prudent to manage for wildlife and allow cat feeding in the same parks.”

Unfair Comparisons
But Hawkins’ findings are insufficient to supports such claims; indeed, his methodology doesn’t allow for them. Hawkins has no idea what the cat area was like prior to his arrival; he merely assumes the populations of birds and rodents would have been identical to those found at the no-cat area, and makes his comparisons accordingly. In fact, there are a number of factors that indicate that the two areas are not as comparable as Hawkins suggests:

  • The cat area was almost a peninsula, with a lake on one side and a residential area (within 0.5 km) on the other. The no-cat area, on the other hand, was located largely in the interior of the parks.
  • Hawkins notes that there were more people in the cat area (of­ten twice as many as were observed at the no-cat area), but dismisses the possibility that their presence may have influenced the numbers of birds and rodents he observed there.
  • The habitat along the 2.2 km transects from which bird counts were conducted varied considerably between the two areas. Compared to the no-cat area, the cat area had 31% less chaparral, 183% more trees, 52% less grass, and 240% more “modified” habitat (it’s not clear what Hawkins means by “modified,” but I assume it refers to habitat that reflects significant human impact).
  • Finally, the presence of pesticides may have played a role. According to a 2002 report (the earliest I was able to find) from the East Bay Regional Park District, “The focus of Lake Chabot’s weed control efforts are vegetation reduction within the two-acre overflow parking lot, picnic sites and firebreaks around park buildings, corp. yard, service yard, and the Lake Chabot classroom.” [4] And it’s clear from Hawkins’ 1999 article that the cat area did include picnic sites: “…over half of the cat scat in this study was collected under and around picnic tables.” [3] Now, Hawkins’ fieldwork was done in 1995 and 1996, but if there was any pesticide use during the study period, it may have affected the results—especially if the pesti­cide was distributed differently across the two areas.

Cats and Birds
“Almost twice as many birds were seen on the no-cat transect as on the cat transect,” writes Hawkins. But it’s not quite as simple as that—the details reveal a rather complex, often uneven count over the course of the study. Nevertheless—and despite the differences between the two areas—Hawkins’ only explanation is the cats. This is especially true for ground-feeders:

The preference of ground feeding birds for the no-cat treatment was striking; for ex­ample, California quail were seen almost daily in the no-cat area, whereas they were never seen in the cat area.

What’s more striking to me is the fact that five of the nine ground-feeding species included in the study showed no preference for either area. But Hawkins scarcely acknowledges the point, and doesn’t even hint at an explanation. “Birds that were known to nest on or near the ground or in shrubs and vines ≤ 1.5 m in height” also showed no preference between the two areas (though no nest counts were conducted).

The picture painted by Hawkins is that bird species absent from the cat area represent species killed off by the cats. But it’s generally accepted that cats are opportunistic hunters, catching whatever prey is readily available and easily caught. [5–8] Fitzgerald and Turner, for ex­ample, argue that “domestic cats (both house and feral ones) are best described as generalist resident preda­tors, exploiting a wide range of prey, and able to switch readily from one prey to another.” [9] So, how is it that some species were present at the cat site while others were not? Again, Hawkins offers no explanation.

In fact, it’s clear from Hawkins’ study that the cats aren’t much of threat at all to the birds—even vulnerable ground-feeding and ground-nesting species—in the cat area. Of the 120 scat samples found by searching the cat area, “65% were found to contain rodent hair and 4% feathers.” [2] This finding comes toward the end of the study, when the cat population was at its greatest—and still, only 4% contained feathers. And this could easily represent one cat and one bird.

One final point about the birds: Hawkins suggests (without explanation) that the olive-sided flycatcher, American robin, and Stellar’s jay—all of which showed no preference for either the no-cat area or the cat area—may have been responding to a “specialized habitat.” Could it be that the birds not seen in the cat area were also responding to a specialized habitat—by “migrating” to a place with less human activity (e.g., the no-cat area), for example? Once again, Hawkins has no comment.

Cats and Rodents
The fact that scats indicated rodents were predated to a greater extent than birds is hardly surprising [5, 6, 9, 10], but it should be noted the 65% figure represents the frequency of occurrence, and not a predation rate (a topic I address in greater detail here).

Hawkins’ analysis didn’t reveal whether the rodent hair was that of deer mice, harvest mice (both of which were found less often in the cat area than in the no-cat area), house mice (found more often in the cat area), or California voles (which showed no preference for either area). In any case, it’s not clear that the cats were responsible for the presence or absence of any of these rodent species. Again, the selective dietary habits suggested by Hawkins simply don’t fit with the domestic cat’s profile as a “generalist resident predator.”

Two additional points that might explain the differences Hawkins observed concern the habitat of the cat area. First, there’s the nearby lake and residences—potential sources of pollution that could affect nearby plant and animal life. Secondly, there’s the issue of possible pesticide use mentioned previously. As I say, it’s largely conjecture on my part; at the same time, though, it’s easy to imagine its potential impact on small mammals (and ground-feeding birds, for that matter).

Finally, Hawkins suggests that certain bird species were responding to specialized habitat—perhaps the rodents were simply doing the same.

Cats
Hawkins used track plates (baited devices that detect the presence of mammals by way of preserved “footprints”) for “estimating a relative cat presence index,” but found only one cat track in 200 track plate nights. And, “in 560 days of exposure, no scat was found in any of the sand boxes.” [2] Now, the cats were seen at feeding stations and on the rodent grids of the cat area—as many as 26 during a one-week period toward the end of the study. But clearly, they were not where Hawkins was expecting them to be. If, after two years at the study site, Hawkins was unable to get a better handle on the presence of the cats, how can he be so sure of their behavior when it comes to predation?

If, as Hawkins argues, the differences observed between the two areas are a result of the cats’ predatory behavior, then one would expect the number of birds and rodents to decrease as the number of cats increases. Yet, Hawkins’ findings don’t bear this out.

And then there are the unanswered questions about the cats—for example:

  • Where did these cats come from—were they illegally dumped, the result of nearby residents’ unsterilized cats breeding? Did they belong to the residents?
  • Were the cats sterilized? (Their increasing numbers would suggest that they weren’t.)
  • Were these cats part of a managed TNR colony? (Local newspaper reports indicate a long-standing battle between TNR advocates and opponents. [11–13])

Considering the central role these cats played in Hawkins’ two-year study, he knew surprisingly little about their behavior—including various factors that surely had an impact on his findings.

*     *     *

In their recent comment, Lepczyk et al. suggest that conservation biologists and wildlife ecologists “look to the evolutionary biology community” [1] for an example of how to influence policy. For feral cat/TNR opponents interested in shaping policy, it seems Hawkins’ study has become quite popular. [14–17] Actually, Nico Dauphiné and Robert J. Cooper take its already-tenuous claims one step further, citing Hawkins’ work (actually a 2004 conference paper that summarizes his dissertation [18]) as evidence that “the continuous predation pressure exerted by exotic predators in exponentially high densities can and has resulted in numerous local extinctions of continental land birds.” [8]

But is Hawkins’ methodology one that evolutionary biologists would advocate—or even recognize? Not likely.

Hawkins draws conclusions—infers important causal relationships—without any evidence of what “pre-treatment” conditions were like. And ignores entirely his own findings when they contradict his conclusions. Rather than beginning his inquiry with questions to answer, it seems Hawkins had his answer from the outset. At best, his work is an interesting pilot study—generating research questions for a more rigorous, less biased investigation.

References
1. Lepczyk, C.A., et al., “What Conservation Biologists Can Do to Counter Trap-Neuter-Return: Response to Longcore et al.” Conservation Biology. 2010. 24(2): p. 627-629.

2. Hawkins, C.C., Impact of a subsidized exotic predator on native biota: Effect of house cats (Felis catus) on California birds and rodents. 1998, Texas A&M University.

3. Hawkins, C.C., Grant, W.E., and Longnecker, M.T., “Effects of Subsidized House Cats on California Birds and Rodents.” Transactions of the Western Section of the Wildlife Society. 1999. 35: p. 29–33.

4. Brownfield, N.T., 2002 Annual Analysis of Pesticide Use East Bay Regional Park District. 2003, East Bay Regional Park District. www.ebparks.org/files/stew_pest_report_02.pdf

5. 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(5): p. 475–487.

6. Fitzgerald, B.M., Diet of domestic cats and their impact on prey populations, in The Domestic cat: The biology of its behaviour, D.C. Turner and P.P.G. Bateson, Editors. 1988, Cambridge University Press: Cambridge; New York. p. 123–147.

7. Lepczyk, C.A., Mertig, A.G., and Liu, J., “Landowners and cat predation across rural-to-urban landscapes.” Biological Conservation. 2003. 115(2): p. 191-201.

8. Dauphiné, N. and Cooper, R.J., Impacts of Free-ranging Domestic Cats (Felis catus) on birds in the United States: A review of recent research with conservation and management recommendations, in Fourth International Partners in Flight Conference: Tundra to Tropics. 2010. p. 205–219.

9. Fitzgerald, B.M. and Turner, D.C., Hunting Behaviour of domestic cats and their impact on prey populations, in The Domestic Cat: The biology of its behaviour, D.C. Turner and P.P.G. Bateson, Editors. 2000, Cambridge University Press: Cambridge, U.K.; New York. p. 151–175.

10. Woods, M., McDonald, R.A., and Harris, S., “Predation of wildlife by domestic cats Felis catus in Great Britain.” Mammal Review. 2003. 33(2): p. 174-188.

11. Chui, G., Stray Cats Live Harsh Lives in Area Parks, in San Jose Mercury News. 1985. p. 1

12. Bogue, G., Those Poor Cats Need a Human Assist, in Contra Costa Times. 1997: Walnut Creek, CA. p. A02

13. n.a., Spring controversy: What to do with feral cats?, in San Mateo Daily Journal, The (CA). 2001.

14. Longcore, T., Rich, C., and Sullivan, L.M., “Critical Assessment of Claims Regarding Management of Feral Cats by Trap–Neuter–Return.” Conservation Biology. 2009. 23(4): p. 887–894.

15. ABC, Domestic Cat Predation on Birds and Other Wildlife. n.d., American Bird Conservancy: The Plains, VA. www.abcbirds.org/abcprograms/policy/cats/materials/predation.pdf

16. Winter, L. and Wallace, G.E., Impacts of Feral and Free-Ranging Cats on Bird Species of Conservation Concern, G.E. Wallace, Editor. 2006, American Bird Conservancy. www.abcbirds.org/newsandreports/NFWF.pdf

17. Ash, S.J. and Adams, C.E., “Public Preferences for Free-Ranging Domestic Cat (Felis catus) Management Options.” Wildlife Society Bulletin. 2003. 31(2): p. 334–339.

18. Hawkins, C.C., Grant, W.E., and Longnecker, M.T. Effect of house cats, being fed in parks, on California birds and rodents. in Proceedings Of The 4th International Symposium On Urban Wildlife Conservation. 2004. Tucson, AZ: University of Arizona. http://cals.arizona.edu/pubs/adjunct/snr0704/snr07042l.pdf

The Work Speaks—Part 4: Mean Spirited

4 Jun 2010

In April, Conservation Biology published a comment authored by Christopher A. Lepczyk, Nico Dauphiné, David M. Bird, Sheila Conant, Robert J. Cooper, David C. Duffy, Pamela Jo Hatley, Peter P. Marra, Elizabeth Stone, and Stanley A. Temple. In it, the authors “applaud the recent essay by Longcore et al. (2009) in raising the awareness about trap-neuter-return (TNR) to the conservation community,” [1] and puzzle at the lack of TNR opposition among the larger scientific community:

“…it may be that conservation biologists and wildlife ecologists believe the issue of feral cats has already been studied enough and that the work speaks for itself, suggesting that no further research is needed.”

In fact, “the work”—taken as a whole—is neither as rigorous nor as conclusive as Lepczyk et al. suggest. And far too much of it is plagued by exaggeration, misrepresentations, errors, and obvious bias. In Part 3 of this series, I discussed the distinction between compensatory and additive predation. Here, I’ll focus on how feral cat/TNR researchers often misuse averages to characterize skewed distributions, and how that error overestimates the impact of free-roaming cats on wildlife.

Something’s Askew
When a data set is skewed, it is inappropriate to use the mean, or average, as a measure of central tendency. The mean should be used only when the data set can be considered normal—the familiar bell curve. As Woods et al put it:

“the simple average number of animals brought home is not a useful measure of central tendency because of the skewed frequency distribution of the numbers of prey items brought home…” [2]

Studies of cat predation routinely reveal a positively skewed distribution; a few cats are responsible for many kills, while many of the cats kill few, if any, prey. So when researchers use the mean to calculate the total number of prey killed by cats in a particular area, they overestimate the cats’ impact.

How common is this? Very [see, for example, 3-9]. Of the many cat predation studies I’ve read, only a few [2, 10, 11] properly account for the skewed nature of this distribution. And others [12-17] often take these inflated figures at face value—as evidence of the impact cats have on wildlife. Published repeatedly, the erroneous estimates take on an undeserved legitimacy.

The proper method for handling skewed distributions involves data transformations, the details of which I won’t go into here. The important point is this: in the case of a positively skewed distribution, the back-transformed mean will always be less than the simple mean of the same data set.

Big Deal
Depending on the particular distribution, the difference between the simple mean and the back-transformed mean can be considerable. Let’s use the 2003 study by Woods et al. [2] to illustrate. In the case of mammals killed and returned home by pet cats, the back-transformed mean was 28.3% less than the simple mean. Or, put another way, the simple mean would have overestimated the number of mammals killed by 39.5%. Similarly, when all prey items were totaled (as depicted in the illustration above), the simple mean would have overestimated the total number off all prey (mammals, birds, herpetofauna, and “others”) by 46.9%.

On the other hand, the figures for birds appear to break the rule mentioned above. In this case, the back-transformed mean (4.1) is actually a bit higher than the simple mean (4.0). How can this be? In order to log-transform the data set, Woods et al. had to first eliminate all the instances where cats returned home no prey—you can’t take the logarithm of 0. So, they were actually working with two data sets. Now, the second data set—which includes only those cats that returned at least one prey item—is also highly positively skewed. As a point of reference, its simple mean was approximately 5.6 birds/cat, which, compared to the back-transformed mean, is an overestimation of 37.5%.

By now, it should be apparent that log-transformed means have another important advantage over simple means: because you have to eliminate those zeros from the data set, you are forced to focus only on the cats that returned prey home—which, of course, is the whole point of such studies! And in the case of this study, Woods et al. found that 20–30% of cats brought home either no birds or no mammals. And 8.6% of the cats brought home no prey at all over the course of the study.

Transforming a data set (and then back-transforming its mean) is simpler than it sounds, but Barratt offers a useful alternative, rule-of-thumb method (one echoed by Fitzgerald and Turner [18]):

“…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.” [10]

So, whereas Dauphiné and Cooper (and others) suggest increasing such estimates by factors of two and three (“predation rates measured through prey returns may represent one half to less than one third of what pet cats actually kill…” [14]), they should, in fact, be reducing them by half.

Cat Ownership
There are other instances in which simple averages are used to describe similarly skewed distributions—with similar results. That is, they overestimate a particular characteristic—and not in the cats’ favor.

Cat ownership, for example, is not a normal distribution. Many people own one or two cats; a few people own many cats. This is precisely what Lepczyk et al. found in their 2003 study:

“The total number of free-ranging cats across all landscapes was 656, ranging from 1 to 30 per landowner…” [6]

In fact, about 113 (I’m estimating from the histogram printed in the report) of those landowners owned just one cat apiece. About 70 of them owned two cats. Only one—maybe two—owned 30 cats. And yet, Lepczyk et al. calculate an average of 2.59 cats/landowner (i.e., 656 cats/253 landowners who allow their cats outdoors), thereby substantially overestimating cat ownership—and, in turn, predation rates (which calculations are based upon the average number of cats/landowner).

Lepczyk et al. are not the only ones to make this mistake; several other researchers have done the same. [4, 5, 7-9]

Outdoor Access
The amount of time cats spend outdoors is also highly positively skewed, as is apparent from the 2003 survey conducted by Clancy, Moore, and Bertone. [19] Their work showed that nearly half of the cats with outdoor access were outside for two or fewer hours a day. And 29% were outdoors for less than an hour each day.

Among those researchers to overlook the skewed nature of this distribution are Kays and DeWan, who calculate an average of 8.35 hours/day. This greatly overestimates potential predation, and leads them to conclude—erroneously—that the actual number of prey killed by cats was “3.3 times greater than the rate estimated from prey brought home,” [9] as was discussed previously.

Compound Errors
Clearly, these errors are substantial—in some cases, doubling the apparent impact of cats on wildlife. Of course the errors are even more significant when one inflated figure is multiplied by another—as when Lepczyk et al. [6] multiply the average number of prey items returned by the average number of outdoor cats per owner. The resulting predation figures may well be four times greater than they should be! (Actually, there are additional problems with the authors’ predation estimates, which I’ll address in a future post).

*     *     *

The fact that such a fundamental mistake—one a student couldn’t get away with in a basic statistics course—is made so often is shocking. The fact that such errors slip past journal reviewers is inexcusable.

References
1. Lepczyk, C.A., et al., “What Conservation Biologists Can Do to Counter Trap-Neuter-Return: Response to Longcore et al.” Conservation Biology. 2010. 24(2): p. 627-629.

2. Woods, M., McDonald, R.A., and Harris, S., “Predation of wildlife by domestic cats Felis catus in Great Britain.” Mammal Review. 2003. 33(2): p. 174-188.

3. Coleman, J.S. and Temple, S.A., On the Prowl, in Wisconsin Natural Resources. 1996, Wisconsin Department of Natural Resources: Madison, WI. p. 4–8. http://dnr.wi.gov/wnrmag/html/stories/1996/dec96/cats.htm

4. Baker, P.J., et al., “Impact of predation by domestic cats Felis catus in an urban area.” Mammal Review. 2005. 35(3/4): p. 302-312.

5. Baker, P.J., et al., “Cats about town: is predation by free-ranging pet cats Felis catus likely to affect urban bird populations?“ Ibis. 2008. 150: p. 86-99.

6. Lepczyk, C.A., Mertig, A.G., and Liu, J., “Landowners and cat predation across rural-to-urban landscapes.” Biological Conservation. 2003. 115(2): p. 191-201.

7. Crooks, K.R., et al., “Exploratory Use of Track and Camera Surveys of Mammalian Carnivores in the Peloncillo and Chiricahua Mountains of Southeastern Arizona.” The Southwestern Naturalist. 2009. 53(4): p. 510-517.

8. van Heezik, Y., et al., “Do domestic cats impose an unsustainable harvest on urban bird populations?“ Biological Conservation. 143(1): p. 121-130.

9. Kays, R.W. and DeWan, A.A., “Ecological impact of inside/outside house cats around a suburban nature preserve.” Animal Conservation. 2004. 7(3): p. 273-283.

10. 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(5): p. 475–487.

11. Barratt, D.G., “Predation by House Cats, Felis catus (L.), in Canberra, Australia. I. Prey Composition and Preference.” Wildlife Research. 1997. 24(3): p. 263–277.

12. May, R.M., “Control of feline delinquency.” Nature. 1988. 332(March): p. 392-393.

13. Jessup, D.A., “The welfare of feral cats and wildlife.” Journal of the American Veterinary Medical Association. 2004. 225(9): p. 1377-1383.

14. Dauphiné, N. and Cooper, R.J., Impacts of Free-ranging Domestic Cats (Felis catus) on birds in the United States: A review of recent research with conservation and management recommendations, in Fourth International Partners in Flight Conference: Tundra to Tropics. 2010. p. 205–219

15. Longcore, T., Rich, C., and Sullivan, L.M., “Critical Assessment of Claims Regarding Management of Feral Cats by Trap–Neuter–Return.” Conservation Biology. 2009. 23(4): p. 887–894.

16. Winter, L., “Trap-neuter-release programs: the reality and the impacts.” Journal of the American Veterinary Medical Association. 2004. 225(9): p. 1369-1376.

17. Clarke, A.L. and Pacin, T., “Domestic cat “colonies” in natural areas: a growing species threat.” Natural Areas Journal. 2002. 22: p. 154–159.

18. Fitzgerald, B.M. and Turner, D.C., Hunting Behaviour of domestic cats and their impact on prey populations, in The Domestic Cat: The biology of its behaviour, D.C. Turner and P.P.G. Bateson, Editors. 2000, Cambridge University Press: Cambridge, U.K.; New York. p. 151–175.

19. Clancy, E.A., Moore, A.S., and Bertone, E.R., “Evaluation of cat and owner characteristics and their relationships to outdoor access of owned cats.” Journal of the American Veterinary Medical Association. 2003. 222(11): p. 1541-1545.

The Work Speaks—Part 2: Sample-Minded Research

27 May 2010

In April, Conservation Biology published a comment authored by Christopher A. Lepczyk, Nico Dauphiné, David M. Bird, Sheila Conant, Robert J. Cooper, David C. Duffy, Pamela Jo Hatley, Peter P. Marra, Elizabeth Stone, and Stanley A. Temple. In it, the authors “applaud the recent essay by Longcore et al. (2009) in raising the awareness about trap-neuter-return (TNR) to the conservation community,” [1] and puzzle at the lack of TNR opposition among the larger scientific community:

“…it may be that conservation biologists and wildlife ecologists believe the issue of feral cats has already been studied enough and that the work speaks for itself, suggesting that no further research is needed.”

In fact, “the work”—taken as a whole—is neither as rigorous nor as conclusive as Lepczyk et al. suggest. And far too much of it is plagued by exaggeration, misrepresentations, errors, and obvious bias. In my previous post, I presented examples of researchers “reinterpreting” the work of others to better fit their own arguments. For the next few posts, I’ll focus on some of the major flaws in the feral cat/TNR research itself—beginning with the reliance, by some, on small sample sizes.

Size Does Matter
There are all kinds of reasons for small sample sizes, perhaps the most common being limited resources (e.g., time, funding, etc.). And they are often a fact of life in real-world research, where investigators have less control over conditions than they might in a laboratory environment. Studies employing small sample sizes are not without value; indeed, they often serve as useful pilot studies for future, more comprehensive, work. They do become problematic, though, when broad conclusions are drawn from their results. Below are three (among many!) examples of such studies.

Impressive Estimates
In “Free-Ranging Domestic Cat Predation on Native Vertebrates in Rural and Urban Virginia,” [2] published in 1992, the authors estimated that the state’s 1,048,704 cats were killing between 3,146,112 and 26,217,600 songbirds each year. “This number,” they note, “is certainly inaccurate to some degree, although the estimates are impressive.” [2] Impressive? I suppose. Maybe incredible is more fitting—since the study from which they were derived included exactly five cats, four “urban” and one “rural.”

Mitchell and Beck acknowledged “the limitations of extrapolation to large areas from relatively small data sets such as ours,” suggesting that their work was intended to provoke future “careful and detailed studies that can reveal truer estimates of the impact of this introduced species.” Hawkins [3] and Dauphiné and Cooper [4], however, seem to take them at their word, regardless of any disclaimers.

Many Cats, Multiple Seasons
In a recent study on Catalina Island, the researchers “examined the home-range behavior and movements of sterilized and intact radiocollared feral cats living in the interior” [5] of the island. Although Guttilla and Stapp concede that “sample sizes, especially for males, were relatively low” despite having “tracked many cats across multiple seasons,” they nevertheless come to some rather dramatic conclusions. Among them: “sterilization likely would not reduce the impact of feral cats on native prey.” [5]

So what do the authors mean by many and multiple? Actually, there were just 27 cats in the study (of an estimated 614–732 on the island). “Four cats were tracked during all four seasons, 9 cats were tracked for three consecutive seasons, 4 cats were tracked for 2 consecutive seasons, and the remaining cats were tracked for 1 season.” [5] And these numbers were effectively cut in half, because the researchers were comparing sterilized and non-sterilized cats. At best, this is a pilot study—though it’s already morphed into something more substantial in the mainstream media.

Myth vs. Math
In their 2004 study, “Ecological Impact of Inside/Outside House Cats Around a Suburban Nature Preserve,” Kays and DeWan observed hunting cats, concluding that their kill rate (13%) is “3.3 times greater than the rate estimated from prey brought home.” [6] Not surprisingly, this figure has been used as an instant multiplier (much in the same way William George’s work has been misused) for researchers interested in “correcting” (inflating?) prey numbers. [4, 7-11]

But this ratio, 3.3, hinges on the hunting behaviors of just 24 cats—12 that returned prey home, and another 12 (11 pets and 1 feral) that were observed hunting for a total of 181 hours (anywhere from 4.8–46.5 hours per cat). It’s interesting to note that the cat observed the most (46.5 hours) was only a year old—the youngest of the 12 observed, and likely the most active hunter. This factor alone could have had a significant influence on the outcome of the study.

Also, as several studies have shown [7,8,12,13], the distribution of prey catches tends to be highly skewed (many cats catch few/no prey, while a few catch a lot). In other words, the distribution is not the familiar bell curve at all—making it inappropriate to use a simple average for calculating estimations (a topic I’ll address in detail later). What’s more, with only 12 cats being monitored, how can we be sure their behaviors accurately represent any real distribution at all?

But the key to their calculation is the average time spent outdoors. This, too, tends to be a highly skewed distribution [14, 15], although—curiously—Kays and DeWan’s data suggest otherwise. By way of example, a 2003 survey conducted by Clancy, Moore, and Bertone [15] revealed that nearly half of the cats with outdoor access were outside for two or fewer hours a day. And 29% were outdoors for less than an hour each day. A survey conducted by the American Bird Conservancy revealed similar behavior, reporting that “35% keep their cats indoors all of the time” and “31% keep them indoors mostly with some outside access.” [14]

Kays and DeWan’s average of 8.35 hours/day, then, seems rather out of line with other studies. This, in addition to a number of unknowns (e.g., influence of time of day/night on hunting success, actual time spent hunting by each cat, etc.) raises serious questions about their conclusions.

By way of comparison, using an average of 2.5 hours/day (which is not out of line with the surveys described above) would yield a ratio of 1:1. In other words, no difference between predation rates predicted by actual hunting observation and those predicted by way of prey returned home. Which is not to say that I agree with Kays and DeWan’s underlying methods—we don’t know the possible effects of seasonal variation, for example, or differences in habitat. I’m only pointing out how sensitive this one factor—with its enormous consequences—is to the amount of time cats actually spend outdoors (and, just to introduce one more complication: I’d be very surprised if the amount of outdoor time cats spend hunting is normally distributed; it, too, is probably skewed).

Ironically, while the authors express disappointment that “biologists have rarely sampled both cat and prey populations in such a way that direct effects on prey populations can be shown,” [6] they seem to have had no misgivings about how their work—suffering from its own sampling issues—might be used to misrepresent those same effects.

*     *     *

Next, I’ll discuss the difference between compensatory and additive predation, and how that affects predictions of feral cat impacts on wildlife.

References
1. Lepczyk, C.A., et al., “What Conservation Biologists Can Do to Counter Trap-Neuter-Return: Response to Longcore et al.” Conservation Biology. 2010. 24(2): p. 627-629.

2. Mitchell, J.C. and Beck, R.A., “Free-Ranging Domestic Cat Predation on Native Vertebrates in Rural and Urban Virginia.” Virginia Journal of Science. 1992. 43(1B): p. 197–207.

3. Hawkins, C.C., Impact of a subsidized exotic predator on native biota: Effect of house cats (Felis catus) on California birds and rodents. 1998. PhD Dissertation, Texas A&M University.

4. Dauphiné, N. and Cooper, R.J., Impacts of Free-ranging Domestic Cats (Felis catus) on birds in the United States: A review of recent research with conservation and management recommendations, in Fourth International Partners in Flight Conference: Tundra to Tropics. 2010. p. 205–219

5. Guttilla, D.A. and Stapp, P., “Effects of sterilization on movements of feral cats at a wildland-urban interface.” Journal of Mammalogy. 2010. 91(2): p. 482-489.

6. Kays, R.W. and DeWan, A.A., “Ecological impact of inside/outside house cats around a suburban nature preserve.” Animal Conservation. 2004. 7(3): p. 273-283.

7. Baker, P.J., et al., “Impact of predation by domestic cats Felis catus in an urban area.” Mammal Review. 2005. 35(3/4): p. 302-312.

8. Baker, P.J., et al., “Cats about town: is predation by free-ranging pet cats Felis catus likely to affect urban bird populations?“ Ibis. 2008. 150: p. 86-99.

9. van Heezik, Y., et al., “Do domestic cats impose an unsustainable harvest on urban bird populations?“ Biological Conservation. 143(1): p. 121-130.

10. Nelson, S.H., Evans, A.D., and Bradbury, R.B., “The efficacy of collar-mounted devices in reducing the rate of predation of wildlife by domestic cats.” Applied Animal Behaviour Science. 2005. 94(3-4): p. 273-285.

11. MacLean, M.M., et al., “The usefulness of sensitivity analysis for predicting the effects of cat predation on the population dynamics of their avian prey.” Ibis. 2008. 150(Suppl. 1): p. 100-113.

12. Churcher, P.B. and Lawton, J.H., “Predation by domestic cats in an English village.” Journal of Zoology. 1987. 212(3): p. 439-455.

13. Woods, M., McDonald, R.A., and Harris, S., “Predation of wildlife by domestic cats Felis catus in Great Britain.” Mammal Review. 2003. 33(2): p. 174-188.

14.  ABC, Human Attitudes and Behavior Regarding Cats. 1997, American Bird Conservancy: Washington, DC. http://www.abcbirds.org/abcprograms/policy/cats/materials/attitude.pdf

15. Clancy, E.A., Moore, A.S., and Bertone, E.R., “Evaluation of cat and owner characteristics and their relationships to outdoor access of owned cats.” Journal of the American Veterinary Medical Association. 2003. 222(11): p. 1541-1545.

The Work Speaks—Part 1: Lost in Translation

24 May 2010

In April, Conservation Biology published a comment authored by Christopher A. Lepczyk, Nico Dauphiné, David M. Bird, Sheila Conant, Robert J. Cooper, David C. Duffy, Pamela Jo Hatley, Peter P. Marra, Elizabeth Stone, and Stanley A. Temple. In it, the authors “applaud the recent essay by Longcore et al. (2009) in raising the awareness about trap-neuter-return (TNR) to the conservation community,” [1] and puzzle at the lack of TNR opposition among the larger scientific community:

“…it may be that conservation biologists and wildlife ecologists believe the issue of feral cats has already been studied enough and that the work speaks for itself, suggesting that no further research is needed.”

In fact, “the work”—taken as a whole—is neither as rigorous nor as conclusive as Lepczyk et al. suggest. And far too much of it is plagued by exaggerations, misrepresentations, errors, and obvious bias. For the next few posts, I’m going to present a sampling of its more serious flaws, beginning with how some researchers “reinterpret” work of others to suit their own purposes.

Tell It Like It Is
Studies of cat predation frequently cite the work of William G. George, who, in 1974, published a paper documenting his meticulous observations of the hunting behavior of three cats on his southern Illinois farm. “The results,” wrote George, “established a basis for examining the possibility that cat predation may result in depleted winter populations of microtine rodents and other prey of Red-tailed Hawks, Marsh Hawks, and American Kestrels.” [2]

Thirty years later, David A. Jessup interpreted things rather differently, giving George’s work an additional—and unwarranted—degree of certainty. Gone are the doubts that George expressed—first, regarding the impact of cat predation on rodent and other prey populations; second, regarding the relationship between these populations and the raptors that feed on them. For Jessup—who offers no additional evidence—it’s all very straightforward: “Feral cats also indirectly kill native predators by removing their food base.” [3]

More recently, Guttilla and Stapp seem to prefer Jessup’s take: “Human-subsidized cats… can spill over into less densely populated wildland areas where they reduce prey for native predators (George 1974).” [4]

If any additional work has been done on the subject (surely there are more cats in the area these days; how are the voles and raptors faring?), it seems to have gone unnoticed. Instead, Jessup, Guttilla, and Stapp (and others, too, no doubt) have simply rewritten George’s conclusion to suit their own purposes. Perhaps their version makes for a better story, but it’s rather poor science.

Credit Where Little/None Is Due
When the Lancet recently retracted a 1998 paper linking vaccinations to autism in children—“research” that sparked the ongoing backlash against vaccinations—it was headline news. The move prompted this criticism from one member of the British Parliament: “The Lancet article should never have been published, and its peer review system failed. The article should now be expunged from the academic record…”

At the risk of drawing too many parallels between the two papers, I think the same can be said for Coleman and Temple’s infamous “Wisconsin Study.” (On the other hand, it does serve a useful purpose as a red flag.) Actually, as Goldstein et al. point out, Coleman and Temple’s paper was never peer-reviewed (not necessarily a deal-breaker in my book, but such publications do warrant additional scrutiny), but achieved its mythical status by being cited ad nauseam in peer-reviewed journals, as well as the mainstream media.

Does anybody actually believe the numbers suggested by Coleman and Temple? Stanley Temple (one of the co-authors of the recent anti-feral cat/TNR comment in Conservation Biology) himself admitted their published figures “aren’t actual data; that was just our projection to show how bad it might be.” [5]

I don’t think Longcore et al. [6] or the editors at Conservation Biology put much stock in the Wisconsin Study—so why continue to publish “projections” that have been so thoroughly discredited? Because doing so strengthens their case, at least among those who don’t know any better—especially people outside the scientific community, including many journalists, policy makers, judges, and the general public.

In their recent comment, Lepczyk et al. suggest that conservation biologists and wildlife ecologists “look to the evolutionary biology community” [1] for an example of how to influence policy:

“When local policies or regulations are put forth that promote the teaching of creationism or intelligent design, the evolutionary biologists have responded in force from across the nation and world.” [1]

Let’s set aside for the moment all the baggage associated with their analogy. My question is this: Is the evolutionary biology community still publishing bogus “projections” from 13 years ago? I doubt it.

Check Your Premises
In their recent paper (available for download via the American Bird Conservancy (ABC) website), Dauphiné and Cooper arrive at their absurd figure of “117–157 million free-ranging cats in the United States,” [7] in part, by way of Jessup’s “estimated 60 to 100 million feral and abandoned cats in the United States.” [3]

So where does Jessup’s figure come from? We have no idea—there’s no citation. And Jessup is no authority on the subject—having conducted no studies or reviews of studies that quantify the feral cat population. What’s more, his “estimation” is among the highest figures published. Yet this is the shaky foundation upon which Dauphiné and Cooper attempt to build their subsequent argument.

The authors then add to the (dubious) number of feral cats the proportion of pet cats allowed outdoors. They refer to a 2004 paper by Linda Winter, director of ABC’s Cats Indoors! campaign, in which it was reported, “A 1997 nationwide random telephone survey indicated that 66% of cat owners let their cats outdoors some or all of the time.” [8]

That’s an interesting way to put it—Winter makes it sound like two-thirds of pet cats are essentially outdoor cats. But the survey—commissioned by ABC!—actually indicates that “35% keep their cats indoors all of the time” and “31% keep them indoors mostly with some outside access.” [9] The difference in wording is subtle, and hampered by imprecision—it all comes down to the meaning of some.

Winter’s 2004 paper implies that there are twice as many outdoor pet cats as was indicated in the original survey—an interpretation Dauphiné and Cooper seem to embrace. Had they looked further—and to a less biased source—they might have been able to get a better handle on the degree of outdoor access. For example: a 2003 survey conducted by Clancy, Moore, and Bertone [10] revealing that nearly half of the cats with outdoor access were outside for two or fewer hours a day. And 29% were outdoors for less than an hour each day.

Do these “part-timers” have the same impact on wildlife as feral cats? Dauphiné and Cooper would have us believe they do.

[Note: For a closer look at the flaws in Dauphiné and Cooper’s paper, download “One Billion Birds,” by Laurie D. Goldstein.]

The lesson? Credible research begins with a solid foundation; a weak foundation—one plagued with unsubstantiated claims—on the other hand, leads to pseudoscience.

Or worse. ABC’s Senior Policy Advisor, Steve Holmer, cited Dauphiné and Cooper’s bogus numbers when he spoke to the Los Angeles Times about his organization’s involvement with the legal battle against TNR. It’s like the Wisconsin Study all over again.

When All Else Fails, Look It Up
Though this would seem to be utterly obvious, it apparently bears repeating: Don’t cite work you haven’t actually read.

Isn’t this emphasized in all graduate (indeed, undergraduate, too) programs? What grad student isn’t, at one time or another, tempted to take the easy way out—ride the coattails of somebody else who’s (presumably) done the real work? In addition to the ethical implications, such shortcuts tend to invite more immediate troubles, too. Again, George’s work (described above) provides an excellent case study. Below are some examples of how his work has been referenced in the cat predation literature:

“It is very unlikely that cats bring home all of the prey that they capture. What proportion they bring home has been little studied. George (1974) on a farm in Illinois USA found that three house cats, all adequately fed, brought home about 50% of the prey that they killed.” [11]

“George found that about 50% of prey were indeed brought home, with the other 50% being eaten, scavenged by other animals or simply not found.” [12]

“These approximations are probably underestimates, assuming that cats do not bring back all the prey that they kill.” [13]

Trouble is, George never said these things; what he said was:

“… the cats never ate or deposited prey where caught but instead carried it into a ‘delivery area,’ consisting of the house and lawn. The exclusive use of this delivery area was verified in 18 to 70 mammal captures per cat, as witnessed between early 1967 and 1971.” [2]

In 2000, Fitzgerald and Turner pointed out the fact that George’s work was being misrepresented, noting that the erroneous 50% figure “has been reported widely, though it is unfounded.” [14] Nevertheless, the myth persists—even in 2010.

“In Illinois, George (1974) found that only about half of animals killed by cats were provided to their owners, and in upstate New York, Kays and DeWan (2004) found that observed cat predation rates were 3.3 times higher than predation rates measured through prey returns to owners. Thus, predation rates measured through prey returns may represent one half to less than one third of what pet cats actually kill…” [7]

As Dauphiné and Cooper demonstrate, the “reinterpreted” version of George’s work makes for a very convenient multiplier—suddenly, every kill reported is doubled (or tripled, if Kays and DeWan are to be believed—and they’re not, but that’s a topic for another post). Never mind the fact that it has no basis in actual fact.

Getting a copy of George’s study isn’t difficult, especially with the inter-library loan services available today. To reference it—to use George’s work so that your own appears more credible—without ever having actually read it, is simply inexcusable. But citing it blindly suggests more than laziness—it points to a certain coziness that has no place in scientific discourse. Too much Kool-Aid drinking, and not enough honest research.

*     *     *

Scientists can (and do) look at identical results and come to very different conclusions. But misinterpreting, misrepresenting, or dismissing the conclusions of others, is something else altogether. As the above examples (and there are many, many more!) illustrate, this happens far too often in the feral cat/TNR literature. And if we can’t believe what researchers are saying about the work of others, why would we believe what they say about their own work?

Next, I’ll focus on some of the major flaws in the feral cat/TNR literature—beginning with small sample sizes…

References
1. Lepczyk, C.A., et al., “What Conservation Biologists Can Do to Counter Trap-Neuter-Return: Response to Longcore et al.” Conservation Biology. 2010. 24(2): p. 627-629.

2. George, W., “Domestic cats as predators and factors in winter shortages of raptor prey.” The Wilson Bulletin. 1974. 86(4): p. 384–396.

3. Jessup, D.A., “The welfare of feral cats and wildlife.” Journal of the American Veterinary Medical Association. 2004. 225(9): p. 1377-1383.

4. Guttilla, D.A. and Stapp, P., “Effects of sterilization on movements of feral cats at a wildland-urban interface.” Journal of Mammalogy. 2010. 91(2): p. 482-489.

5. Elliott, J., The Accused, in The Sonoma County Independent. 1994. p. 1, 10

6. Longcore, T., Rich, C., and Sullivan, L.M., “Critical Assessment of Claims Regarding Management of Feral Cats by Trap–Neuter–Return.” Conservation Biology. 2009. 23(4): p. 887–894.

7. Dauphiné, N. and Cooper, R.J., Impacts of Free-ranging Domestic Cats (Felis catus) on birds in the United States: A review of recent research with conservation and management recommendations, in Fourth International Partners in Flight Conference: Tundra to Tropics. 2010. p. 205–219

8. Winter, L., “Trap-neuter-release programs: the reality and the impacts.” Journal of the American Veterinary Medical Association. 2004. 225(9): p. 1369-1376.

9. ABC, Human Attitudes and Behavior Regarding Cats. 1997, American Bird Conservancy: Washington, DC. http://www.abcbirds.org/abcprograms/policy/cats/materials/attitude.pdf

10. Clancy, E.A., Moore, A.S., and Bertone, E.R., “Evaluation of cat and owner characteristics and their relationships to outdoor access of owned cats.” Journal of the American Veterinary Medical Association. 2003. 222(11): p. 1541-1545.

11. Churcher, P.B. and Lawton, J.H., “Predation by domestic cats in an English village.” Journal of Zoology. 1987. 212(3): p. 439-455.

12. May, R.M., “Control of feline delinquency.” Nature. 1988. 332(March): p. 392-393.

13. Crooks, K.R. and Soule, M.E., “Mesopredator release and avifaunal extinctions in a fragmented system.” Nature. 1999. 400(6744): p. 563.

14. Fitzgerald, B.M. and Turner, D.C., Hunting Behaviour of domestic cats and their impact on prey populations, in The Domestic Cat: The biology of its behaviour, D.C. Turner and P.P.G. Bateson, Editors. 2000, Cambridge University Press: Cambridge, U.K.; New York. p. 151–175.