Many people in western societies like to keep cats as pets, and with that simple observation we run into two problems that require resolution. First, cats are killers of wildlife, particularly birds but also an array of other small prey. Most people do not believe this, because cats are adored and make good, if somewhat disinterested pets. So, my first point might be that if you think cats are not killers, I invite you to keep another cat like a mountain lion for a pet. But we need some data on the kill rate of cats. Before we begin this search, we should note that cats can be kept inside dwellings or in cat runs with no access to birds or other prey. If this is the case, no problem exists for wildlife, and you can skip to the bottom of this blog for one other issue to recognize.
How much mortality can be traced to cats roaming out of doors? This will include normal house cats let out to roam at night, as well as wild cats that have been discarded by their owners into the wild. There is extensive literature on cats killing birds. If you want a brief introduction Greenwall et al. (2019) discuss a nesting colony of Fairy Terns, a threatened species of Australian seabird, along a beach in southwestern Australia. With detailed observations and photographic data, they recorded the complete failure of all 111 nests in this colony with the loss of all tern chicks in the early summer of 2018. The predator was a single desexed feral cat. Many local governments allow the capture of feral cats with the protocol that they are desexed and then released back into the environment. Clearly desexing and release does not remove the problem.
The domestic cat has been spread world-wide, so that the cat problem is not a local one. Li et al. (2021) completed a survey of feral cat kill rates in the eastern part of China and found that the minimum annual loss of wildlife to feral cats was in the range of 2.7-5.5 billion birds, and 3.6-9.8 billion mammals, as well as large numbers of amphibians, reptiles, and fish. In gardens in Western Europe cat predation on ringed birds studied with precise data showed that up to 25% of dead birds were killed by cats, but these data varied greatly among species (Pavisse et al. 2019). For the European Robin which often feeds on the ground 40% of all ringed birds were killed by cats, for the European Greenfinch the figure was 56% of ringed birds killed. These are just two examples of an extensive literature on cat kills going back many years (Calvert et al. 2013).
What can we do about this predation? As with too many conservation issues the answer is simple but difficult to implement: Ban all cats from free-ranging unless they are on a leash and under control. Keep cats in the house or in special cat runs that are confined outdoors. Ban completely stupid programs of catching feral cats, sterilizing them, and releasing them back to the wild to continue their killing. Cats may make marvellous pets but need to be kept indoors. Many people would support these measures but many cat owners would disagree about such measures. Some progress is being made in urban environments in which some suburbs do not permit cats to roam freely.
Feral cats are a serious issue in Australia because they attack many threatened birds and reptiles (Doherty et al. 2019). In this case a federal environmental policy to kill 2 million cats is popular but from a conservation viewpoint still a poor policy. We do not know if killing 2 million cats is too much or too few, and without specific goals for conservation and careful monitoring of bird populations widespread killing my not achieve the goal of protection for threatened species. Eradications of cats on islands is often feasible, but no mainland eradication is currently possible.
As conservation biologists know too well, when humans are the problem, wise policies may not be implemented. So, the second issue and the bottom line might be to consider the human costs of cat ownership. Adhikari et al. (2020) report a highly significant association between the risk of dying from colon cancer and cat ownership. These results are not confounded by sedentary lifestyle, cigarette smoking or socio-economic status. In a similar study Adhikari et al. (2019) found that living with a cat significantly increased the death rate from lung cancer among women. The cause of these associations cannot yet be deciphered but are postulated to result from mycotoxins, toxic secondary metabolites produced by fungi (moulds) in cereal crops used in cat food. Aflatoxin is a mycotoxin that produces well-known chemicals that are seriously toxic to animals and humans.
These kinds of studies of associations arising from surveys can be tossed off by the typical comments ‘these-things-do-not-concern-my cats’ or ‘that there is no proof of the exact cause’ so if you are concerned you might investigate the literature on both mycotoxins and the diseases that cats carry.
It is up to humans to solve human problems, but up to conservation biologists to point out the detrimental effects of household pets and their feral cousins on wildlife. The present situation is a complete policy failure by governments at all levels. Good science is relatively easy, good policy is very difficult.
Adhikari, Atin, Adhikari, A., Jacob, N. K., and Zhang, J. (2019). Pet ownership and the risk of dying from lung cancer, findings from an 18 year follow-up of a US national cohort. Environmental Research 173, 379-386. doi: 10.1016/j.envres.2019.01.037.
Adhikari, Atin, Adhikari, A., Wei, Y. D., and Zhang, J. (2020). Association between pet ownership and the risk of dying from colorectal cancer: an 18-year follow-up of a national cohort. Journal of Public Health 28, 555-562. doi: 10.1007/s10389-019-01069-1.
Calvert, A.M., Bishop, C.A., Elliot, R.D., Krebs, E.A., Kydd, T.M., Machtans, C.S., Robertson, G.J., 2013. A synthesis of human-related avian mortality in Canada. Avian Conservation and Ecology 8: 11. doi 10.5751/ACE-00581-080211.
Doherty, T.S., Driscoll, D.A., Nimmo, D.G., Ritchie, E.G., and Spencer, R. (2019). Conservation or politics? Australia’s target to kill 2 million cats. Conservation Letters 12, e12633. doi: 10.1111/conl.12633.
Li, Yuhang, Wan, Yue, Shen, Hua, Loss, S.R., Marra, P.P., and Li, Zhongqiu (2021). Estimates of wildlife killed by free-ranging cats in China. Biological Conservation 253, 108929. doi: 10.1016/j.biocon.2020.108929.
Greenwell, C.N., Calver, M.C., and Loneragan, N.R. (2019). Cat Gets Its Tern: A Case Study of Predation on a Threatened Coastal Seabird. Animals 9, 445. doi: 10.3390/ani9070445.
Pavisse, R., Vangeluwe, D., and Clergeau, P. (2019). Domestic Cat Predation on Garden Birds: An Analysis from European Ringing Programmes. Ardea 107, 103-109. doi: 10.5253/arde.v107i1.a6.
Hi Charley, my concern about statements about how many billion native animals cats take each year, only show that they do kill natives, but they don’t show the impact on the population nor do they consider the other factors influencing the conservation status of native species, especially the loss or degradation of suitable habitat. For many species, native habitat is a small percentage of the original – no suitable habitat , no natives! It also seems to escape many re feral cats and Australia, that many species that are endangered on the mainland are doing quite well on big islands such as Tasmania and Kangaroo Island, both of which have healthy cat populations but no foxes! I wonder what the explanation might be.
Re domestic cats, I wrote an article for the local press a couple of years back.
ACT Cat Containment Program – is it justified?
The ACT Government is proposing to introduce legislation requiring that all domestic cats be contained by their owners. Personally, I am not a fan of cats and would prefer that they not be in Canberra. But I am also aware that many Canberrans love their cats and that cats are an important companion animal that undoubtedly save millions in health costs through the mental health benefits that they give their owners. I have worked in the pest management for more than 30 years and understand that managing pests is a complex social, environmental and economic issue that needs to addressed strategically and one that takes into account the many and varying attitudes towards pests. The ACT has adopted this rational and strategic approach to planning and managing pests. Hence it is interesting to compare the cat management proposal against the approach that is out in the ACT Pest Animal Strategy, 2012-2022. How does the Government’s proposal stack up?
In essence the ACT pest strategy requires those planning and implementing pest management to follow a series of steps
1.Define the problem. What damage are domestic cats causing to native wildlife in Canberra? Yes they take native birds and lizards although they also feed on rats, mice and rabbits, especially at night. But diet studies only indicate what animals may be at risk, they do not show whether the level of predation is affecting the overall population. As Professor Chris Dickman from Sydney University who has written extensively on cats and their management says: The difficulty in assessing the impact of cats on populations of native species, as opposed to predation on prey individuals, is the teasing out of the relative contributions of all the variables that can lead to reductions in the abundance, distribution and densities of species. These variables include climatic events (drought, fire, flood, etc), habitat modification, disease, and food resource distribution and density. It is not sufficient to simply document the diet of cats and assume that this equates to impact at the population level, although several studies have attempted to do this. Such extrapolations are inappropriate for several reasons. First, cats may prey most readily upon the ‘doomed surplus’; that is, the individuals in a prey population that are too young, old, or weak to reproduce. Their removal clearly makes no difference to a population’s rate of increase. Second, remaining prey individuals may respond to the removal of conspecifics by showing improved survival and increased reproductive output, thereby compensating for individual losses at the population level. Third, cats may eat individuals of a particular prey species but have positive effects on prey population growth and size if they depress other species that would otherwise impact on the prey species more strongly. Such indirect interactions are pervasive in natural systems, and are likely to have particularly strong effects in predator-prey systems (Denney and Dickman, 2010; Dickman 2014).
Interestingly, the ACT has the ideal situation for testing the effectiveness of cat containment. It would be a simple and relatively inexpensive scientific study to compare the abundance and diversity of native birds and reptiles in suburbs with cat containment, Forde and Bonner, with that in similar suburbs with no cat containment. The suburbs would need to be matched for age, housing density and have similar mix of garden plantings.
2.Quantify the extent of the problem and look at other factors that may be causing similar damage.
Accurately quantifying the damage requires a study similar to that outlined above, or better still to compare areas where cat density has been reduced to areas where there has been no cat control. But what other factors are of concern? Well in urban Canberra there are many stray cats, foxes and dogs all of which predate native wildlife. Then there is the quality of habitat for native wildlife; the areas that provide food and nesting and roosting sites that all species need to survive. It is interesting that Canberra is built on old grazing and cropping land that was cleared of native grassland and woodland. Further (admittedly degraded) woodland has been cleared to build shopping centres, ovals, roads industrial centres and houses, not ideal habitat for native animals. To some extent landscaping of gardens has restored some of this habitat and studies have shown that as new suburbs age and are landscaped, the number and variety of native birds increases greatly, and in the presence of cats.
3.Consult all stakeholders.
Given the wide diversity of views about cats and their importance as companion animals, before any wide scale management of them and costs imposed on the community. Studies by Grayson et al (2002) and Lilith et al (2006) conclude that some forms of cat management are seen to be acceptable; including registration and sterilisation, but that many owners would be reluctant to keep their cats out of particular areas or to contain them. This suggests that such actions should only be taken after extensive community consultation and education.
4.Set clear time limited objectives in terms of the reduction of damage to wildlife
This is not possible at present because the level of damage due to cats is not known nor is the extent to which cat predation needs to be reduced.
5.Identify cat management options and assess the risks and the costs and benefits.
Currently there are very few techniques available for managing urban cats. Poisons are not suitable nor acceptable nor is government trapping which also is very labour intensive and expensive. That leaves containment. It is likely to be expensive, between $1,000 and $2,500 per household plus installation and maintenance, but even if it is implemented how will compliance be ensured? It is likely to be costly both in terms of the number of rangers required, the difficulty in proving that the owner had not accidently let their cat escape, nor will it be easy to distinguish domestic from stray cats. Then there are the financial and social costs of enforcing penalties through the courts. The containment proposal needs to be carefully examined in terms of the costs and benefits. Also it would help to identify the assumptions that must hold true for the strategy to work. For example, will the necessary proportion of cat owners contain their cats; will the reduced predation of rats and mice by cats lead to an increase in their numbers and the damage that they cause; do all cat owners have the funds to purchase and install and maintain cat containment enclosures?
6.Monitor and evaluate.
This is not possible unless the areas where cats are contained can be compared to matched areas where there is no cat containment, what science calls treatment versus non treatment studies.
In summary the proposal to introduce legislation that requires cat owners to contain their cats needs to be assessed against the process set out in the ACT Pest Strategy. This includes extensive open and two-way community debate about the proposal amongst all those concerned both those for and those against the proposal. It may be that other actions such encouraging residents to better landscape urban areas to provide more high quality habitat would have a much greater benefit in more diverse and abundant native wildlife than cat containment. It would also be much less controversial.
References
ACT Government (2012) ACT Pest Animal Strategy 2012-2022. Department of Environment and Sustainable Development, Canberra. http://www.environment.act.gov.au/cpr/conservation-strategies/pams2
Braysher, M (2017) Managing Australia’s Pest Animals: a guide to strategic planning and effective management. CSIRO Publishing, Clayton, Victoria.
Denny, E and Dickman, C (2010) Review of cat ecology and management strategies in Australia. A report to the Invasive animals Cooperative Research Centre, Canberra.
Dickman, C.R (2014) Measuring and managing the impacts of cats IN Glen, A. and Dickman, C. Eds Carnivores of Australia, past, present and future. CSIRO Publishing, Collingwood.
Grayson, J, Calver, M, Styles, I (2020) Attitudes of suburban Western Australians to proposed cat control legislation. Australian Veterinary Journal 80: 536 -543.
Lilith, M, Calver, M, Styles, I, Garkaklis, M (2006) Protecting wildlife from predation by owned domestic cats: application of a precautionary approach to the acceptability of proposed cat regulations. Austral Ecology 31: 176-189.