Danyang Jiang

Friends but also troublemakers – studies have shown that outdoor domestic cats are threatening our urban ecosystem by hunting other animals. With an increasing population of outdoor cats in urban cities, we need to take action to build sustainable cities and communities, for us and our cats.

Cats in the urban cities

Humans love cats. The population of cats owned by households in Europe has been increasing steadily over the recent years and has reached approximately 110.15 million in 2020, making them the most preferred pets (Cat Population in Europe 2010-2020, 2021). With the fast speed of urbanization, domestic cats enter urban cities with humans and occupy their places, both indoors and outdoors.

Figure 1. The number of pet cats in Europe from 2010 to 2020 (in 1000s). Source: FEDIAF

Though most cats have their owners, a large number of stray cats are wandering in the cities. Many of the stray cats used to have a home, but have ended up homeless because of being lost or abandoned. Another important cause of stray cats is free-ranging domestic cats without neutering. Domestic cats are very adaptive in terms of food, habitat and climate, and they are fertile breeders – they reach reproductive age between 7 and 12 months and can have up to three offspring each year (Trouwborst et al., 2020). Free-ranging domestic cats mate with strays, produce offspring, and thus form larger stray cat colonies. At the same time, due to humans’ feeding, stray cat colonies can maintain and expand.

There is no rigorous estimation of stray cats worldwide, while an estimation suggests that the population of outdoor cats in the U.S. is around 32 million. Among those outdoor cats, roughly 76% live in urban areas, with a rather high estimated density of 100 cats/km² (Rowan et al., 2019).

Table 1. Estimates of the number of outdoor cats in the USA. Adapted from (Rowan et al., 2019)

The ecological threats are so critical that domestic cats (Felis catus) have been listed as one of the world’s worst non-native invasive species by the International Union for Conservation of Nature (IUCN) (Simberloff & Rejmanek, 2019). Given the high numbers and densities of cats in cities, coupled with the hunting instinct, outdoor domestic cats have become wandering hunters that threaten the urban ecosystem.

Cats are opportunistic hunters. They pray a wide range of animals including birds, mammals, reptiles, amphibians, fish, and invertebrates like butterflies (Trouwborst et al., 2020). Quite a lot of outdoor cats are fed cat food, so they kill less prey per day than wild predators. But they tend to stick close to home – their home ranges are so small that they have a highly concentrated impact on local wildlife (Pet Cats Have up to ‘10-Times Larger Impact on Wildlife than Wild Predators’, 2020).

Figure 2. Cats who roam outdoors tend to stick close to home, having a big impact on local wildlife. Source: Louise LeGresley/Getty Images

Domestic cats’ predation impact has been quantified by several studies. In Canada, predation by domestic cats is ‘probably the largest human-related source of bird mortality, with approximately 100–350 million birds killed each year (Blancher, 2013). Similarly, each year domestic cats kill an average of 377 million birds (Woinarski et al., 2017) and 649 million reptiles (Woinarski et al., 2018) in Australia. In the U.S., domestic cats annually kill 95–299 million amphibians, 258–822 million reptiles, 1.3–4.0 billion birds, and 6.3–22.3 billion mammals (Loss et al., 2013). It is suggested that outdoor cats are likely the single greatest source of anthropogenic mortality for U.S. birds and mammals (Loss et al., 2013).

Outdoor cats can even contribute to the extinction of species. As an example, Tibbles the cat possibly caused the extinction of Lyall’s wren in New Zealand by decimation (“How Tibbles the Cat Possibly Caused an Entire Species to Go Extinct”, 2021). Threaten is large, especially in urban cities, where the ecosystem is rather fragile due to human perturbation and low levels of biodiversity.

Figure 3. Probability distribution of estimated bird and mammal mortality caused by all free-ranging cats in mainland areas of the contiguous United States. (Loss et al., 2013)

What could be the solution

To solve the threats caused by outdoor cats in urban cities, a solution would be giving a call to cat owners to keep their cats indoors. Although indoor cat life brings several health risks: obesity, diabetes, depression, etc., a tradeoff exists, because there are also outdoor dangers: car accidents, catfights, diseases, and also the huge threats to the local ecosystem we discussed above. For those who are concerned about cat’s happiness and the state of nature, it is suggested to set outdoor restrictions and to install catios that enable cats to run and climb but separate them from wildlife (Pet Cats Have up to ‘10-Times Larger Impact on Wildlife than Wild Predators’, 2020).

Figure 4. A catio is a great way to ensure the safety and happiness of cats but eliminate their ecological threats at the same time. Source: (Ramos, 2015)

Another suggestion is to get outdoor cats neutered as soon as possible so that stray cat colonies in cities would be limited. Cats from households are suggested to be neutered, especially if they have access to the outdoor communities. As for the stray cats, the Trap, Neuter, Release (TNR) strategy is generally preferred to reduce the population of stray cats. However, the TNR method cannot take effect immediately, and the effect in large stray cat colonies has not been supported by peer-reviewed scientific studies (Loss et al., 2013; Mofield, 2020).

Cat collars with bells, in addition, are generally considered a good idea to prevent outdoor cats from hunting easily. Besides alerting the prey of approaching cats, cat collars with bells can also help the owners locate them and help others identify the cat when he gets lost (“Should I Put a Bell on My Cat’s Collar?”, 2017).

Figure 5. Cat wearing a collar with a bell. Source: Jirakarn Dhejsawatdhirat/Shutterstock

Conclusion

The number of cats in our cities is continuously growing. The number of outdoor cats is growing as well, consisting of not only the free-ranging pet cats but also the stray cats. Due to the hunting instinct among a large number of cats in our cities, the urban ecosystem faces huge threats. It is suggested to keep cats indoors, get outdoor cats neutered, and use cat collars with bells to decrease the ecological impact of outdoor domestic cats. We should take action to build sustainable cities and communities, along with our adorable cat friends.

References

Blancher, P. (2013). Estimated Number of Birds Killed by House Cats (Felis catus) in Canada. Avian Conservation and Ecology, 8(2). https://doi.org/10.5751/ACE-00557-080203

Cat population in Europe 2010-2020. (2021). Statista. https://www.statista.com/statistics/516041/cat-population-europe-europe/

How Tibbles the cat possibly caused an entire species to go extinct. (2021, May 7). ZME Science. https://www.zmescience.com/science/how-tibbles-the-cat-possibly-caused-an-entire-species-to-go-extinct/

Loss, S. R., Will, T., & Marra, P. P. (2013). The impact of free-ranging domestic cats on wildlife of the United States. Nature Communications, 4(1), 1396. https://doi.org/10.1038/ncomms2380

Mofield, G. (2020). A Modern Cat-tas-trophe? The Relationships and Impacts between Feral Cat Management and Sustainable Urban Development: A Literature Review. 34.

Pet cats have up to ‘10-times larger impact on wildlife than wild predators’. (2020, March 12). BBC Science Focus Magazine. https://www.sciencefocus.com/news/pet-cats-have-up-to-10-times-larger-impact-on-wildlife-than-wild-predators/

Ramos, M. (2015, October 26). Catio Spaces Helps Cat Owners Build Safe Outdoor Havens For Their Feline Friends! IHeartCats.Com. https://iheartcats.com/catio-spaces-helps-cat-owners-build-safe-outdoor-havens-for-their-feline-friends/

Rowan, A. N., Kartal, T., & Hadidian, J. (2019). Cat Demographics & Impact on Wildlife in the USA, the UK, Australia and New Zealand: Facts and Values. Journal of Applied Animal Ethics Research, 2(1), 7–37. https://doi.org/10.1163/25889567-12340013

Should I Put a Bell on My Cat’s Collar? Pros and Cons. (2017, April 28). Fluffy Kitty. https://thefluffykitty.com/should-i-put-a-bell-on-my-cats-collar

Simberloff, D., & Rejmanek, M. (Eds.). (2019). 100 of the World’s Worst Invasive Alien Species: A Selection From The Global Invasive Species Database. In Encyclopedia of Biological Invasions (pp. 715–716). University of California Press. https://doi.org/10.1525/9780520948433-159

Trouwborst, A., McCormack, P. C., & Martínez Camacho, E. (2020). Domestic cats and their impacts on biodiversity: A blind spot in the application of nature conservation law. People and Nature, 2(1), 235–250. https://doi.org/10.1002/pan3.10073

Woinarski, J. C. Z., Murphy, B. P., Legge, S. M., Garnett, S. T., Lawes, M. J., Comer, S., Dickman, C. R., Doherty, T. S., Edwards, G., Nankivell, A., Paton, D., Palmer, R., & Woolley, L. A. (2017). How many birds are killed by cats in Australia? Biological Conservation, 214, 76–87. https://doi.org/10.1016/j.biocon.2017.08.006

Woinarski, J. C. Z., Murphy, B. P., Palmer, R., Legge, S. M., Dickman, C. R., Doherty, T. S., Edwards, G., Nankivell, A., Read, J. L., & Stokeld, D. (2018). How many reptiles are killed by cats in Australia? Wildlife Research, 45(3), 247. https://doi.org/10.1071/WR17160