Fido and Felix – dogs and cats – are man’s best friends. Today, we estimate close to a billion dogs and a billion cats worldwide1,2. Through the process of domestication and artificial selection we greatly altered the natural physiology and behaviour of our four-legged companions, which is why we can no longer consider them to be “wildlife”. We also introduced dogs and cats across regions of the world where they never occurred, including islands, and this by definition makes them an invasive species. The impact of free-roaming, stray and feral dogs and cats is of particularly concern, as these individuals are out and about and the chances that they will interact with wildlife are higher. Global estimates suggest that most of the billion dogs and cats are free-roaming, stray or feral1,2.
Image credits: https://bit.ly/2pKewlu
There are four main ways that dogs and cats can interact and impact the conservation of wildlife populations. First, through predating native species. A highly cited, 2003 study published in Mammal Review suggested that 92 million wild animals can be killed by an estimated 9 million cats in the UK in just 5 months3. More recent figures from the U.S. suggested that free-ranging domestic cats may kill between 1.3–4.0 billion birds and 6.3–22.3 billion mammals annually4. Feral cats on islands are responsible for at least 14% global bird, mammal, and reptile extinctions and are the principal threat to almost 8% of critically endangered birds, mammals, and reptiles, according to a 2011 study5. Similarly, a recent study suggested that dogs contributed to 11 vertebrate extinctions and that they are severely threatening the conservation of at least 188 threatened species worldwide – mostly through predations6.
(a) Number of extinct or threatened vertebrate species that are, or were, negatively impacted by domestic dogs Canis familiaris; (b) Percentage of extinct or threatened vertebrate species that are, or were, affected by different types of dog impact. Image credits8
Other ways that dogs and cats impact wildlife populations include disturbance (e.g., chasing, harassment), disease transmission, competition for resources and hybridisation. The hybridisation issue has received particularly little attention by the media, and it is a largely unknown or unheard-of issue by the public. Interbreeding with domestic cats is the very issue that has led the Scottish wildcat on the verge of extinction7, while interbreeding with dogs is currently a main threat to European wolf populations8. Also known as a type of anthropogenic hybridisation, this interbreeding issue occurs when dogs or cats mate with their wild counterparts and have a fertile hybrid litter. In the long term, such hybridisation can lead to the genetic extinction of wild species, which is complementary to the issue of demographic extinction.
A subtler issue which is very much unexplored scientifically and which particularly concerns free-ranging, stray and feral dogs, is how dogs may exacerbate human-carnivore conflicts by depredating livestock, while wolves may get the blame.
There is also a fifth, and perhaps most controversial point, on how dogs and cats impact wildlife. This time are our dogs and cats at home that are concerned. Dogs and cats both eat meat-heavy diets, meaning they contribute to the consumption of animal products, the meat farming industry and all the pollutions, toxins and other emissions that it creates. A 2017 study published in PLOS One suggested that cats and dogs are responsible for 25 to 30 percent of the environmental impact of meat consumption in the United States9.
Tackling conservation issues related to dogs and cats is complex, not only for the sheer number of their populations, but also because as man’s best friends they have intricate relationships with us which means that any management intervention deeply touches upon and interweaves with ethical issues (e.g., lethal, non-lethal removal from the wild).
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- Woods, M., McDonald, R.A. and Harris, S., 2003. Predation of wildlife by domestic cats Felis catus in Great Britain. Mammal review, 33(2), pp.174-188 https://doi.org/10.1046/j.1365-2907.2003.00017.x
- Loss, S.R., Will, T. and Marra, P.P., 2013. The impact of free-ranging domestic cats on wildlife of the United States. Nature communications, 4, p.1396.
- Medina, F.M., Bonnaud, E., Vidal, E., Tershy, B.R., Zavaleta, E.S., Josh Donlan, C., Keitt, B.S., Le Corre, M., Horwath, S.V. and Nogales, M., 2011. A global review of the impacts of invasive cats on island endangered vertebrates. Global Change Biology, 17(11), pp.3503-3510 https://doi.org/10.1111/j.1365-2486.2011.02464.x
- Doherty, T.S., Dickman, C.R., Glen, A.S., Newsome, T.M., Nimmo, D.G., Ritchie, E.G., Vanak, A.T. and Wirsing, A.J., 2017. The global impacts of domestic dogs on threatened vertebrates. Biological conservation, 210, pp.56-59 https://doi.org/10.1016/j.biocon.2017.04.007
- Okin, G.S., 2017. Environmental impacts of food consumption by dogs and cats. PloS one, 12(8), p.e0181301 https://doi.org/10.1371/journal.pone.0181301