Apocalypse Meow

By EcoEvo@TCD • Reviews • 4 Feb 2013


Cats eh? You either love them or you hate them it seems. Well the latest research published in Nature by Loss et al. (2013) will give those who hate them plenty more reason to do so. While those who love their cats may just sit that little bit less comfortably next to their feline companions.

Let me start by making a few things clear. Cats are predators, they are an invasive species which have been introduced to islands all over the world, by man. In many places domestic cats have become feral, i.e. reverted to living in the wild, which has led to huge increases in their numbers in some places, which can have a devastating effect on indigenous wildlife populations. For a more detailed and somewhat depressing example of where this has occurred read about Stephens Island in New Zealand. Famously a lighthouse keeper’s cat had been blamed for the extinction of an entire species on this island though it seems that reports may have been somewhat exaggerated in this case.

The report in Nature is more scientifically robust than urban legends about a lighthouse keeper’s cat though. Figure 1 below shows just how devastating domestic cats can be on local wildlife populations. The graphs show the estimates of predation by domestic cats on (a) birds and (b) small mammals.

Figure 1. Estimates of cat predation on US birds and mammals (from Loss et al. 2013)

Figure 1. Estimates of cat predation on US birds and mammals (from Loss et al. 2013)

The numbers are startling, an estimated 2.4 billion birds are killed by cats every year in the US and 12.3 billion mammals. Incredible numbers I’m sure you will agree, there is however a caveat; only 31% and 11% for birds and mammals respectively are caused by what the writers class as “owned cats”, cats which are regularly kept indoors and well fed. While the majority of the mortality is thought to be caused by free roaming “unowned” cats. Incidentally there has recently been some debate about wind farms and their impact on local bird populations but this excellent blog and another recent Nature piece put the numbers into perspective in terms of other anthropogenic causes of bird deaths.

As those responsible for the domestication and introduction of cats, we can’t lay all the blame at the feet of our feline friends. First of all we need to somehow effectively manage the populations of feral and “unowned” cats and while this has been attempted with the Trap-Neuter-Return movement, it has been viewed as a response based on regarding feral cats as part of the native fauna rather than the invasive aliens that they are , therefore largely unscientific and ineffective. Secondly pet owners can take several measures of their own, neuter or spay your cat while keeping your cat indoors at night can vastly reduce their impact on local wildlife.


Keith McMahon: mcmahok[at]tcd.ie

Photo credit

wikimedia commons

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6 Responses

  1. It would be interesting to know what people think about this issue and what should be done about it? Do cat owners care? Would conservationists like to see cats eradicated? That’s exactly what they’ve proposed in New Zealand? Please chip in?

  2. Aoibheann

    As a cat lover (erm, I’m slightly obsessed) this topic does make me quite uncomfortable. Taking personal responsibility for your pet is critical. Our fella is neutered and he stays in at night, although he’s managed to bring home a few birds nonetheless.

    There have been repeated calls by groups here for a government-sponsored TNR program to deal with feral colonies, but it’s just not on their agenda. Extermination by (often privately-contracted) pest control companies is the usual method of dealing with feral cats. So TNR programs tend to rely on volunteers for funding and manpower, and they don’t get a lot in the way of sympathy from joe-public.

    There were a couple of similar studies carried out in the UK on cat predation (links below). I heard an interview with one of the authors who said that it is possible that cats are merely replacing native predators that have declined in the UK and are acting as a control on rodent populations. Which is interesting…

    In terms of islands and other sensitive areas where feral cats are demonstrably decimating native species, I think eradication is probably the only way to deal with them. Though I couldn’t physically participate in that myself!



  3. Some good points there Aoibheann, the problem with TNR is that even if you have 80-90% capture rates, the 10% who can still reproduce are very soon able to replace those non-reproducing individuals, especially when you consider tghe size of litters and how soon those litters themselves become able to reproduce. TNR then becomes an all or nothing exercise, not to mention extremely costly. As for the argument about replacing natural predators? I don’t buy that for a second, there are just far too many feral cats for that to be a fair excuse. They may well control pest rodent populations but there are some rodents who don’t need controlling and it is these that we should be concerned about. I agree with you on the island eradication idea, it has been very successful in New Zealand. Overall though it really is a complex issue and a sensitive one.

  4. Aoibheann

    On reflection, those UK studies were based on responses from domestic cat owners and say nothing about the impact of feral colonies on wildlife really

    And Id like to point out I didn’t say I believed her re. the replacement of natural predators!!

    Of course TNRing 100% is impossible and costly (Im sure there’s a model that would tell us how quickly numbers would actually recover at different levels of TNR effort). But is the objective then to keep numbers down and reduce impact of feral cats on native wildlife or is it total eradication?

    If it is eradication, and you achieve it, how do you stop feral colonies becoming re-established when ‘owned’ cats become feral/are abandoned/are not sterilized? Should cat owners be legally obliged to neuter and spay their cats or have a license to breed them? Should cat ownership be banned altogether? Whatever any legislation might say, enforcement would be a huge issue. And the larger the country/landmass, the harder it will be to deal with.

    Either way, in this country there is no official policy that attempts to deal with the problem either through TNR or eradication. And those advocating for TNR are coming at the issue from the perspective of the welfare of the feral cats, rather than the protection of native species. And that probably means that getting a policy for culling feral cats through over and above a TNR approach would be difficult?

    Also, it would be interesting to know the differences between urban and rural feral cats, in terms of their densities and impact on wildlife. Would urban colonies have less of an impact on native biodiversity than rural colonies? If so, perhaps you tackle the rural populations first?

    Finally, please don’t murder my cat!

  5. I promise I won’t murder your cat. I think realistically eradication of the feral cats (by what ever means) is key, then education of cat owners and the general public. I think if cat owners knew what their cats get up to a lot of them would think twice about letting them out. Aside from the damage cats do to the local wildlife it also increases the risk of those cats contracting parasites and diseases transmitted by the animals they hunt. So its better for all if cats are just kept indoors. Not an ideal solution, but what solutions are.

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