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The commentary of the last few days following Dr. Gareth Morgan’s media release has focussed too much on the messenger and how the message was delivered rather than understanding the problem and potential solutions. Some have described his views as extreme but whether or not they are extreme is less important than whether or not they are supported by evidence. Desicion-making in environmental policy should follow the evidence like it does in medicine, law and engineering. Let us move beyond the messenger and message and discuss evidence for a cat problem – is there a problem? – and the potential solutions that evidence informs – what solutions are best?

Cat-astrophy measured

Studies show that there is enormous variation between our domestic cats in their propensity to kill native wildlife. Some cats rarely kill while others kill frequently. If your cat is killing it will sometimes return home with its kill, although some cats do this more than others and some not at all. In Dunedin, only about 1 in 4 domestic cats bring prey home [1]. In a UK study about 40% of cats came home with their prey [2].

The interesting use of micro-cameras on domestic cats in the US – KittyCams – showed that only a quarter of cats’ prey was brought home. About 30% of prey was eaten and 50% left uneaten at the kill-site. Thus, even if you never see you cat with prey, it may still be killing.

New Zealand studies of domestic cat killing have relied on estimates from documenting the prey a cat brings home and so certainly under-estimate how much killing domestic cats do. Nevertheless, these studies are instructive for their local relevance.

NZ studies show that 13 animals are brought home per year by cats. If that is a quarter of total kill then the figure inflates to around 50 animals a year – one per week per cat. But about a third of NZ domestic cats do not hunt [1]. The reason why so many animals can be killed but a large number of domestic cats not hunt is that some cats kill an awful lot.

In the US study with KittyCams on 55 cats, 16 cats (29%) killed animals at an average rate of 2 animals per week. Most killed just one or two animals a week but some killed as many as 5 a week. Goodness – that would approximate 250 wildlife killed a year by one cat! Most domestic cats are not killing or killing only a little, but a few kill a tremendous amount.

We do not know at this time for New Zealand what most influences some domestic cats to hunt more than others. Characters of the cat like age and breed, or the owner’s behaviour like how much time they spend home might be important. More research is required on this question because it may be that there are small measures an owner can take to substantially reduce a cat’s killing. Nevertheless, we do know that you can reduce your cats kill rate of birds by as much as 50% by making it wear a bell [3] – please do. Cat collars with bells are a pre-cautionary, albeit imperfect, solution until we can contribute other solutions.

Fantail – highly vulnerable to predatory cats. Source: www.doc.govt.nz

We also know that if native wildlife is more common where your cat lives, and the cat hunts, then they will kill more native wildlife [1]. If you live near a park or reserve, or well treed neighbourhood, your cat is likely to be killing more native animals. If you live in an area without native wildlife – perhaps because introduced predators like cats prevent them from living there – then your cat will be killing native wildlife less often. The success of your local biodiversity santuary or your the neighbour’s attempts to restore native wildlife to the area is likely to depend on reducing the number of rats, stoats, ferrets and cats where you live.

The Common Skink is a very widespread lizard species in New Zealand. Source: Trent Bell, www.landcare.org.nz.

You might also be surprised how much native wildlife is in yours and your neighbours’ gardens. In surveys of Wellington and Dunedin suburbs native birds and reptiles are surprisingly common, although most residents are unaware of the reptiles. The US KittyCam study found that the prey brought to the residence underestimated the number of reptiles killed – they are more likely than birds to be left where they are killed.

Some, like the SPCA’s Robert Kerridge, claim that well-fed domestic cats do not kill wildlife or the kill is trivial. The evidence shows that this is not true. In our neighbourhoods enough domestic cats are owned such that there will inevitably be some who kill a lot. Some of those killed will be native animals, especially where wildlife is common around local parks and reserves.

Nevertheless, even if domestic cats kill, it is still possible that the number they kill is not substantial enough to prevent native populations from thriving. Fortunately, there is evidence from New Zealand and internationally that also addresses this question. In a follow-up post I answer the question ‘do domestic cat populations exterminate native animal populations?’

 

Bibliography

1 van Heezik, Y., et al. (2010) Do domestic cats impose an unsustainable harvest on urban bird populations? Biol. Conserv. 143, 121-130

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

3 Gordon, J.K., et al. (2010) Belled collars reduce catch of domestic cats in New Zealand by half. Wildl. Res. 37, 372-378