Rats & Mice

Gareth MorganEnvironment

Our recent cat camera survey has rekindled the debate on wandering cats, and the risks they pose by killing native wildlife and spreading disease, let alone to themselves. The response from some was to express concern about that old chestnut – that managing domestic cats might let loose a plague of rodents.

This response is typical of hard core cat lovers. While our native birds are getting slaughtered coming out of Zealandia and other wildlife hotspots, they waste time on ill-informed claims trying to point the finger elsewhere. The fact is that we have always advocated cat management as part of a wider programme of eliminating all predators. At the moment in Wellington the Council controls all predators to some degree – except cats. It’s time to tidy up the rats & mice and deal to the most potent predator of them all, cats.

Let’s take a moment to disentangle the misinformed claims made around cats and rats. The study most often cited by cat supporters was a study of one single cat on the edge of the city. That study found that cat brought home more rats and mice than native birds. Shazam! Since rats eat birds and eggs, cats must be good for native wildlife, right? Well, no.

For now, lets overlook the fact that it was one study and one cat, what do those results really tell us? Firstly we know from cat camera studies that cats bring home one in five pieces of prey that they kill. They eat one third, and leave the rest where they kill them to rot. So the data used for this study is literally what the cat dragged in. There are huge species biases here – for example cats are more likely to eat the native lizards they kill. So any study like this grossly underestimates lizard kills.

Secondly, any ecologist knows that to understand the total population impact of a predator, births are just as important as deaths. Rats breed 20-30 times faster than native birds, so to have the same impact on the rat population as the bird population a cat would need to kill many times more rats and mice than birds. We don’t see this in any data. On the other hand we do know that cats kill birds faster than they breed – in urban areas they kill enough to cause local extinctions.

Thirdly, cats are opportunistic hunters. In other words they kill whatever is around. If there are birds, they kill birds. If there are mice, they kill mice. If there are rats, the really brave ones might occasionally kill a rat. The fact that cats kill more rodents than birds is more an indication of what is around rather than what cats prefer to hunt. Given that cats can cause local extinctions of birds, it should be no surprise they kill more rats.

Finally, our study showed that cats are far more prevalent than rats and mice in Wellington. We photographed 613 cat visits, compared with 27 hedgehogs, 9 dogs and a grand total of one single rat. We are a long way from having to worry about rats over cats as the main predator in Wellington. This sentiment has been echoed by the very scientists who are most concerned about an explosion of rat populations if cats are removed (like Landcare’s John Innes). The densities of cats in urban areas are far higher than those needed to control rodents. And there is no ecologist we know of that approves of cat colonies as supported by the SPCA – these are clearly an ecological disaster.

We know that cats are major predator to birds coming out of Zealandia. We know this from the autopsies that have been done of the birds that fly out of Zealandia and return in body bags. Cat lovers need to face up to the facts and start keeping their cats inside.

Ecosystems are incredibly complex, and it is hard to know what the impact of removing one species will be. But what we do know is that our native wildlife flourishes when their predators are kept in low numbers. These predators include cats AND rats. So achieving our vision of a Predator Free New Zealand will ultimately require both.

catloverThe fact is that humans are far more effective at controlling rat numbers than cats are. Only human trapping can get rat numbers down to the level below which saddleback could survive outside Zealandia. As mentioned above, there are no records of cats making rats extinct, the best they can do is control them. There is no case to answer in terms of cats being necessary to control rats, it’s an uninformed argument.

The real limitation on rat populations is the availability of food, not the prevalence of cats. We are seeing this in the beech forests of the South Island right now with the mast season. In short the best way to make our backyards safe for native wildlife is to limit food for rats, trap rodents in areas where food is available (like compost heaps) and keep our cats inside.

Relying on cats to control rats is just plain lazy – we have to get out there and do it ourselves. Then one day we might be able to entrust control of our few remaining rodents to the resurgent numbers of morepork and karearea.


Rats & Mice was last modified: December 15th, 2015 by Gareth Morgan
About the Author

Gareth Morgan

Facebook Twitter

Gareth Morgan is a New Zealand economist and commentator on public policy who in previous lives has been in business as an economic consultant, funds manager, and professional company director. He is also a motorcycle adventurer and philanthropist. Gareth and his wife Joanne have a charitable foundation, the Morgan Foundation, which has three main stands of philanthropic endeavour – public interest research, conservation and social investment.