By - Wayne Linklater 26/11/2013 5


Just because native species persist in our backyards despite being hunted by predators, like cats, it does not mean their populations are healthy. Our backyard can be a sink-hole for native species. Each killed is replaced by recolonisers who are themsleves be killed (photo source: inhabitat.com)

Cats might be a “beneficial urban predator” according to Dr. John Flux. His conclusion and its representation in media, however, has relied heavily on the 17-year record of prey John’s cat, Peng You, brought to him.

But there are five reasons why John’s record cannot be used to conclude that cats are a beneficial urban predator. I addressed the first two in my previous post: cats bring only a small and biased sample of what they actually kill.

John’s conclusion also depends on his observation that native species, like fantails and skinks, persisted in his backyard even though his cat killed them. This, John asserts, is evidence that his cat had no impact but also leads to the third reason why John’s logic and conclusion are flawed:

3. Native animals re-colonise the homes of their killed neighbours

Native animals killed by cats in our backyards are often replaced by animals from other backyards and natural habitats. Young animals, for example, disperse from their parent’s habitat to establish their own breeding territories or home ranges as adults. They will colonise where there is unoccupied habitat and space for them. If a cat, and other backyard predators, are making space available by killing, other native animals may re-colonise that space only to themselves be killed and eventually also replaced.

John’s backyard could have been an all-you-can-eat buffet for his cat – depleting the wider landscape of native animals. Our backyards with cats can be like a sink-hole for native animals. John makes the mistake of assuming that just because he can see the same native species in his backyard for 17 years of killing by his cat that the wider population of natives is not impacted. On the contrary native populations could still be in a cat-induced decline or not increasing as fast as they could without cats.

Our backyards can be breeding-grounds for native animals that grow to recolonise the backyards of our neighbours and the landscapes natural places if, as well as planting native species, we control the predators of native animals in our back yards too. That includes keeping our cats indoors or choosing not to have one in your home (Photo source: www.doc.govt.nz).

Reversing the sink-hole

We can also turn this process around to consider generating a positive outcome. Without exotic predators, like his cat and rats, John ‘s backyard could have been a place producing more native animals that dispersed to colonise his neighbours’ backyards and the wider landscape.

All John needed to do was control his cat and trap for other predators like rats. How wonderful would that be!

John’s backyard could have contributed to the restoration of native animals in his area, but instead he had a cat, and let the rats be.

When concluding that his cat was a beneficial urban predator, John forgot to think outside his own property to his neighbours’ properties and farther afield. What happens in our backyards has consequences beyond the boundaries of our own properties – for better and worse, depending on whether we support and tolerate exotic predators where we live.

Is your backyard predator-free?

It could be.


5 Responses to “Presence of native species, not evidence all is hunky-dory”

  • To be fair, 17 years is a long time, and one might well expect a local depletion of at least the less dispersable species (e.g. skinks) after 17 years of cat predation, assuming that his isn’t the only cat in the vicinity. It is not as if the number of prey individuals out there is FIXED, and the cat is slowly knocking them off, one by one. The prey species are probably recruiting from within the area (i.e. breeding) as well as recruiting from outside the area (and also, of course, breeding outside the area as well). Having said that, it can be hard to tell a native skink from a rainbow skink, so one has to make sure that one’s identifications of the prey species are reliable.

    • Thank you Stephen for your comment. Importantly, your comment identifies several of the uncertainties that would need to be addressed to prove John’s cat was a beneficial urban predator, or that this is true of cats generally.
      In particular, that where cats are there is:
      1. no local depletion of native species
      2. local recruitment and dispersal by native species (not local extermination and re-colonisation from outside the hunted area)
      We would also need to know:
      3. how many cats, other than John’s, occupied the local area and
      4. what and how much they killed, and lastly,
      5. what other predators were present and how much they killed and were themselves killed by cats.
      Proof requires that these things be measured and compared with a similar area that does not have cats, or cats are controlled.
      None of these uncertainties are addressed by John’s article. None of the 5 is measured or compared with a control area without cats.
      The idea of ‘cats as beneficial urban predator’ was given greater credence during the debate than the scant evidence warrants. John might be right. But we should not inform debate with guesswork or construct environmental policy from anecdote. John’s “beneficial urban predator” is guesswork based on anecdote. At best it is a hypothesis. Science is now required.