… just because our cats kill it does not necessarily mean that native animal populations cannot co-exist, or even thrive, where our feline companions live.
Two considerations determine whether cat-kill exterminates native prey populations:
1. how many cats there are, and
2. how fast native animal populations in our backyards reproduce themselves or recolonize our backyard (immigration) when the previous native animal is killed.
The more cats there are, the more native animals can be killed, and the more likely that cat-kill will exceed the capacity of native animal populations to replace themselves. But some native animals are faster reproducers than others – they raise larger numbers of offspring. Fantail may raise up to 5 clutches of 2 to 5 eggs each year but Kereru (NZ pigeon) is more likely to have just one clutch and egg each year. Habitat will also influence how fast native animals can replace themselves. The native animals in neighbourhoods with better habitat, because they are treed or near regenerating bush, may reproduce more.
Thus, the resilience of native animal populations to cat predation or, to put it from the cat’s perspective, achieving a sustainable hunt (harvest) of native prey, depends on the balance we strike in our neighbourhoods between having cats and habitat for native animals.
If we measure how fast native animals replace themselves, and combine the estimate with known cat-kill rates, we can know if native animal populations will co-exist with our cats. Fortunately, these measures and comparisons have been made.
In Reading, United Kingdom, 6 common bird species were sometimes brought home dead by domestic cats in greater numbers than were alive in the adult and breeding population . Cats, therefore, had the capacity to significantly reduce the size of local populations for common urban birds. Not all studies suggest that the amount of killing by cats is enough to exterminate native animal populations. Enough do, however, for us to want to conduct similar studies in New Zealand.
Fortunately, this has already been done in Dunedin by Dr. Yolanda van Heezik and her research group at the University of Otago. Domestic cats in Dunedin City killed silvereyes, bellbirds and fantails – 3 native bird species - at a rate faster than those birds could establish and reproduce . This was also true for exotic birds like house sparrow, blackbird and song thrush. So severe was the kill rate for fantail that cats were catching more than were bred.
Thus, the persistence of these native and exotic birds in Dunedin, and perhaps other New Zealand cites, may depend on them colonising from somewhere else. The cat densities in Dunedin responsible for such kill rates are typical of other New Zealand cities. About 35% of Dunedin households had at least one cat resulting in about 223 cats per urban square kilometre. At these cat densities Dunedin’s native birds were not resilient to predation and the cat harvest was not locally sustainable.
Mr Robert Kerridge, CEO SPCA, is reported to have said that “fewer than half of New Zealand’s domestic cats killed other animals. The ones that did caught far more rodents than birds”. What he says is only sometimes true. And even when it is true the kill-rate of native animals may still be large enough to cause native animal populations to decline. The density of cats in our cities generates a kill rate sufficient to exterminate native animals from our backyards and neighbourhoods.
The solution to cat-kill will most likely come from reducing the ability of cats to kill or the number of cats, and improving our neighbourhoods as habitat for native wildlife. How we achieve these in ways that are acceptable to our diverse community, including many cat owners, will be the subject of a future post.
For the next post, however, I will treat the problem of unintended consequences. Even if we accept that cat-kill needs to be reduced, might the environmental costs out-weigh the benefits for native wildlife? Some, for example, have cautioned that a reduction in cat-kill will just mean rats become a more serious problem because cats also hunt rats. Could cats be good because they are the lesser of two evils?
I will meet this problem head on next.