Feature

The Cat's Breakfast

Farmers want to introduce myxomatosis to control rabbits, while DSIR scientists recommend using cats to do it. Both schemes have implications for native species not considered by proponents of either view. By Dr Charles Daugherty and Dr David Towns

Introduced rabbits occupy most areas of New Zealand. In at least a few areas of South Island high country -- central Otago, the Mackenzie Basin and Marlborough -- they are abundant enough to pose a significant local threat to agriculture. Now the government is moving nearer to a decision to allow the introduction of myxomatosis, long advocated by some farmers to control rabbit populations.

Another possible control which avoids the use of myxomatosis is proposed by DSIR scientists who have studied New Zealand mammal populations for many decades (Dr Bob Brockie, The Final Solution?, NZ Science Monthly, March 1991).

They recommend enhancement of cat populations in problem areas by provision of shelter and food throughout the winter months. The cat population would thus have a head-start on its rabbit prey, sufficient to prevent the spring build-up of rabbits to pest proportions each year. According to this view, cats and other mammalian predators comprise a "cost-free natural system of rabbit control" that only needs a little help to do the job in the remaining problem areas.

The DSIR plan might work -- and it just as likely might not. Despite decades of research worldwide, ecologists still know far too little about factors controlling wild populations to predict future numbers of pest species in most instances. Within New Zealand, the complex interactions of many predator species with each other and with a variety of native and introduced prey species have barely been studied.

Collateral Damage

In our view, the greatest danger of the cat scheme is the virtual certainty of what military people call "collateral damage" -- that is, unintended deaths of innocent bystanders.

To some of New Zealand's rarest and most spectacular invertebrates and lizards, the uplands of Otago, the MacKenzie Basin and Marlborough are far from the "bleak country" referred to by Brockie. Species such as the large Otago skink (Leiolopisma otagense), the grand skink (Leiolopisma grande), the black-eyed gecko (Hoplodactylus kahutarae), and several kinds of large weta have special adaptations to cope with climate. But they have no adaptations to cope with cats.

Cats can be very efficient predators of lizards and large insects. During a Wildlife Service survey of Otago's large lizards in December 1983, a cat was killed mid-morning while it was searching the same rock outcrops as the survey party. The cat's stomach contained 14 lizards, all undigested and therefore the result of that morning's hunting. These 14 lizards represented three species -- the grand skink, the common skink, and a species that was then unknown to zoologists.

Extrapolation of the possible effects of even a few cats on local lizard populations from this single piece of evidence is speculative. It does indicate that one cat could easily kill hundreds of lizards over a summer. The more effective cats prove to be in reducing rabbit populations, the more they would need to turn to alternative prey sources such as lizards and insects.

Unnatural Predators

Furthermore, the presence of cats in New Zealand is not natural. Cats have been in New Zealand only about 200 years; lizards and wetas at least 20 million years and probably far longer. Cats and other mammalian predators constitute an unnatural menace to terrestrial ecosystems, one whose effects are disastrous.

Biologists last century warned that stoats and ferrets should not be introduced to control rabbits, as farmers wanted, because of the harm they would cause to native species. This warning went unheeded, with exactly the predicted results. It is no small irony that a similar scheme to use introduced predators is being proposed again, and again native species will suffer.

In focusing on the adverse effects of cats on lizards and other innocent bystanders, we are not advocating the introduction of myxomatosis to control rabbits. Rather, we believe that the arguments over rabbits, cats and myxo divert attention from a more complex problem -- the proper use of high country lands. The myxomatosis scheme and the cat-enhancement scheme are intended to have the same effect. They are to reduce rabbit populations for the sake of growing more sheep. In either case, native species will be the real meat in the sandwich.

Debate Misses Point

The present debate tacitly accepts the view that sheep farming is the best use of these areas. But the result of a century of sheep farming has been the conversion of broad areas of grassland ecosystems into wastelands. The rabbit alone is not the villain. Over-intensive farming of sheep that removes the native tussock landcover and replaces it with exotic plants is the real problem.

Rational debate should thus not focus on rabbits, which are only a symptom and not the disease. Ecological and environmental as well as financial costs of sheep farming in the South Island high country should be clearly acknowledged. Possibilities for use of this land other than sheep farming should be considered.

Further land degradation and extinction of native species will be the price paid for continuing present land management regimes.

Dr Charles Daugherty is an ecologist and herpetologist with Victoria University.
Dr David Towns is an ecologist and herpetologist with the Department of Conservation.