Feature

The Final Solution?

Rabbit control strategies have focused on myxomatosis and poisoning, but DSIR scientists suggest a simpler alternative.

By Dr Bob Brockie

Cats, not myxomatosis, are the answer to the South Island's burgeoning rabbit population. That's the message from the DSIR and other scientists.

Until the 1940s, most New Zealand farms were overrun with rabbits. Dropping poison from planes, improved fencing and stock management helped to bring down rabbit numbers. They were kept down because farmers ceased simply burning off their scrub and weeds, and turned to top-dressing and hormone weedkillers to improve their pasture. Rabbits thrived in the old short burnt pasture, but did badly in tall improved grass. Consequently, their numbers fell away in the 1950s.

Wild cats and other predators took control, and still hold rabbits in check over at least 95% of the country. You can now drive from North Cape to Bluff and scarcely see a rabbit where once they swarmed.

Rabbits do remain a problem in small areas of the South Island, mainly near Alexandra, in pockets of the Mackenzie Basin and in upland Marlborough. The climate, soils and plant cover in these places favour the rabbit and put predators at a disadvantage. Running high numbers of sheep on this country also favours rabbits.

Poisoning No Answer

Poisoning does not control the pests in these threatened places. Indeed, so much poison has been used that most rabbits have learned to avoid it. Some appear to have become genetically resistant to the usual poisons. The time has come, say many disgruntled runholders and pest controllers, to bring in myxomatosis and the rabbit flea needed to spread the infection.

Few New Zealand scientists support the farmers' demands for myxomatosis. They know that, despite the disease, rabbits are far more numerous in Britain and Australia than they are in New Zealand. The biologists argue that the South Island rabbit problem is being exaggerated. It's seen as a local phenomenon, not a national scourge like possums. Myxomatosis could destabilise a cost-free natural system of rabbit control which operates very successfully over the entire North Island and over most of the South Island.

Leading field biologists suggest that if the balance were tipped in favour of wild cats, these predators would soon get on top of the rabbits and hold their numbers down.

This suggestion has not been made lightly, but follows many years of research on cats, ferrets and rabbits on farms, forests and islands around the country.

Wild Diets

Over the last 20 years, biologists Mike Fitzgerald, Phil Moors, Ray Pierce and Nigel Langham have examined over a thousand cat and ferret droppings. They have counted and weighed the fragments of bone, fur, insects, feathers and egg shell in these scats. This has enabled them to build up a very detailed picture of the diets of these animals in different kinds of landscape.

Wild cats and ferrets prefer rabbits to all other foods. In the open farmland of the Wairarapa, rabbits make up some 77% of the cats' diet, and 81% in the Mackenzie Basin. Above all, cats and ferrets prefer young rabbits, which they will eat all year round if available.

On farms in the Wairarapa, North Canterbury and near Wanganui, biologists John Gibb and Morgan Williams asked Pest Boards to leave rabbits alone with no poisoning or shooting for a year or two. It might be expected that, left to themselves, rabbit numbers would explode. In fact, their numbers remained much the same as before, or actually fell without any human interference. Something other than the Pest Boards was holding the rabbits in check. When cats were removed from farms, rabbit numbers shot up within a year.

Island biology is always instructive. Islands without cats, such as Whale and Brown Islands, have serious problems with rabbits. On Motuihi Island, cats were removed in 1978-79 and rabbit numbers soon shot up. When cats arrived on Mangere Island, rabbits died out.

Population Dynamics

Over most of New Zealand, rabbits breed all year round. This provides wild cats with a diet of young rabbits throughout most of the year. Fewer youngsters are produced in winter, so the predators turn to rats, mice, insects and birds to see them through the coldest months. This alternative prey ensures that cats survive the winter and that enough predators are on hand to control young rabbits when they start appearing in spring.

The uplands of Otago, the Mackenzie Basin and Marlborough are not like the rest of New Zealand. In these cold places, grass stops growing and rabbits produce no young at all for several months through the winter. Few rodents, birds or insects are available in this bleak country, so the cats have very little alternative prey during the coldest months. Poisoning rabbits in winter makes matters worse. The few cats around feed on the poisoned carcasses and are themselves poisoned.

While studying the ecology of the black stilt in the Mackenzie Country, Ray Pierce found that cats and ferrets almost disappeared after rabbits were poisoned in winter. When the rabbits started breeding again each spring, few cats or ferrets remained to attack them. Consequently, in these cold areas of the South Island, the pests range out of human and animal control.

Cat Control

DSIR biologists are looking for a farm in Central Otago where they can run experiments on cat control of rabbits. No new cats would be released on the farms, but the few resident animals would be provided with simple shelters and food -- shot rabbits -- through the winter.

The biologists are confident that, given the right conditions, more cats will stay where they are wanted over winter and prevent the buildup of young rabbits in spring.

There are concerns that encouraging cats may prove a threat to wild birds, such as the black stilts which survive only in Otago's riverbeds. Ironically, using cats to control rabbits may help reduce cat predation on rare birds. As rabbits are eaten off the landscape, the number of cats will also fall away. This has been demonstrated in field studies in the North Island.

Critics claim that cats, ferrets and stoats did not control rabbits when they were released here last century and ask why they should do the trick now? Few people realise that these animals do a very good job, holding rabbits in check over most of the country. In addition, few people appreciate the peculiar circumstances in these upland areas which conspire against natural predators.

Dr Bob Brockie is a Research Associate with Victoria University's School of Biological Science.