Feature

The Cat's Lunch

DSIR ecologists have considered the implications for native species of using cats to control rabbits, and believe that lizards and wetas may yet be better off.

By Dr John Flux and Dr Bob Brockie

Most of the criticisms of cat control of rabbit populations raised by Drs Daugherty and Towns [April 1991] arise from a misunderstanding. Our proposed scheme should stabilise cat numbers. It would also prevent any periodic explosions of hungry predators, which probably cause the most "collateral damage" to desirable wildlife.

We are not advocating long-term "enhancement of cat populations", but an eventual reduction of cat numbers by at least half and as much as 80% from present densities. There would also be a great decline in numbers of ferrets, stoats and weasels, all of which eat more lizards than cats do.

We agree that "ecologists still know far too little about factors controlling wild populations to predict future numbers in most instances". But more ecological work, here and overseas, has been done on rabbits than on any other wild animal. We recall John Gibb's classic prediction in 1963 -- subsequently proved correct -- that stopping all Rabbit Board control over 3,000 acres in the Wairarapa would have no effect on rabbit numbers. However, we do not advocate that people should start using cats to control rabbits, but merely that an experiment is needed first.

Experiments Needed

The experiment will preferably involve two 20 km² study areas, one with a high rabbit population (10-20/ha) and the other with few rabbits. The aim is to demonstrate that cats can keep rabbit numbers from rising again. Simple shelters will house one cat family per square kilometre, a density much lower than where rabbits are currently abundant.

The aim is not to increase cat numbers so that they get on top of the rabbits, but to decrease rabbit numbers to below the cats. The cats will be managed as semi-domesticated rabbit hunters and fed shot rabbits in winter to encourage them not to wander off to adjacent farms with more rabbits.

Their feeding habits will be closely watched and the numbers of other predators and rabbits studied. In the long term, improved pasture will prolong the rabbit breeding season and provide more varied prey, so feeding the cats in winter would no longer be necessary.

Rabbit Densities

The difference in density of rabbits in Central Otago compared with the rest of New Zealand is hard to appreciate. Until the 1980s, poison followed by night shooting was used in the problem areas to keep numbers down, but the procedure had to be repeated annually. If it was not, reduced rabbit populations of one per hectare could recover to 20/ha within a year.

When the government cut funds, expensive night shooting was stopped and bait-shy rabbits multiplied. The simplest solution is to revert to night shooting, selling carcasses to cover costs (as we have advocated for 30 years without success).

Densities of up to 60 rabbits/ha are reported in Otago. It is unlikely they could get higher, because of poor grassland productivity --  on good pasture, rabbits can reach stable populations of 200/ha. In general, problem areas carry 10-30 rabbits/ha, or about 200 times the density in other parts of the country.

Given the enormous biomass of young rabbits, one would expect high numbers of predators in Central Otago, and this is what we find. Harriers, for example, are at least ten times as abundant there as elsewhere. Because of seasonal shortages of food, lack of alternative prey and erratic supply of of rabbits following poisoning, the predators never get on top of the situation.

Instability Dangers

The present instability must be more harmful to native fauna than a stable, reduced number of cats. Where rabbits are at low density under cat control, ferrets and weasels are generally absent and stoats are rare. Cats eat stoats, weasels, rats and mice, and probably do more good than harm by keeping more serious predators away. Hence our choice of cats, rather than ferrets or stoats or promoting all predators equally in Central Otago.

If the cats cause trouble we will know about it. Two people stationed at Alexandra will record exactly what goes on, and the removal of errant moggies is simple if they are semi-domesticated.

Of course cats, rats, ferrets, stoats and weasels should not have been introduced in the first place, but it is not possible to remove them from the mainland now. Perpetual control of everything, as exercised by British gamekeepers, is too expensive. It seems better to adjust existing populations to stable densities at which they do the least damage. Hence, this is not "a similar scheme to use introduced predators" comparable to the original imports of such animals. Rather, we aim to establish the most favourable balance of predators, rabbits and native wildlife that will allow these degraded lands to recover.

Farming To Blame

Finally, we would be upset if the grass saved from these rabbits was merely fed to sheep. Rabbits recycle nutrients which are removed from the land in sheep exports and are not often replaced. The real culprit is over-intensive farming, as Daugherty and Towns point out.

The proper use -- if any -- of this land is going to be a never-ending political debate. In the meantime, environmental degradation continues, rabbits increase and calls for myxomatosis intensify. Both myxomatosis and viral haemorrhagic disease could destabilise the free and very efficient rabbit control by predators over most of New Zealand. Then we would have hungry predators eating native species from North Cape to Bluff, as rabbit populations oscillated from one epidemic to the next.

"The DSIR plan might work -- and it just as likely might not." We rate our chances better than that because whether it works or not, we may learn why and how the system functions. We could even find that the cat killed after eating 14 lizards for breakfast was going to eat a weasel for lunch -- a weasel that would have lived on all winter, preying regularly on skinks and invertebrates.

Dr Bob Brockie is a Research Associate with Victoria University's School of Biological Science.
Dr John Flux is an animal ecologist with DSIR Land Resources.