Convincing local experiences have shown that predator or competitor destruction is likely to be practical only on small, not-too-rugged islands, usually no larger than about 500 ha. However, special circumstances have prompted us to attempt destruction, or at least control, on much larger and more difficult islands. It is implicit that the predators or competitors are exotic, not indigenous. Recently, on those rare islands that are inhabited but still free of either black or Norway rats, we have set up permanent bait stations (at which sodium fluoroacetate—"1080"—is used as the poison) on wharves and jetties in the hope that such a precaution will, with the addition of a propaganda campaign calling for the regular fumigation of visiting vessels, prolong the charmed lives that these fortunate islands have so far enjoyed. It goes without saying that we ask that the greatest care be taken when expeditions land stores on uninhabited, rat-free islands which, if by "rat-free" we mean also free of R. exulans, are even rarer in our seas.
The sociolegal approach is effective only when ecosystems or communities have not been seriously modified, otherwise it is no substitute for either of the other two measures discussed.
Some Case Histories
Translocation
Last century, an endemic monotypic genus of wader—the New Zealand shore plover (Thinornis novaeseelandiae)—was widespread and occasionally very common around the coasts of the North and South islands and the Chatham Islands. As a result of European settlement and the accompanying predation by feral cats and rats, the species now occurs only on South East Island in the Chatham group (860 km east of the mainland), where it at present seems safe, since there are no rats on the island and it is now a reserve. However, the population numbers only about 120 individuals. Because calamities can always occur (for example, ship rats recently reached shore on three important islets off the southwest coast of Stewart Island), the Wildlife Service is anxious to spread the shore plover to other suitable islands, if they can be found. The species is not a migrant and is rather sedentary. The first translocation attempts failed, probably because mainly adult birds were used, and we are now continuing our studies of the species with the thought in mind, among others, that success may come if young birds are used instead; the question is—how young?
As is widely known, the New Zealand Wildlife Service has been remarkably successful in recent years in translocating one species of the endemic wattlebird family—the forest-dwelling saddleback (Philesturnus carunculatus)—to other islands than the four small ones it had been reduced to by the early 1960's; three of these islands were the ones recently invaded by ship rats, referred to above.
Predator Control
Some 25 km off the North Island's east coast lies the 3,000-ha, very rugged and forested Little Barrier Island, which has now been a reserve for the protection of flora and fauna for about 80 years. Before that, it had been almost continually occupied by Maoris since their arrival in New Zealand, and about one-third of its forest was felled or burnt, especially after European settlement of the adjoining New Zealand mainland began.
Most unusually, Little Barrier is now free of any grazing or browsing mammals, and has only the Polynesian rat (a reminder of the Maori occupation) and feral cats (a European legacy) to impair its extreme importance as a reserve. The rats have been unmolested by man because, rightly or wrongly, they are considered ineffective predators generally; however, their impact has probably been under-rated. More than half a century of trapping and hunting of cats by successive caretakers on the island has not effectively reduced that population.