The Legacy of Big South Cape Island

Forest and Bird, Issue 313, 1 August 2004, Page 32

The Legacy of Big South Cape Island

DON MERTON

—DON MERTON

Forty years on,

remembers an ecological catastrophe

which changed the way we protect our islands and endangered birds. Historic pictures by the author.

orty years ago an event occurred on island near Stewart Island that changed forever the way we perceive, manage and protect our offshore islands — and our living heritage. The accidental introduction of rats to Big South Cape Island/Taukihepa had far-reaching implications not only for New Zealand, but for island nations around the world. Before the diasaster, as a junior officer with the former Wildlife Service, I was privileged to spend a month on Big South Cape Island — at 939 hectares, the largest of the ‘muttonbird islands’ off the southwestern coast of Stewart Island. Big South Cape at that time was the final refuge for a number of native animals formerly abundant and widespread on the mainland and Stewart Island. The island was home to the last populations of three bird species and a bat which had become extinct on all but Big South Cape and two tiny adjacent islands — Solomon and Pukaweka. Although inhabited by some 300 ‘muttonbirders’ for around three months each year, and somewhat modified, no mammal had been introduced and the island retained its full quota of native wildlife. Big South Cape was a remarkable bird island then — and a very special place for both Maori and Pakeha. As a consequence of this visit (and many other field trips in remote areas), I tried to fathom just how and why our native wildlife — especially birds peculiar to New Zealand — could flourish here but be in such a predicament on the mainland. Although habitat destruction and

fragmentation were well advanced on the mainland, we still had hundreds of thousands of hectares of seemingly intact native forest and other habitats. Yet massive extinctions and retractions in range had occurred and, in contrast to islands like Big South Cape, our mainland forests were largely silent. Some leading biologists (educated in Europe or North America) were adamant that the ecological collapse and extinctions we'd experienced were not a consequence of predation — predators, they explained, were a natural part of the scheme of

things....They argued that, as in Europe and North America, extinction and the numbers of birds endangered within New Zealand were primarily due to habitat destruction and fragmentation. My colleagues and I were not convinced. It was the rat invasion of Big South Cape Island that clinched our arguments, and changed forever the way we perceive, protect and manage our islands and their native plants and animals: In March 1964, muttonbirders returning to Big South Cape reported that a rat plague was causing immense damage to property and wildlife. Ship rats Rattus rattus had reached their island and had ‘irrupted.This was disastrous as the island was the final and only refuge for such rarities as the South Island saddleback, Stead’s bush wren, the Stewart Island bush snipe and (unbeknown to us at the time) the greater short-tailed bat. My Wildlife Service colleagues and I were very concerned and anticipated a biological disaster. We had the support of Forest and Bird and the Southland Acclimatisation Society in this, but only a few scientists agreed. Some of the most knowledgeable and respected biologists at that time genuinely believed the rats did not pose a significant threat to resident wildlife, and vigorously opposed any suggestion to intervene. They took the view that the birds would adjust to rats as they apparently had in Europe, and maintained that ‘if we intervene we will change the ecology in a way that we cannot predict. We should

intervene only after research has shown there is in fact a problem, They overlooked the fact that our birds are different, having evolved without fourlegged predators. In spite of this — and thanks to the tenacity of senior Wildlife Service staff members Brian Bell and Frank Newcombe, together with concerned

scientists including Sir Charles Fleming — we eventually succeeded in gaining permission to mount a rescue mission. Sadly, by the time we reached Big South Cape, five months had passed and many land-bird populations had already been effectively destroyed. Our small team, led by Brian Bell, succeeded in saving the South Island

saddleback by transferring some of the remnainder to two small neighbouring pest-free islands — Kaimohu (8 hectares) and Big (Stage) (18 hectares). Tragically, it was too late to save the bush wren, the snipe and the bat — all of which were quickly exterminated along with at least one invertebrate species. On our first visit to Big South Cape Island, in 1961, 18 species of landbird were present. By 1965, following the colonisation and irruption of rats, eight species of the island’s landbirds had undergone a dramatic decline. Five of these were subsequently lost from the island — two becoming globally extinct. Not only did the rats exterminate the bush wren, the snipe and saddleback; more common species such as the Stewart Island fernbird and robin became locally extinct, and others such as the bellbird and kakariki underwent major population declines. Fortunately, the saddlebacks we rescued and released on Kaimohu (15 birds) and Big (Stage) (21 birds), thrived so that it has since been possible to establish them on numerous other islands off Stewart and the South Island, including Breaksea in Fiordland — after rats were removed. Sadly, the six bush wrens we released on Kaimohu did not persist. The last two

were seen there in 1972. And, the two Stewart Island snipe caught died in captivity before the constant wintery gales and big seas abated sufficiently for us to transfer them. The bush snipe would eat only live invertebrates and we were unable

to find enough natural food for them (I later carried out trials involving the related Chatham Island snipe and found that these birds can in fact be maintained in captivity, but require copious quantities of live food by night and day). The rats on

Big South Cape had reduced that island's invertebrate populations to a level where we had great difficulty finding sufficient natural foods to feed the captive birds. The tragedy of Big South Cape was a valuable and timely lesson for conservation workers. The experience served to convince even the most sceptical that — unaided — rats are capable of inducing ecological collapse and extinctions on islands. The Big South Cape disaster also had a massive, enduring impact in shaping future conservation policy and practice both within New Zealand, and in other island nations around the world. Refined over the decades, the techniques of predator mitigation, eradication and control have now reached a level where — with ongoing vigilance — it is possible to maintain the rodent-free status of biologically important islands. Forty years on, we can eradicate rats, stoats and other aliens from large islands (both offshore and mainland) so as to restore ecological values and processes. We can then reinstate predator-sensitive species, such as kaka, kokako, kiwi — and even saddleback — within mainland habitats.

is now a world-renowned

expert in the rescue and recovery of endangered species. He works from Wellington, with the Department of Conservation.