insects at Risk

Forest and Bird, Issue 294, 1 November 1999, Page 32

insects at Risk

GEORGE GIBBS

GEORGE GIBBS argues that too many protected predators are changing the nature of our offshore sanctuaries. he offshore island sanctuaries set aside to protect our threatened wildlife may be the scene of New Zealand’s next extinctions. Burgeoning numbers of predators, both native and introduced, may be causing the accelerated extinction of their prey, including several rare insects and other invertebrates. On the tuatara islands of Cook Strait, growing numbers of this prehistoric reptile are changing the very nature of their environment. On Little Barrier, the introduced kiore, or Pacific rat, and the saddleback, have brought the ‘world’s largest insect? — the wetapunga — to the brink of extinction. If we are not to lose an increasing number of our insect species, thought needs to be given to the number of their predators. It is ironic that while conservation of New Zealand’s native animals and plants is progressing in leaps and bounds, with each successful island transfer or pest eradication, the threats to our native insects, spiders and other small creatures can be increased. Much of our invertebrate diversity has never been ‘described’ in the scientific sense. Some creatures, such as the Cromwell chafer beetle, have only tiny localized areas of distribution. In such cases, extinction of a species can occur without anyone noticing, but the changes in our offshore nature reserves are closely observed. Scientific records over the years indicate that invertebrate numbers can be affected by burgeoning populations of predatory animals. Several rarities, including the giant weta, are in danger of extinction unless we adjust the pressure on them. New Zealand has a long list of extinctions. For birds, the list is almost a world record; nothing short of calamitous. Less is known about the loss of invertebrate animals.

While birds leave bones, animals which do not have a bony skeleton leave few traces. It is almost impossible to list the invertebrates that have recently become extinct. Nevertheless, we should attempt it. With the emphasis of conservation moving from individual species to whole communities, we should be devoting far more effort to this ‘silent majority’ of small invertebrate animals. To set goals for New Zealand conservation, scientists strive for the ‘pre-humar’ state. The first ‘unnatural’ extinctions in New Zealand are associated with the introduction of kiore, the Pacific rat, which Richard Holdaway has suggested might have occurred as long ago as 2000 years. Although this small rodent had little impact on the larger birds, there is clear evidence that it had a devastating effect on some of the larger terrestrial invertebrates, and on reptiles, amphibians and small flightless wrens. The impact of kiore can still be seen today by comparing the fauna of islands where kiore are present against those without. The scarcity of larger invertebrates and reptiles on kiore-infested islands is immediately apparent. There are several recent cases where an insect or a snail has disappeared from a lim-

ited and well-studied area. And a disturbing feature is emerging: invertebrates are becoming extinct within our nature reserves! Believe it or not, sanctuaries which receive heavy doses of ‘conservation management’ are the places where species are being lost. One reason for this is that sanctuaries are chosen for their unique or rare examples of animals and plants. Another is that we tend to know more about the biology of nature reserves than the country at large. The irony is that they are selected for the express purpose of saving species from extinction. Small areas, such as islands, can only maintain limited populations of a limited number of species. Sometimes, when a disturbance occurs, the balance will be destroyed allowing certain species to increase in numbers at the expense of others which may go extinct. Probably the best-known example of

such disturbance, from New Zealand, occurred on Big South Cape Island in 1962-63 when ship rats got ashore, leading to the total annihilation of a wren, a snipe, and a bat, and the local extinction of two other birds and a weevil. That example is well known because it involves vertebrate species. Our preoccupation with vertebrates, however, can lead to disaster for insects.

akapourewa (Stephens Island) in Cook Strait is still one of our prime wildlife sanctuaries despite extensive modification when a lighthouse was constructed in 1892-94. The forest was cleared for farm animals and cats were introduced. Extinction of no less than 14 land bird species followed including the Stephens Island wren, South Island kokako and South Island thrush. At least

one reptile and a large carnivorous paryphantid snail were not immune to the impact. When the cats were finally eradicated in 1925, all had not been lost. The survival of a number of reptiles, including tuatara, the world’s rarest frog (Hamilton’s frog), and some notable insects, gave the island the status of a Wildlife Sanctuary in 1966. The island’s trump card was tuatara which burgeoned to reach an estimated 50,000 animals as a result of the increased area of open sunny slopes where eggs can incubate more successfully than in forest. This extent of increase in a top predator species confined to the limited area of an island was bound to have serious repercussions on its prey species. Sure enough, when we examine the fate of the previously rich fauna of forest invertebrates we find they have suffered badly. The giant carnivorous land snail Powelliphanta hochstetteri obscura, was last seen alive on the island in 1905. A Stephens Island endemic, the large carabid beetle Mecodema punctellum, was last seen in 1931. Another similar carabid beetle Mecodema costellum, described as being ‘very abundant under logs and stones’ by the early naturalist G.V. Hudson in 1934, is now extremely scarce. The list goes on,

and includes the Cook Strait giant weta and the Stephens Island endemic ngaio weevil, which are still present but in low numbers. The situation is complex because not only have tuatara numbers escalated to an estimated 1500-2000 per hectare in the bush areas but, over the same time period, the forest logs and deep moist litter required by some of these invertebrate species have all but disappeared. The combination of forest loss followed by increased predation has upset the delicate balance of this island community I believe the list of extinctions of largebodied invertebrates on Stephens Island will continue to grow unless conservation management faces up to the issues. Re-

establishment of forest habitat is a longterm goal but it will not happen fast enough to stop more extinctions. To give these ailing creatures a fighting chance, tuatara numbers should be reduced. Why not adopt a management policy that considers the optimal tuatara carrying capacity of Takapourewa with regard to the other elements of the fauna and relocate some, as is done in Africa with elephants? A similar thing is happening on another rat-free island in the narrows of Cook Strait. North Brother Island is a tiny island with low, windswept shrub cover on its upper slopes, crowned with a lighthouse and accompanying buildings. From my first visit in 1957, I recall the abundance of the nocturnal Duvaucel’s gecko, the scarcity of tuatara and the large numbers of beetles sheltering under rocks. I returned in 1993 on a census survey of tuatara. Over 400 tuatara were counted (160 per hectare) but one of the beetle species (the Cook Strait click beetle) seen on the previous trip could not be found and has since been ‘presumed extinct’ so far as this island is concerned. Why should this happen in an ‘undisturbed nature reserve? I can only conclude that the increase in exposed soil areas, resulting from human lighthouse activities over a century, has boosted tuatara numbers to the point where they are exterminating their food species, in this case one of New Zealand’s listed ‘endangered’ invertebrate species. auturu (Little Barrier Island) in the Hauraki Gulf is undoubtedly New Zealand’s foremost nature reserve. Here, on a much larger and more complex island, the impacts of single events are less likely to have such far-reaching results. Yet it is the pending risk of extinction of New Zealand’s most significant invertebrate that stimulates me to write this article. The wetapunga Deinacrida heteracantha is often claimed to be the world’s heaviest insect. Whether correct or not, it is certainly the moa of our insect world. Although its sole population occurs in our top wildlife sanctuary, it could be doomed to extinction in the very near future. In the days of Sir Walter Buller, wetapunga were found all over the Northland peninsula, on Waiheke, Great Barrier and Little Barrier Islands. Buller witnessed its extinction on the mainland North Island prior to 1900, attributing it to the invasion of Norway rat. When first studied on Hauturu by Aola Richards in 1958-61, it was widespread around the bunkhouse area at a time when the island was infested with both kiore and cats.

Since then the cats have been exterminated and a number of visits have been made by entomologists with the express purpose of finding and studying this remarkable insect. It is clear from reports that wetapunga numbers have been steadily declining to the point where it is now very difficult to find one, let alone any number of them for a population study. In 1994 and 1995, we could not find sufficient adult weta in the two-week search periods to do the work properly. Two animal species that we did see frequently were kiore at night and saddleback by day. Both are known to be expert weta predators and together could well be the reason for decreasing wetapunga numbers. In the days of Aola Richards’s study, the cats kept the rats in check and saddleback had not been re-introduced. Today it appears that wetapunga cannot sustain the twopronged attack. Either predator by itself might be tolerable. The commonly-held notion that ‘saddleback are native and therefore wetapunga must be adapted to their predation’ might no longer apply in the presence of kiore. How long can this declining weta population persist? I had been assured by the Department of Conservation that an eradication programme for kiore was planned and should take place in the spring of 1999.

If I successful, we might see wetapunga plucked from the brink of extinction. The latest, and most devastating news, however, is that the planned kiore eradication did not go ahead this year due to some cultural differences of opinion about the value of kiore which are regarded as a taonga (or treasure) by the tangata whenua, Ngati Wai. here are two lessons here for New Zealand conservation. The first is that our overwhelming focus on vertebrates, especially birds and tuatara, is not only taking resources away from other organisms and communities but is actually threatening the invertebrates. The second is the myth that only the aliens are ‘bad guys’ — that all native fauna and flora evolved in harmony and therefore can live together without threatening one another. However, following ecological disturbance on the confined space of an island, a native predator — even a tuatara — can wreak just as much havoc amongst native prey species as a rat or a stoat, if there are too many of them. The Department of Conservation must address these issues or face the ironic situation of having further extinctions on managed conservation areas. Imagine the

shame of conservation-conscious New Zealanders should they learn that wetapunga, the world’s heaviest insect, has become extinct on our top island sanctuary because we could not make a critical management decision and get rid of the rats. As a major plank of the much heralded ‘Biodiversity Strategy — Our Chance to Turn the Tide’ we need to develop a conservation philosophy which recognizes that in some (exceptional) cases there can be too many of even our beloved vertebrates for the good of the greater ecological community. In the case of Little Barrier/Hauturu, the cultural issues which threaten native biodiversity must also be resolved or we risk still more extinctions.

is an entomologist, and

Associate Professor of Zoology at Victoria University of Wellington. He is author of New Zealand Butterflies, The Monarch Butterfly and New Zealand Weta.