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The nature of New Zealand's biodiversity

New Zealand has a relatively small number of indigenous flowering plants and vertebrate animals by tropical or continental standards. This is partly because our isolation from continental landmasses limited large-scale recolonisation by these groups after the global extinction catastrophe of 65 million years ago. It is also partly the result of a long period of submergence between 60 million and 30 million years ago, when up to four-fifths of the landmass sank below sea level, leaving just a few islands (Cooper and Milliner, 1993). The ice ages of the past two million years also had an effect, reducing habitat and fragmenting populations into ecological 'islands'.

These factors limited the number of major plant and animal groups in New Zealand, but they also contributed to the uniqueness of the surviving groups. Cut off from the rest of the world, evolution took an eccentric course here, leading to a high percentage of endemic, or unique, species. This high rate of endemism is what makes New Zealand's biodiversity both special and highly vulnerable.

The Age of the Birds

When New Zealand parted company with the prehistoric super-continent, Gondwana, about 80 million years ago, it was a mammal-free forested raft of birds, reptiles (including dinosaurs), frogs and strange invertebrates, some of them 'living fossils' from the previous Jurassic age. From then until the recent arrival of humans New Zealand had the longest period of isolation of any non-polar land mass on Earth. Our endemic biodiversity reflects this. It is dominated by plant, animal and fungal groups which arose more than 100 million years ago and which tend to be overlooked because of their lack of colour and size. These include flatworms, snails, mites and spiders, primitive insects (solitary bees and wingless crickets), and many species of fungi, mosses and liverworts. The 'higher' plants and animals are relatively under-represented, but those which do occur are mostly unique.

Following the extinction of our dinosaurs, pterosaurs and plesiosaurs 65 million years ago, the land became dominated by birdsto such an extent that they may have affected the distribution and composition of the country's vegetation (Atkinson and Greenwood, 1989; Clout and Hay, 1989; Wellman, 1994). For example, moa browsing may have maintained a relatively open forest understorey. Seabirds are known to have had a profound influence on the lowland and coastal vegetation, as shown by the high number of rare and declining plant species in these habitats now that seabirds are less abundant (Norton et al., in press).

Some birds evolved into unique new forms (e.g. a giant penguin, the world's largest eagle, the flightless kakapo ground-parrot, the mammal-like kiwi and the celebrated giant moa, taller than any other bird). Flightless birds filled roles that small mammals elsewhere may fill, foraging on the ground, living in burrows and hollows. Their only predators were eagles. Poisonous insects and spiders were rare, and snakes were absent. Surrounding the birds were a range of other plants and animals, some of which still retained their ancient Jurassic forms (e.g. the fern Loxoma, kauri and podocarp trees, freshwater mussels, freshwater crayfish, the strange worm-come-insect Peripatus, giant wetas and giant snails, lung-bearing slugs, primitive native frogs, and tuatara). In the absence of mammals, giant wingless crickets (wetas) became the ecological equivalents of mice (see Box 9.4). While the rest of the world had entered the Age of Mammals, New Zealand had taken a unique evolutionary path and entered the Age of Birds.

Box 9.4: Jurassic giants

Giantism evolved in various groups of New Zealand animals (Daugherty et al., 1993). The giant birds, such as Haast's eagle and the moas, are extinct but many invertebrate giants still hang on in the face of rat predation and habitat destruction. Most are confined to isolated locations or rat-free islands (Meads, 1990). The world's heaviest insect is among them - the giant wingless cricket, or weta, of Little Barrier Island. At lengths of more than 8 cm and weights of up to 70 g, this relative of primitive grasshoppers is as heavy as 4 or 5 mice. Known to Māori as the wetapunga or 'god of ugly things', it is the largest of our dozen giant weta species (Deinacridaspp.). It has a docile temperament and lives high in the tree tops. In Jurassic times, 190 million years ago, giant wetas were widespread around the world. Now New Zealand is their only holdout.

While all seven species of the smaller tree weta (Hemideina spp.) are quite common, the giants face an uncertain future. All but one, the alpine scree weta (D. connectans) of the South Island mountains, are listed as threatened. Another group of wingless crickets, the large elephant or tusked wetas, is also under extreme threat. Named for their 2 cm tusks, these wetas can grow up to 8 cm long and live in ground burrows. Three species have been discovered. One is confined to a few hectares of forest on Middle Mercury Island, near the Coromandel peninsula, one is scattered in parts of Northland and one was recently discovered in the Raukumara mountains in Bay of Plenty.

New Zealand's other large insects cannot challenge the weta or the several species of tropical beetle that reach lengths of 15 cm or more. Nevertheless, by normal insect standards, some still qualify as giants. They include: our largest beetle, the huhu or longhorn beetle (Prionoplus reticularis), which grows to 5 cm in length; Helm's stag beetle (Dorcus helmsi) which can reach up to 4 cm; various large weevils (Anagotus spp.); and the enormous Buller's moth (Aoraia mairi) whose 6 inch (15 cm) wingspan caught the eye of Sir Walter Buller while hunting huia birds in the Ruahine Ranges 120 years ago. It has not been seen since (Meads, 1990).

Myriapods (millipedes and centipedes) are a less flamboyant class of arthropods than the insects, but they also have monsters, such as the giant pill millipede (Procyliosoma tuberculata ). Unlike its smaller relatives in Europe, which were swallowed as folk medicine, this pill would be hard to wash down with a glass of water. Females can grow up to 5 cm long and 2.5 cm wide. They feed on rotting leaves and are widespread in the North Island and northern South Island. They reach full size, however, only on rat-free islands. Then there is the giant centipede ( Cormocephalus rubriceps) which, at 25 cm, looks like the caterpillar from Hell with its armour-plating and jointed legs. However, mothers are protective of their young, even carrying them around. They capture prey (usually insects, spiders, snails and slugs) with claw-tipped pincers which inject a lethal poison. They can even kill small lizards. Though still present in the North Island, full-sized ones are only found on rat-free islands (Meads, 1990).

Spiders belong to the third class of land arthropods, the Arachnids. Our largest, the Cave spider (Spelungula cavernicola), may be a 'missing link' between primitive spiders, which arose 350 million years ago, and the modern 'true' spiders. Its 3 cm body and 30 cm leg-span have startled more than one speleologist in the limestone caves of the Tasman Region where it lives. It is very rare and is the only spider protected by the Wildlife Act 1953 (Faulls, 1991). Another group of indigenous giants confined to the Tasman Region are the giant red flatworms (Geoplana spp.), which can reach lengths of 20 cm. They hunt by smothering their victims (mainly slugs and snails) in slimy mucus. Once abundant, they are rarely seen today.

Far more widespread are the nine species of giant land snail (Powelliphanta spp.) whose colourful shells can be up to 10 cm in diameter. Some of these snails have human-scale lifespans, taking up to 15 years to reach maturity and living 40 years or more. Their reproductive rates are correspondingly low. The giant snails belong to the oldest family of carnivorous land snails on earth, having originated about 200 million years ago. They prey mainly on earthworms, slugs and other snails. Although they are scattered across habitats ranging from forest to alpine grassland, they have been driven into confined areas and nearly all are listed as threatened. Related to the giant snails are the giant leaf-veined slugs (Pseudaneitea gigantea) which, at 15 cm, are the largest of our native slugs. These have also been driven into relic areas of forest and tussock grassland.