The biodiversity of New Zealand, a large Pacific archipelago, is one of the most unusual on Earth, due to its long isolation from other continental landmasses. Its affinities are derived in part from Gondwana, from which it separated 82 MYA, some modest affinities with New Caledonia and Lord Howe Island, both of which are part of the same continental plate as New Zealand and in part from Australia. More recently a component has been introduced by humans. New Zealand's biodiversity exhibits high levels of endemism, both in its flora and fauna. The islands historically have no native mammals except for bats, the main component of the fauna being insects and birds. Its flora is dominated by Gondwanan plants, comprising historically of forests, most famously the giant kauri.
Evolution of New Zealand's biodiversity
The break up of the supercontinent of Gondwana left the resulting continents and microcontinents with shared biological affinities. New Zealand, along with New Caledonia began to move away from Antarctic Gondwana 100 MYA, the break being complete by 82 MYA. It has been moving northwards since then, changing both in relief and climate. At some points it has been mostly underwater, with as little as 18% of the present surface area being above the water. Of the original biodiversity that it carried with it from Gondwana several groups remain: most predominantly plants, such as the podocarps and the Southern beeches, but also a distinctive insect fauna, New Zealand's unusual frogs and the tuatara, as well as some of New Zealand's birds.
The two sources of New Zealand's biodiversity following separation from Gondwana have been speciation and air- or sea-borne immigration. Most of these immigrants have arrived from Australia, and have provided the majority of New Zealand's birds, bats and some plant species (carried on the wind or inside the guts of birds). Some of these immigrants arrived long enough ago that their affinities to their Australian ancestors are uncertain; for example, the affinities of the unusual Short-tailed Bat were unknown until fossils from the Miocene were found in Australia. It has been suggested that the unusual adzebill is related to the Kagu of New Caledonia. The link between the two island groups also includes affinities between skink and gecko families.
Elements of New Zealand's Biodiversity
The history, climate and geology of New Zealand has created a great deal of diversity in New Zealand's vegetation types. The main two types of forest have been dominated by podocarps and southern beech. Podocarps (Podocarpaceae), an ancient evergreen gymnosperm family of trees, have changed little in the last 190 million years. Forests dominated by podocarps form a closed canopy with an understory of hardwoods and shrubs. The forests of southern beeches, from the genus Nothofagus, comprise a less diverse habitat, with the beeches of four species dominating the canopy and allowing a single understory. In the north of New Zealand the podocarp forests were dominated by the ancient giant kauri. These trees are amongst the largest in the world, holding the record for the greatest timber volume of any tree. The value of this was not lost on early European settlers, and most of these trees were felled.
The remaining vegetation types in New Zealand are grassland of grass and tussock , usually associated withe the subalpine areas, and the low shrublands between grasslands and forests. These shrublands are dominated by daisies, which can become woody and 3m high.
No mammals, other than bats and marine mammals, reached New Zealand before humans did. The Short-tailed Bat (from the monotypic family Mystacinidae ), having arrived in the late Oligocene, has had plenty of time to evolve, and has begun to fill the role of a small terrestrial mammal, flying out from roosts at night but frequently foraging on the ground. Some plants have evolved with the bats and are fertilised on the ground by the bats.
Birds comprise the most important part of New Zealand's vertebrate fauna. It is uncertain if many birds in New Zealand are descended from Gondwanan stock, as DNA evidence suggests that even the ratites (the kiwis and the moa) arrived after the split from Antarctica. Recent studies suggest that New Zealand wrens are Gondwanan descendents. DNA studies seem to indicate that the wrens are the most ancient of all passerines, spliting from the ancestoral passerine stock at the time New Zealand become an isolated land mass. In the absence of mammals, birds diversified into the niches usually filled by mammals in other ecosystems.
The Moa, of which there were 10 species, were large browsers, which were in turn the prey species of a giant eagle, the Harpagornis or Haast's Eagle. Both moa and eagles became extinct shortly after the arrival of humans on New Zealand sometime around 1300 CE. It appears that human hunters exterminated the moa populations, which deprived the Harpagornis of their primary food source, leading to the extinction of that species, as well. New Zealand's emblematic Kiwi fills the role of a small forager of the leaf-litter, and the enigmatic adzebill was a universal omnivore. The wattlebirds, Callaeidae, are a family endemic to New Zealand, but many other New Zealand birds show clear affinities to Australia, including the New Zealand Pigeon, the New Zealand Falcon, as well as various parrots, rails, waders, owls, and seabirds, albeit often with a New Zealand twist. Of the 245 species of birds from the greater New Zealand (the main islands along with the offshore islands, also including Norfolk Island), 174 were endemic, roughly 71%. Of these, about 32% of the genera were endemic.
No agamas, iguanas, land turtles or snakes ever reached New Zealand. The fossil record shows one crocodile, possibly a mekosuchine crocodile, in the Miocene, but otherwise the only reptiles to reach New Zealand were skinks and geckos, along with the living fossil, the tuatara. The tuatara, reaching 60 cm, are New Zealand's largest reptile. Frogs, which because of their intolerance for saltwater must have been descended from ancestors that broke off from Gondwana, are the exception to the rule that amphibians are never found on oceanic islands. New Zealand's few wholly freshwater fishes derive from marine ancestors.
New Zealand's invertebrate community displays strong Gondwanan affinities, and has also diversified strongly, if unevenly. There are over a thousand species of snail, and many species of insect have become large and in many cases flightless, especially grasshoppers and beetles. There are, however, less than 12 species of ant. The most famous of New Zealand's insects, the wetas, are ground-living relatives of the crickets that often reach enormous proportions.
New Zealand's biodiversity and humans
The arrival of humans on New Zealand has presented a challenge for the native species that has caused the extinction of many species. Mostly because the species of New Zealand have evolved in the absence of mammalian predators, be they human or otherwise(a situation known as ecological naivety), they have never evolved or have lost the responses need to deal with the threats. As humans arrived they brought with them, intentionally or otherwise, a host of other hanger-ons, starting with the Polynesian Rat, but now including stoats, weasels, Black Rats, Norway Rats, Brushtailed Possums, feral cats and dogs, as well as herbivores such as deer and tahr (a wild goat species from the Himalayas), which detrimentally affect native vegetation.
The date of the first arrival of the Maori in new Zealand is given as around 1300 AD, but some recent evidence suggests that some Polynesian travellers arrived earlier, as Polynesian Rats seemed to have arrived in 500 AD. Their arrival set off a first wave of extinctions, eliminating smaller defenseless ground nesting birds. A second wave of extinctions was triggered by the arrival of the Maori, who hunted many of the larger species, such as the moa, adzebill and several large ducks, for food. The Harpagornis presumably went extinct because of the loss of its food source. A third wave of extinction began with the arrival of European settlers, who brought with them numerous new mammal species, particularly the predatory domestic cat, and initiated more habitat modification. In all, over 50% of New Zealand's bird species are considered extinct, along with a species of bat and several frogs, skinks and geckos; this is second only to Hawaii in terms of proportion of species lost.
Today New Zealand's species are amongst the most threatened in the world. The New Zealand government, through the Department of Conservation, works aggressively to protect what remains of New Zealand's biological heritage. It has pioneered the use of offshore reserves, cleared of introduced species, as safe places for New Zealand's threatened species.
- Worthy, Trevor H., & Holdaway, Richard N. (2002) The Lost World of the Moa, Indiana University Press:Bloomington, ISBN 0-253-34034-9
- Ericson P, Christidis L, Cooper, A, Irestedt M, Jackson J, Johansson US, Norman JA., A Gondwanan origin of passerine birds supported by DNA sequences of the endemic New Zealand wrens. Proc Biol Sci. 2002 Feb 7;269(1488):235-41.