Most of New Zealand has been forested over the last 10,000 years, and it is no surprise that most of the land animals are forest-dwellers. Of the animals that have backbones (vertebrates), forest birds were and still are the dominant native life form.
With the exception of bats, New Zealand has no native land mammals, but there are 32 species of birds living in the bush. There were many more at the time Polynesians arrived, around 1250–1300 AD. Birds had evolved to fill almost every available habitat, in many cases adopting mammal-like characteristics. For example, moa browsed tall shrubs like giraffes.
Loss of species
Polynesians hunted moa and other large flightless birds to extinction. The Pacific rat they introduced also decimated populations of small, ground-nesting birds and insects.
Early European explorers and colonists remarked on the volume of the birdsong. But burning and milling trees reduced and fragmented the bush area available to birds, while introduced predatory animals depleted populations and completely exterminated some species. Around 50% of native bird species are now extinct in the North and South islands.
Some birds still flourish but other species are dwindling. In the early 2000s ornithologists estimated that New Zealand’s forests have:
- 10,000–20,000 great spotted kiwi
- 1,100 little spotted kiwi
- 1,400 kōkako
- 5,000 saddlebacks (kiekie)
- 85 kākāpō.
Generally, the larger an area of forest, the greater variety of plants and animals it holds. The larger national parks are home to 15–20 bird species, but bush remnants (100 hectares or less) seldom support more than 10.
Tuatara, reptiles and frogs
Two species of tuatara (a lizard-like reptile) once lived in New Zealand forests. It is thought that Pacific rats preyed on their young, eradicating them from the mainland. They survive only on offshore islands or in mainland sanctuaries. Some 30,000 live on Stephens Island in Cook Strait. Most of New Zealand’s 80 or so lizard species are forest dwellers.
Frogs were once much more widespread, with seven primitive species common on the forest floor. Four species survive as remnant populations.
Invertebrates: animals without backbones
Insects and their larvae are everywhere in the New Zealand bush, feeding on living, dead or rotting leaves, trunks, roots, flowers, fruit, and seeds. In one 18-month study, nearly 23,000 creatures were observed climbing up and down tree trunks in the forest. Vast numbers of insects, including cicadas, wasps, ants, beetles, moths and flies, spend long larval lives in rotting logs, bush litter, or underground, to emerge and fly briefly as adults. This makes sense, as under the forest litter the environment is fairly constant, but above ground it is often unpredictable and hazardous.
The weight of worms
In conifer–broadleaf forests there is a greater weight of animals living under the ground than above it. Large numbers of insects spend most of their lives as subterranean grubs or caterpillars. The weight of earthworms alone exceeds that of all the birds, possums and insects above ground.
In the forest litter the most numerous animals are mites, caterpillars and grubs, springtails, litter hoppers, woodlice, millipedes, centipedes, beetles, and spiders. Predatory forest animals, whether they are insects, spiders, litterhoppers, native snails, birds or bats, depend on other bush animals for their food.
Wētā are important bush insects, providing food for lizards, and for birds such as kingfishers, moreporks, riflemen, whiteheads, grey warblers and tomtits. Other inhabitants of forest litter include carnivorous snails with large shells (up to 9 centimetres in diameter), kauri snails and an abundance of minute snails.
The biodiversity of insects and other invertebrates is much higher on small offshore islands where mice, rats, hedgehogs and other exotic mammalian predators have never been introduced – or islands from which conservations have since managed to eradicate these pests.