Story: Introduced animal pests
Page 5 – Deer, goats, pigs and wasps
Red deer (Cervus elephas) are the most abundant deer, but seven other species – wapiti, fallow, white-tailed, sika, sambar, rusa deer, and possibly moose – also live in the bush. Chamois and tahr (thar) occasionally stray into the bush from the alpine zone.
Between 1851 and 1923, about 1,000 British red deer were released to provide sportsmen with game. At first the deer multiplied rapidly, and then there was a period of sustained high density. By the 1940s they had colonised most of the bush in the southern and central North Island and most of the remote areas of the South Island. They ate out many of the palatable plants within reach, and began to damage the forests irreversibly. Their diet included large-leaved plants such as broadleaf, five-finger, patē, coprosmas, fuchsia, tītoki, heketara, and hen-and-chicken ferns. They also browsed any grasses or tussock in the bush, scrub or alpine zones.
Several studies have shown that deer may take 90% of seedlings, preventing the regeneration of damaged forest. After eating the palatable plants, deer leave those that are inedible, such as pepper trees, which grow in their place. If deer are excluded from a patch of bush, plants regenerate spectacularly.
Some 40 years after their establishment, deer numbers fell because of government deer-culling operations. Numbers declined further when helicopters were used to shoot deer or to capture them for farming.
Today, red deer inhabit forests from East Cape to Stewart Island. Other deer species have more restricted ranges.
Captain James Cook gave goats (Capra hircus) to Māori in the 1770s and the first sealers and whalers brought more. Early farmers used goats to control blackberry, gorse and briar. But many of the goats escaped and they became populous in the bush. In 2006, goats were scattered at low densities, with concentrations in scrubby hill country in both islands. Most lived in patches of rough, upland, scrubby or bushed country, with the largest groups in Taranaki, the top of the South Island and inland Otago.
Goats are mobile and inquisitive, grazing on plants up to 2 metres high, although they can browse higher by climbing on leaning tree trunks. In the bush they eat mainly broadleaved trees and shrubs, but also ferns, grasses and unlikely plants like tree nettle, bush lawyer and lancewood.
Browsing goats change forests because the plant species they eat are replaced with inedible ones. Continued grazing thins undergrowth and, as each generation of seedlings is destroyed, the course of plant regeneration and succession is disrupted and redirected. On some offshore islands, goats have reduced forest to rank grassland.
A shilling for a pig
In 1925 the New Zealand government introduced a bounty scheme in which one shilling or three rounds of ammunition would be paid for a pig’s snout and tail. During their most populous year, 1947, there were an estimated 123 pigs per square kilometre of bush.
Pigs were introduced to New Zealand by early explorers in the late 1700s. Wild pigs (Sus scrofa) plough up large areas of forest floor, uprooting supplejack, bracken and many other plants. They feed on any forest berries, fruit or succulent stems, and any surface-dwelling animals, such as wētā, litter hoppers, earthworms and centipedes. With up to 40 pigs per square kilometre of bush, large volumes of food are taken which would otherwise feed native species.
German wasps (Vespula germanica) arrived in New Zealand in 1945. They now occur throughout the country and have reached plague proportions in South Island beech forests, especially in north-west Nelson. In the late 1970s European wasps (Vespula europaeus) arrived and quickly became established.
In spring and summer, German and European wasps are carnivores, eating mainly flies, caterpillars and spiders. They threaten the survival of some rare native insects. In late summer and autumn, wasps switch to a diet of nectar. In South Island beech forests they feed on the honeydew produced by scale insects (Ultracoelostoma species). These suck the sap of beech trees and produce up to 4 tonnes of honeydew per hectare of forest each year. Wasps, which have up to 34 nests and 27,000 individuals per hectare of beech forest, take up to 90% of the honeydew, depriving native birds, bats, lizards and insects of this vital winter food.