The effects of introduced mammals
Introduced mammals such as deer, goats, pigs, rats, mustelids and possums have had an immense effect on New Zealand’s native forests and wildlife. Continual browsing by larger animals opens up the forest canopy and clears the undergrowth. Eating out certain palatable plants has changed the makeup of forests. Mammalian predators that hunt by scent (such as rats and mustelids) have had a devastating effect on populations of birds and other small animals.
Because introduced mammals have spread throughout the two main islands of New Zealand, their removal is now extremely difficult. Instead, effort is concentrated on controlling their populations and clearing them from certain high-priority areas.
Deer were introduced from the 1850s. The government supported importing them in the early 1900s to encourage tourists who wanted to hunt. By the 1920s, deer had spread through the forests, where they ate out the undergrowth, causing extensive damage. After a ‘Deer Menace’ conference in 1930, organised culling began, and continued for the next 30 years.
Deer cullers came to represent a male culture of outdoor independence and resilience, later portrayed in the book A good keen man (1960) by Barry Crump. Between 1931 and 1956, over 600,000 deer were killed. However, the real question for conservation is how many remained alive. Deer numbers rose again when hunting slowed down. Since the 1970s, helicopters have been used to shoot deer or capture them for farming.
The Australian brushtail possum (Trichosurus vulpecula) was deliberately introduced for the fur trade from the mid-19th century. It was a protected species except to licensed hunters until 1938. Possums found New Zealand forest more palatable than the eucalypt-laden fare in their homeland, and thrived.
Fur then foe
In the 1920s, school children were given the day off to watch and celebrate the release of Australian possums into the forest at Mt Bruce. The animals were to form the basis of a possum-fur industry in the Wairarapa. Mt Bruce is now a bird sanctuary, and millions of dollars have been spent freeing the area of possums.
Because they eat the upper forest canopy, the damage they caused was not always obvious. Their impact was debated until about 1950, by which time they had spread through the country. They drive small animals out of their dens and nests, and eat eggs and chicks. They also carry bovine tuberculosis, and can infect cattle.
There are probably about 65 million possums in New Zealand in the early 2000s.
New Zealand is one of the few countries where 1080 (sodium monofluoracetate) is used to control introduced animals, especially deer and possums. The poison biodegrades quickly and has little impact on native birds.
Although wearing fur and animal skins is out of favour in most countries, accessories made of possum fur are acceptable in New Zealand – and a good way to demonstrate your support for ridding the country of a damaging pest.
It has been used since the 1950s. In 2006, half of possum control programmes run by the Department of Conservation involved the aerial dropping of 1080, mainly in remote areas. In more accessible regions the focus was to control bovine tuberculosis, and ground trappers put poison in bait stations.
The use of 1080 has been controversial, but it is generally supported by conservationists because there is no effective alternative. In 2007, Department of Conservation director-general Al Morrison stated that ‘if we cannot carry out aerial 1080 operations, more than half the country … will be left to the ravages of possums, rats, stoats, and other pests.’ 1