War on Wallabies

Forest and Bird, Volume 22, Issue 2, 1 May 1991, Page 43

War on Wallabies

by

and

Forest and Bird Central North Island Field Officers

Basil

Ann Graeme,

O SEE HOW WALLABIES can destroy a forest, go to the wire-netted exclosure plots on the shores of Lake Okataina. These plots are tiny sanctuaries that exclude wallaby browsing, and they are filled with seedlings and saplings. There is no better evidence against the wallaby than the surrounding bare forest floor. These Rotorua wallabies are poised to invade our major axial forest ranges. Our forests stand at another crossroad. Will we allow wallabies to escape as another uncontrollable pest, or will we grasp the chance to eradicate them while we can?

Eminent and misguided

Wallabies are "big feet’, belonging to the Macropodidae family of marsupials. Other than bats, New Zealand had no land mammals, and early European settlers did their best to rectify this perceived deficiency. One of the most eminent and misguided enthusiasts for introducing foreign species was Governor Grey. Besides introducing such unsuitable animals as monkeys and zebras, in 1870 he liberated five species of wallaby on Kawau Island. In the same year a Captain Thomas brought Bennett's wallaby to Christchurch from Tasmania. In 1874 two females and a male, probably from this stock, were liberated

in the Hunters Hills, near Waimate in South Canterbury. Today there is still a thriving population in the Hunters Hills as well as four small populations in Kakahu Forest, Pioneer Park and Peel Forest, and at Quartz Creek (between Lakes Hawea and Wanaka). From Kawau the dama wallaby was released to the Rotorua region, and rock wallabies to Auckland’s Rangitoto and Motutapu Islands. Our native forests have borne the brunt of these introductions. Wallabies come from mixed grass and forest habitat in Australia. They prefer forest and scrub margins in New Zealand too, but where pasture is not available, they thrive on forest plants alone. At Rotorua in the Okataina Scenic Reserve they eat out hangehange, tree fuchsia, karamu, pate, five-finger, putaputa weta, mahoe, kamahi, broadleaf and raurekau. These are the fruit and insect-rich understorey plants so important as food for our native birds. If the wallaby pressure is not totally overwhelming, then an impoverished understorey of mangeao, rewarewa, rangiora and silver fern survives. In places on Kawau Island one can see only unpalatable silver fern and poisonous arum lily. On Rangitoto, rock wallabies eat colonising pohutukawa, Kirk’s tree daisy and Astelia banksii. Wallabies clearly devastate native forest. Their low profile as a serious forest pest is

due to their limited distribution, but this is not likely to last. Wallabies are on the move. However, wallaby populations may still be regarded as "pioneer populations", still at a stage where containment and eradication are attractive and cost-effective options. These options threaten to slip through our fingers forever. If the wallaby threat is ignored, these pioneer populations will spread to the extensive forests of the Urewera and axial ranges of the North and South Island. Wallabies are also pastoral pests. Their appetite for grass prompted control operations on Kawau Island and in Canterbury. Wallabies are not known to be tuberculosis carriers but their forest-to-pasture grazing habits suggest they may be. Wallabies have a fail-safe breeding strategy. The female carries a new ‘joey’ in her pouch for almost a year. But backing this up, waiting to mature and occupy the pouch, is a quiescent embryo joey. Within 24 hours of giving birth, the female mates again. She is thus continually in the business of producing young — a biological forerunner to Ford's assembly line. All wallabies, except Bennett's, are very sociable animals. Their social habits, coupled with their breeding potential, mean wallabies stock an area of forest right up to its carrying capacity. The exclosure plots at Oka-

Natural dispersal of Rotorua Wallabies The Rangitaiki River is an imperfect barrier to wallaby penetration of the Urewera with its shallow headwaters in Kaingaroa Forest. Wallabies have already penetrated the Mamaku margin and Rotoehu Forest. These three native forests nurture the largest populations of the endangered kokako on the mainland. Should wallabies escape to the axial ranges then the regenerative capacity of New Zealand’s main catchment forests will be seriously compromised.

taina graphically demonstrate how wallabies have stripped palatable shrubs from the forest floor. This grazing pressure is so relentless that wallabies even feed on leaves falling from the canopy. The exclosure plots also show how, even two years after 1080 poisoning and a 96 percent reduction in wallaby numbers, the grazing pressure of the remaining few wallabies plus a few deer continues to repress regeneration. Recovery of the forest is still doubtful. ‘Control’ that reduces the density of wallabies without a decisive and conclusive push for eradication looks like a loser. Control alone also does little to address the problem of further wallaby dispersal.

Steady dispersal

The mainland population at Rotorua is dispersing. This spread is slow but accelerating. Shelter provided by the extensive pine plantations in the region has probably helped increase the rate of dispersal. "Outriders" of the Rotorua wallaby population now threaten to invade the Urewera, and have already reached the kokako forest of Rotoehu, and Horohoro in the Mamaku. The social nature of wallabies fortunately means that they do not spread as fast as a lot of other pests. Despite this, wallabies have suddenly appeared around Tauranga, New Plymouth, Waitakere range (Auckland), Great Barrier Island, Te Whaiti (Urewera), Paradise valley (Ngongataha), the Paeroa Range and Puaiti Scenic Reserve (between Taupo and Rotorua). This distant wallaby dispersal is not natural, but results from human intervention. Each new population is a further source of wallabies for more liberations. The New Plymouth, Waitakere, Great Barrier, and Tauranga releases were shot and are presumed to have been eradicated. The Urewera release has been investigated and is thought to have died out or been shot by locals. Rotary (Auckland) has sponsored DoC to eradicate wallabies as a co-target with possums on Rangitoto and Motutapu Islands. However other pockets of wallabies remain

untouched. Most alarming of the releases is that at Ngongataha which has been allowed to spread to the margins of the Mamaku at Horohoro. With the exception of the dispersing Rotorua populations, all of our introduced wallaby pockets are constrained into "islands" isolated by pasture or water. There should be no great physical difficulty in eradicating discrete islands of wallabies. However, there are two discouraging precedents of failed attempts at wallaby eradication. In the 1960s the Rabbit Board’s Wallaby Destruction Committee attempted to eradicate wallabies in South Canterbury, and the Forest Service attempted to clear Kawau Island. The Canterbury operation was instigated by farmers concerned at the loss of pasture. The grass margins were retired for a 1080 poisoning programme. This was so effective in the "control" phase that farmers became impatient to reap the rapid regrowth of pasture, and insisted on re-introducing sheep before the final eradication goal was achieved.

Eradication foiled

On Kawau, eradication was foiled both by residents who had grown fond of their wallabies, and by a remarkable and fortuitous rescue of the parma wallaby which was then thought to be extinct in Australia (see box). Public and particularly local commitment to eradication is clearly a crucial factor in any wallaby strategy, and we must take into account the undoubtedly attractive image of wallabies in any campaign against them. Wallabies are a local problem at the moment. They infest a small proportion of the DoC estate. They are a pest to only five Regional Councils, to a minority of establishing pine plantations, and to farmers of the Hunter Hills region of South Canterbury. Clearly should they spread via the Urewera and Mamaku to the ranges of the North Island, then they will become a major burden to most North Island regions. Should they be spread to the South Island ranges, then most South Island regions will be affected. The Department of Conservation would be in no position to deal with a national wallaby plague. Now is the time for action when wallabies are at a vulnerable stage. When we are faced with so many unsolvable pest problems, it makes sense to focus on an achievable target; one that does offer the potential of total eradication. We already have this strategy for notifiable pests and diseases such as foot and mouth, exotic fruit flies and even the relatively unimportant Dutch Elm Disease. We should adopt the same strategy for wallabies as a serious forest pest. All the discrete populations from Kawau Island to Quartz Creek are ready targets for elimination. Of these the Paradise Valley Horohoro populations must be the top priority. The more extensive Rotorua populations require a more complex strategy. The "outriders" threatening to breach the Urewera should be immediately rolled back. Present operations directed at controlling the centre of the population should be diverted to the borders to progressively shrink the population back to core areas, working towards a final eradication phase.

All wallabies should be classified as pests of local importance, but should also be recognised by the Department of Conservation as potentially pests of national importance. A joint regional and national response is urgently required. The Department of Conservation as the Crown agent representing national interests should shoulder the initiation and co-ordination of the national wallaby strategy. Let us stop wasting money on perpetuating fragmented control programmes. Let us decide wallabies are one environmental threat we can solve forever. A success with wallabies may stimulate new initiatives and funding against old foes deer, goats, thar, chamois and possums. A"

References

M. Llewellyn — Impact of dama wallaby on kanuka-dominant forest on Lake Okataina as shown by exclosures (DoC Rotorua report). M. Llewellyn — Assessments of wallaby populations poisoned with 1080-impregnated baits (DoC Rotorua report). V. and M. Vujcich — Summary of research on wallabies of Kawau Island (Auckland University Masters thesis). B. Warburton — Wallabies in New Zealand. History, current status, research and management needs (FRI 114, 1986). B Warburton — Assessment of 1080 gel on foliage for controlling Bennett's and dama wallabies (FRI 1989). D. Hunt and C. Jenkins — (DoC Rotorua) and D. Moore (Bay of Plenty Regional Council).