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A Plague Of Possums

The possum population has reached crisis proportions, threatening both forests and stock, and it seems that 1080 poisoning is the only answer to that threat.

By Vicki Hyde

Is it a dangerous poison that threatens the ecosystem and hunters' livelihoods, or is it the only method of effectively controlling the possums that are laying waste New Zealand's forests?

The perception of 1080 varies widely, but everyone recognises that something has to be done about the burgeoning possum population that is devouring trees and infecting stock with tuberculosis.

Possums were introduced to New Zealand in the 1850s in the hope of developing a local fur trade. Ironically, they were a protected species until 1946, by which time they had spread throughout most of the country. Efforts to keep them out of the far north and the Coromandel Peninsula have failed, and many of the offshore islands have also become homes for the furry invaders.

The slump in the fur market and the removal of government bounties on possum pelts has given the marsupial free reign over the last decade or so. Numbers have increased to a massive 70 million, causing real problems in some areas.

"Mistletoes are rare or absent wherever possums have been present for any length of time, " says Dr Ian Payton of the Forest Research Institute. The possums damage mature trees, eating out the preferred rata, kamahi and pohutukawa before moving on to less palatable fare. In the process they change the composition of the forest and, in some cases, cause a collapse of the local forest ecosystem through the removal of protective canopy vegetation.

In Australia, fossil pollen records show that a genus of pohutukawa once common there fell prey to marsupial attack. In New Zealand, scientists and conservationists are attempting to rank forests for funding priorities to save them from a similar fate.

FRI's John Parkes uses a combination of biological, technical and economic factors to suggest which forests should benefit from the limited funds available for possum management. At around $30 a hectare, with millions of hectares under threat, the process is a difficult one.

Big Kills Not Enough

Past management techniques have involved 1080 aerial drops every six years in selected regions, but Parkes considers this inappropriate. Big drops may remove 80% of the possums, but they soon repopulate. Plants under stress from possums may never get the chance to recover before the numbers build up again.

"It's better to do an initial blitz and take an annual harvest to keep [the possum population] below the threshold, " Parkes maintains. Even worse, he says, are half-hearted attempts which serve only to produce a population resistant to the poison and large enough to still cause damage.

"It's utterly futile to start if you don't plan to continue, " he says. He sees the best option as beginning with large 1080 drops and continuing with annual hunting efforts. Hunters themselves are not so sure about the efficacy of the 1080 drops.

1080 Concerns

"We are by no means convinced that [1080] is the answer, " contends hunter Geoff Nash. Like many people, he is concerned about the potential long-term effects of 1080 on the forest ecosystem. He worries that the poison could affect the soil and water, and suggests that a non-specific poison is no replacement for a hunter.

These concerns are answered by Dr Jim Coleman. 1080 is a naturally occurring toxin that occurs in plants throughout the world, he says, and is comparable to the toxins found in many of our own native plants.

"It degrades in the environment just as other toxins do when the tree falls over and rots, " Coleman says. Bacteria in the soil quickly break the compound down into non-toxic components. Long-term monitoring of 1080 has failed to detect any presence of the toxin in soils or water supplies, even at the parts per million level.

Unlike many other toxins suggested for pest control, 1080 rapidly degrades in the environment and is also rapidly passed from an animal's body. Sub-lethal doses will not accumulate within the tissue to cause on-going harm to the target animal or to those further up the food chain. This makes 1080 strikingly different from DDT and DDE toxins.

In addition, each pellet is potent enough to kill a possum outright. This reduces the chance of possums learning to avoid baits, a problem which has plagued rabbit eradicators in the Mac-kenzie Country recently. 1080 is seen as a humane choice of poison in that the possum curls up and goes to sleep, without any of the distressing reactions common to other toxins.

Careful Targeting

There are problems with making sure that 1080 is eaten by possums, rather than other animals, and a great deal of research has gone into this. Cinnamon is used to mask the flavour and smell of the poison. For some reason, possums like the taste of cinnamon while birds will quickly reject cinnamon-flavoured bait. The bait is coloured green, again to deter birds from selecting it.

The size and shape of the poison pellet have been carefully developed. Small fragments of bait were found to stick to leaves in the upper canopy, making them more likely to be eaten by birds. The pellets now weigh around four grams to ensure that they fall to the forest floor and to make it difficult for birds to pick up.

Monitoring of bird species has shown no adverse effects on bird populations as a result of 1080 drops. In one study of an area with approximately 60 kokako, one individual went missing after a drop, presumably poisoned. This level of risk is deemed acceptable by FRI scientists.

"We ultimately expect a vast improvement as a consequence of improving the forest environment," Coleman asserts. The few losses of individual birds will be more than made up for by providing a better population base as a result of removing the possum hordes.

Other animals within the forest may eat 1080-laced bait. This could be seen as an advantage as rats, stoats, and even goats are often secondary targets in forest management. New Zealand's relatively simple fauna, most of which is introduced, gives 1080 a freedom of use uncommon anywhere else in the world, according to Coleman.

Hunters Still Useful

This is not to say that there is no place for the hunter in New Zealand's forests. Dr Coleman and his colleagues at FRI agree that hunters can play a valuable role in continuing control of possum populations.

"If good hunters are available, if accessibility is good to all parts of the block and if the landform is such that people can freely hunt it, then hunters can do as good a job as an aerial drop of 1080," Coleman says.

However, those rather stringent conditions are rarely met in the field. Small reserves, such as in Southland and Otago, and pasture margin areas around farms would provide appropriate areas for hunter control. But a large bulk of New Zealand's forest reserves are virtually impossible to get into on foot, making some form of aerial-based control mandatory.

Funding Essential

At the root of the problem is funding. Any method of possum control is going to be expensive. Hunting and poisoning are comparable in cost, but have differing degrees of effectiveness. In Westland, 1080 drops in rata forests cut possum populations by 80%. They rose again while further funding was being sought for maintenance of the low levels.

"We need on-going sustained constant effort in perpetuity, " says Parkes. He's looking 200 years ahead, and fighting for funding from year to year. At this stage, the possums look as if they're winning the battle.

Vicki Hyde is the editor of New Zealand Science Monthly.