Possum. An ecological nightmare.
Brushtail possums are a protected species in their native Australia. Across the Tasman, they have established themselves as New Zealand’s most voracious and intractable pest, attacking simultaneously the beauty of our forests and the good name of our farming products.
Vhat’s all se dead animals on se road?” asked the hitchhiker, settling in to the passenger seat of the old Falcon.
“Possums,” the driver answered briskly. “Haven’t they got these little buggers where you come from?”
“No. Vhat’s a possum?” asked the foreigner.
“By crikey,” the driver shouted above the roar from a muffler that was evidently more hole than metal, “doesn’ even know what a possum is!”
Amused by the ignorance of his passenger, the driver sat chuckling behind the steering wheel. By this time the hitchhiker had noticed a strong smell in the car, but decided not to comment on it because the noise of the vehicle had become deafening, making further conversation impossible.
Suddenly the car swung off the highway to follow a metalled side road. The young traveller from Germany didn’t like the idea of leaving the main road, and possibly having to walk for miles to get another lift. And the dust—that was another new experience for him. There weren’t many dirt roads in Germany, and certainly not many cars so badly rusted through that they sucked dust like a vacuum cleaner.
The road went on for miles—too many miles for the hitchhiker’s taste, and soon suspicion started to creep into the young man’s mind. His hand went down to his right boot, where he was keeping a piece of cutlery with which to defend life and backpack.
The car pulled into a driveway, carried on over a rough farm track and coughed to a stop at a huge old woolshed.
The driver reached a burly hand over to his passenger.
“Anyway, Jim’s the name,” he said. “Pleased to meet you,” replied the foreigner, and they shook hands.
“So you don’t know what a possum looks like, eh? Well, come and have a look at this!”
Jim slid open the door of the woolshed. “Hang on a tick. I’ll just crank up the old kero lamp.”
Moments later, a lantern that looked as if it had been stolen from Ali Baba and his gang threw its hissing light on to a remarkable scene. The floor was strewn with piles of possum skins, spilling out over full wool presses and lying in bundles of different colours and sizes. Dangling from the ceiling were a couple of hundred wooden frames which had drying skins tacked on to them on both sides. Nearby was an enormous workbench on which lay an assortment of scraping tools and remnants of flesh and fur. Somewhere in the midst of all this was a sink, a pile of dishes and an unmade bed.
It was all quite an eye-opener for a young greenhorn from overseas, and little did I suspect then that Jim and his possums would play an important part in my life.
Back in Germany, I had just finished a degree in music, and my delicate fingers were more used to the fretboard of a classical guitar than to the drying boards of a possum trapper. But as I stayed with Jim and began to learn the ways of the bush, boyhood dreams of living close to nature started to become reality.
The more I saw of possums, the more they fascinated me. Marsupials aren’t the smartest of mammals, yet hadn’t this little critter sidestepped everything Homo sapiens could throw at it? Introduced only three years before the Treaty of Waitangi, this furry overstayer now reigns from Stewart Island to Cape Reinga, and has a population on the Chathams for good measure.
While its mocking laugh drives orchardists crazy, other people are enchanted by its cudddliness. Some drivers aim for them on the road; others pick up the wounded and take them home as pets.
Both attitudes are understandable. I remember the first time I came face to face with a possum that wasn’t flattened on the road or tacked out on one of Jim’s drying boards. I found it quite hard to accept that these cute animals were the focus of the biggest eradication campaign this country has ever seen. To me, the possum looked like a South Pacific version of the teddy bear.
Its big, dark eyes speak of naive innocence, the little pointed nose hosts a crop of shiny, oversized whiskers, and the fact that the female has a pouch makes the attraction irresistible. Imagine all the little treasures a three-year-old toddler could hide in a furry pouch like that. And if you’ve ever watched a possum eat, you’ll know how captivating they are, holding their food in their “hands” like we do, as they munch away.
Far superior to the teddy, I thought.
But the more you see of what this animal does, the less appealing it begins to look. Each cat-sized possum has a seemingly limitless appetite for plant matter—tasty native New Zealand plant matter—and the damage is everywhere: once-green forests now look grey, magnificent canopy trees sometimes over a thousand years old are reduced to “stags’ heads” of dead branches, birds that once filled the bush dawn with song are rarely heard now, their supplies of nectar-filled flowers and juicy berries thieved after dark by the voracious intruders.
Animal lovers or not, most New Zealanders agree that drastic action is needed to get rid of the pest. Ironically, it is only now that the magnitude of that particular task is becoming apparent, and it looks as if Trichosurus vulpecula will be with us for a long time to come.
Seventy million possums-20 per man, woman and child living in the country. That’s the current estimate of the possum population, and, in spite of vigorous control programmes, the figure seems to be remaining stable.
These 70 million pairs of jaws consume something like 21,000 tonnes of green matter every 24 hours—the equivalent of a big container ship full of leaves, young shoots, berries, flowers and grasses departing from our shores every night.
No forest can be expected to sustain such depletion for long, and our trees have been giving up the ghost in a big way for at least 50 years. Ironically, in the early part of this century very few people considered the possum to be anything but an asset. The prevailing opinion was succinctly voiced by the Auckland Acclimatisation Society, one of many groups charged with introducing beneficial species into what, to Victorian eyes, looked like a bland and empty landscape. Said the Society in a 1917 report: “We shall be doing a great service to the country in stocking these large areas [of rough bush] with this valuable and harmless animal.”
Such was the government’s concern for the welfare of the little Australian newcomer that the possum was given various levels of legal protection from 1889 until 1947. However, right from the start possums sparked controversy. Orchardists, who were already experiencing the effects of the possum’s big appetite in the 1880s, demanded that protection be removed; so did possum trappers, who wanted open slather. Acclimatisation societies, on the other hand, pressured government to retain protection of what they regarded as a valuable resource.
So, in 1911, protection was increased. In 1912, it was withdrawn. In 1913, it was back in place.
It wasn’t until the 1940s that the public began to see the possum’s “great service” had turned into a full-grown threat. Even then, the primary reason for wanting to curb the population was the damage possums did to orchards and erosion control plantings, and their propensity for climbing power poles and shorting out the wires. Right up until the middle of this century it was claimed that possum damage to forests was negligible compared to the benefits of the fur trade.
All this time, while the government was see-sawing on the possum question, legal (up to 1922) and then illegal liberations of possums continued up and down the country. The trapper’s mind, honed by a century of colonial prejudice, could see no reason why New Zealand’s “empty” bush—all it contained were a few birds and the occasional weta shouldn’t provide him with a living from fur.
That mentality persists to the present day. As recently as the mid-1980s possums were deliberately released into the Aupouri Peninsula, and by 1989 they had reached Cape Reinga.
Possums have the country covered, and the Department of Conservation (DOC), which has most of New Zealand’s best stands of native forest in its charge, is tearing its corporate hair out trying to control them. DOC gave up an all-out assault mentality some years ago. Neil Clifton, a conservation officer on the South Island’s West Coast, where 75 per cent of the rata-kamahi forests have suffered severe possum damage, says, “You could throw the government’s entire budget at possums and still not win. They have spread across too wide a front for us to be able to hold them. All we can do is pick a number of Islands’—areas we think we can defend—and then try to preserve them.
Around the country, DOC offices have been drawing up their own lists of “islands.” One of them is the tip of the Coromandel Peninsula, from Colville Bay north, incorporating Mt Moehau, an area with a unique native fauna and flora that includes two rare frogs, the North Island kaka and the North Island brown kiwi.
Because the Moehau region is surrounded by sea on three sides and connected to the rest of the Coromandel Peninsula by a relatively narrow isthmus (about five kilometres across) DOC plans to build an electrified fence across the peninsula, then use intensive hunting and trapping techniques to eliminate all possums north of the fence.
Being German, I can’t help but be reminded of the Berlin Wall, with our Checkpoint Charlie being just north of Colville. Yet the same system that kept the East Germans in might keep the possums out, in which case it would be one of the few victories in an ongoing battle to save our forests.
So far, the only places where possums have been completely eradicated have been islands, the largest being Codfish Island in Foveaux Strait and Kapiti Island, north-west of Wellington.
It took six years to destroy Kapiti’s 20,000 possums, but the effects have been startling. Possum removal has led to a renaissance in vegetation—especially of the highly possum-palatable plants such as fuchsia, toro and kohekohe. In the case of kohekohe, surveys estimate up to a million new seedlings per hectare. Previously, these trees had been prevented from flowering by possums browsing the emerging blossoms.
Bird numbers have also increased. Not only do possums deprive birds of berries, nectar and some insects, but they compete for nest sites in hollow trees, and have even been observed to devour young birds and eggs. (As one scientist remarked, “It could be expected that an animal endowed with teeth and curiosity sufficient to induce it to open sealed tins of condensed milk would not be averse to sampling any other unusual object encountered.”)
By denuding trees, possums considerably reduce the numbers of insects living in the canopy, as well as the armies of organisms inhabiting leaf litter on the forest floor. Tim Lovegrove of the Zoology Department at Auckland University has been monitoring birds on Kapiti for many years. “Total numbers of birds doubled between 1982 and 1986,” reports Lovegrove. “Bellbird, wood pigeon, kakariki, robin, whitehead and kaka all increased dramatically while tui and weka stayed about the same. The only bird which declined a little was the fantail, because it prefers forest margins. With the thickening up of the bush, Kapiti has become less suitable for fantail.”
The latest island to receive possum treatment is Rangitoto, in Auckland’s Hauraki Gulf. This young volcano, which erupted a mere 500 years ago, supports a thriving pohutukawa forest which, in recent years, has been under threat from possums and their kangaroo-like cousins, wallabies. In November 1990 DOC blitzed the island with an aerial drop of cereal baits laced with poison, and achieved a spectacular 93 per cent kill rate. The surviving animals (estimated to number no more than 600) are expected to be mopped up using hunters and dogs.
By comparison with islands, mainland forests are much more difficult sites for possum eradication because of the problem of reinvasion. In these forests, an initial kill, no matter how successful, must be followed up by regular maintenance control to prevent a new population from establishing itself in the possum-free area.
Nevertheless, in September 1990 DOC achieved a showcase extermination campaign in one of the country’s most revered forest sanctuaries: Waipoua, home of Tane Mahuta, New Zealand’s largest kauri, whose Maori name means “god of the forest.” More than 100 tonnes of poisoned bait was sown across 17,000ha of forest in the largest aerial bombardment the country has ever seen. The target: 200,000 possums. The result? An 86 per cent kill rate, and, a year later, a rejuvenating forest.
With a sharp metallic clang, the two steel arches of the trap snapped together. A cry of pain—not from a possum, but from a young man whose face was rapidly turning red with embarrassment. He had just caught his thumb in a Victor leg-hold trap.
This little incident happened six years after I had met Jim, and the youngster was one of eight trainees in a wildlife control scheme which I was to be tutoring for the next 20 weeks.
The setting for our training programme could hardly have been better. Twenty kilometres inland from Kawakawa, in Northland, on a 800ha block with an adjoining 320ha of bush reserve, the place was absolutely riddled with possums.
My selection as tutor followed a growing interest in wild animal control and the plight of the trees. Coming from a country where forests die from acid rain, I found a browsing animal a much more targetable threat to deal with, and a hunter’s life had strong appeal after an overdose of concrete and pollution.
A bruised thumb notwithstanding, my eight possum-busters were soon ready to catch their first possums. I chose the bush edge for our first trap line. Possum numbers are always highest at the forest-pasture margin—probably because pasture is a higher quality food than trees and shrubs—and as we approach the bush, their regularly worn “runs” are easy to spot in the adjoining paddocks.
Like most animals, possums prefer to travel on paths—runs or pads, as they are known—and a well-worn run will be the site for our first trap. We lay the trap against the base of a fence post and whack a nail through the chain to secure it. Then we put a bit of lure in the front and at the back. The lure is a very individual concoction of all sorts of ingredients: peanut butter, strawberry jam, curry powder, cinnamon, wholemeal flour, icing sugar, peach oil, aniseed—you name it.
I once walked into a local health food shop and gathered the most interesting oils I could find. When I told the shop owner what they were for, he started to have serious worries about the effect this sale would have on his karma.
There is no limit to a trapper’s imagination as to what a possum might find irresistible. The result is usually the same: a strong smelling, flower based brew which leaves the person handling the stuff smelling as if he just walked out of a cheap brothel.
Once the trap and lure are in place, we build a fence, a kind of miniature palisade, on either side of the trap. This will help deter any ground-feeding birds, and with the back of the trap against a fence post, and the sides guarded with palisades, the possum has only one option to get to the lure in the back.
We walk on until we reach a sad-looking kohekohe that has had 90 per cent of its foliage browsed back to the stems, and its trunk shredded by claw marks. Usually the kohekohe is one of the lushest greens in the bush, and seeing this decimated specimen is heartbreaking.
We decide to put our next trap under this tree, and we make an extra-good job with it. First we scratch the ground at the base of the tree. This will excite the possum’s curiosity by creating additional smell. Next we build a little platform out of that scratched dirt, to give the trap the best possible surface to sit on. We then set the trap in the normal way, but about half a metre up the tree we make a series of scratches in the bark, each one about 25mm long and 5mm away from the next one. This was one of Jim’s tricks—he was convinced that a male possum uses this sign (made by his two big upper incisors) to mark his territory. The theory is that by imitating his signature, he’ll get so excited that he will step into the trap for sure!
We carry on until we’ve set all our traps, and on the way back bets are placed as to how many possums we will find in the traps tomorrow.
The next morning sees us back out on the line. Some 60 per cent of the traps have caught. The size of the animals is small, but that is not unusual for Northland. Possum size varies around the country, with the largest animals being found in the coolest, most southern areas.
Nevertheless, the sweet smell of success lies just about as heavily in the air as the smell of our lures. It isn’t until we get back to base camp that I realise that the usual rush for a big feed isn’t happening that day, and some of the trainees are looking a bit pale. For many of them it had been the first time they had actually killed an animal.
Soon we are talking about the need to kill—a subject that always gets a lot of emotions going. I am glad to see such sensitive reactions from these young people, because, contrary to popular belief, I think that no hunter enjoys killing. In the case of wildlife control it is a matter of re-establishing the balance of nature. Each dead possum helps save the lives of threatened bird and plant species. Until we have other technologies, like sterilisation or some form of biological control, we will have to resort to the physical killing of the animal
Every time I have had enough of the smell of death in this line of work I think of the trees and how the stand there with the dignity of someone who’s been around for a while. and I look at the branches, crippled by possums, their trunks shredded by sharp claws and their leaves eaten faster than they can be replaced.
It’s the possums or the trees, and it’s that simple.
Although it is now well known that possums kill trees by defoliating them, their cumulative effect on our vegetation is more subtle and pervasive. Possums are picky eaters with definite dietary preferences. At any one time, some half dozen plant species will constitute 75 per cent of a possum’s diet.
More than 70 native tree species, together with numerous shrubs, ferns and grasses, are eaten by possums. But the species topping the menu vary depending on the area. If the favourites—fuchsia, wineberry, rata, kamahi, five-finger, titoki and kohekohe—aren’t available, the possum will make do with totara, tawa, mahoe and pate. Should these, too, be absent (perhaps by dint of previous overeating), he’ll settle for coprosmas, lancewood, kaikomako, rangiora and red or hard beech, and so on.
Possums are quite happy with introduced tree fodder as well, including pines (they go for the pollen-bearing catkins and strip the bark to get at the sweet tissues underneath), orchard fruit and erosion control trees such as poplars.
Among the list of unpalatable trees and shrubs are Dracophyllum, karaka, rewarewa, matai, silver beech, miro and rimu (perhaps like eating a toothbrush?). Possums can’t stomach kauri either, probably because the leaves are full of terpenes.
The problem with possum browsing is not just in what they eat, but also in how they eat. Possums will return night after night to a single tree, systematically stripping it before moving on to another of the same species. This behaviour has puzzled scientists, who look out on a strip of coastal pohutukawa, for example, and wonder why one is chosen for destruction while its neighbour remains untouched. No clear answer has yet been forthcoming, but the effect is to give the tree no chance to regenerate after each night’s onslaught.
Pohutukawa, northern rata and southern rata are all members of the genus Metrosideros. They all put on a spectacular flowering display—lighting up the forest with showers of red and orange blossom—and they are all being hammered by possums.
The decline of the coastal pohutukawa, beloved by North Islanders as the “New Zealand Christmas tree” because of its December flowering, has aroused widespread concern, resulting in the formation of a task force called Project Crimson. This group funds pohutukawa research and encourages community efforts to protect existing trees and replant areas with new seedlings.
Despite these and other efforts to save and regenerate, most forest ecologists are pessimistic about the future of our native bush. Although possums have been with us for 150 years, nowhere in New Zealand have they and the forest yet reached an equilibrium. Forests continue to be downgraded as larger tree species are browsed to death, to be replaced by scrub, grasses and gorse. If the trend continues, then forests will end up being full of species unpalatable to possums, and the New Zealand bush will be a sickly and uninviting shadow of its former self.
To be fair, the possum does not create this devastation unassisted. While he labours at devouring the high canopy, goats and deer masticate the understorey. Natural disa, ters, such as damaging wind, drought, heavy rains and earthquakes, also damage the forest. Most probably it is the combination of these factors over time that results in heavy forest dieback.
Although the problem has not been long in public consciousness, massive damage was apparent in many areas (particularly Westland and the axial mountains of the southern and eastern North Island) in the 1940s and ’50s. Maximum destruction generally coincided with peak possum numbers. Moribund forests provide little possum food, so numbers of animals drop and the population, like some deadly Mexican wave, moves slowly outward.
Possums hate getting wet, so rivers present a real barrier to their spread. Improved foot tracks and bridges into remote areas have speeded their progress in recent decades.
The southern west coast and Fiordland are too wet for possums to be really comfortable, so these areas have been relatively spared. In Northland there were few early releases of possums, and their spread has been slow and late relative to other parts of the country. Northland populations are probably still rising, or just peaking now with an estimated 10-15 million possums.
Carl Cooper, pest control officer for the Northland Regional Council, is responsible for dealing with all pest-related problems from Wellsford to North Cape. Most of his time is taken up with possum control, although wild pigs, goats and sheep also claim his attention. “A levy on all rates in Northland goes to the Regional Council for pest control,” he says, “so I end up with a budget approaching $1m a year.”
The lion’s share of the money is spent on contracting teams of professional cullers to poison possums. Lately they have been trying to reduce possum numbers in pine plantations inland from Whangarei. Possums can do significant damage to young pine trees, explains Carl, not just by eating them, but by using them as aerial thoroughfares. “Possums dislike the dense undergrowth around these young trees, so they travel along the rows from treetop to treetop, nibbling shoots and breaking branches as they go. It’s just a matter of time before they split the leading shoot, and then the tree is worthless.”
Once you start looking, the damage to the crowns of the trees is obvious. Twenty years ago, when pines were planted out at 10 times their final density, deformed trees were simply removed during thinning, but now that far fewer trees are planted initially the problem is becoming more serious. “My men have been working in here—poisoning along tracks and sending in trained terriers to find possums in their dens on the ground among the dense scrub during the day. But we’ll never win, and that’s why . . .” He points to some large native blocks adjoining the pine plantings. Carl’s men don’t trap there, and as he explains this anomaly a note of frustration enters his voice.
“Those native blocks are actually our biggest problem, but not in the way it sounds. DOC owns a lot of the bush in the North, and we can’t trap in their bush. Being funded entirely from rates, our job is to control pests on ratepayers’ land—and the government doesn’t pay rates. It’s a Catch 22 situation. We can’t trap on DOC land, and DOC doesn’t have the money to do it themselves. As a result, possums on their land wreck their own forests and reinvade cleared private land nearby.”
It’s a problem the Department lives with constantly: in the bush fighting possums; in Wellington fighting for a bigger slice of the budget. Even with improved aerial poisoning techniques such as computer-aided guidance systems (which allow pilots to fly grid lines to an accuracy of a few metres, and thus reduce wastage) it still costs around $20 per hectare to bomb possums from the air. DOC presides over 5.35 million hectares of national parks and reserves, and it doesn’t take a membership in Mensa to see that DOC ‘s possum budget of $3.1 million goes nowhere near addressing the problem.
Tonight, three of us are out to do some spotlighting. I have selected an area that has been trapped previously, and the spotlight is to give us a better idea of the numbers of possums left. It is a calm, warm night, and a good cover of clouds disguises the light of the moon. The little beam of my torch throws out just enough light to show us our way.
Suddenly, a noise. Something is running up a tree. We all stop in our tracks. I switch on the powerful spotlight. Mounted underneath the barrel of the rifle, a .22 magnum, the beam is instantly probing the spot where the noise was last heard. A big grey trunk is all we can see.
“That fellow must have had a bad encounter with a spotlight before,” someone remarks. And true, possums do get spotlight-shy. For an animal that’s half a sandwich short of a picnic, they learn remarkably fast.
A big moth is attracted by the light, and casts eerie shadows on the trees. All of a sudden, out of nowhere, completely silent, a morepork darts through the night, catches the moth and disappears, leaving behind a startled bunch of great white hunters.
After we compose ourselves, I shine the light into the branches of the old puriri again, and there it is. Stunned by the bright beam, the possum looks straight down the barrel. I bring the rifle up to my shoulder, take aim and pull the trigger. The animal hits the ground with a thump. We walk over to examine the carcass. It’s a big grey female, so routinely I check the pouch for young. In the north, with its warmer climate, abundant food supply and lower possum density, as many as 80 per cent of the females breed twice a year.
In optimum conditions, a female joey can virtually step off its mother’s back at seven months and fall pregnant a couple of months later. Whether its offspring survives, however, depends on the population density and the carrying capacity of the forest.
In the Orongorongo Valley, east of Wellington, live the most-studied possums in the country. Scientists from the nearby land resources division of the Department of Scientific and Industrial Research have been trapping, tagging, radio-tracking and otherwise monitoring this population for 25 years, and have built up a revealing picture of possum population dynamics based on this work.
It is the females who run the place, says Bob Brockie, one of the senior possum scientists at the DSIR. They control the home ranges, and hand them down from mother to daughter.
“It normally works like this: if a female produces a daughter, then that daughter will stay and establish a home range which overlaps with that of her mother. If a son is born, then at the age of 10-12 months he will usually leave and look for a new home range. The net result is that the core population consists largely of groups of related females, with the males dispersing outwards, and thus reducing inbreeeding in the population.
“Where the carrying capacity of the forest has been reached, and there is no more room for additional home ranges (as is the case in the Orongorongos),about a quarter of the resident population will die each winter.”
New Zealand possums are a pretty unsociable lot. Apart from the occasional hissing, paw-slashing scrap when animals cross paths, each individual keeps to itself for most of the year. The exception is during the mating season, when young bucks will be competing for available females. At this time of year you can often find cotton ball-sized fluffs of fur in the bush—evidence both of increased contact between males and failed attempts to approach females which are not in oestrus.
While possums generally go out of their way to avoid each other, they have no interest in defending a large territory for the sake of it. They do, however, advertise their presence by rubbing trees with their scent glands (particularly glands under the chin and a large gland on the sternum which produces a distinctive stripe on the fur).
Australian brushtail possums are more territorial than their New Zealand counterparts, a fact that is attributed to scarcity of den sites, the more open nature of the eucalypt forest in which they usually live, and the lower nutritional value of Australian vegetation. To a species which had adapted to such conditions, New Zealand’s dense, lush forest, with den sites aplenty and a veritable smorgasbord of plant matter, must have seemed like possum heaven, and the animal’s territorial instincts probably collapsed under the sheer pressure of numbers.
The brushtail possum is only one of 20 or more “possum” species in Australia, but it is the most common, and one of the largest. (The tiny honey possum, by comparison, is no bigger than a mouse.)
Aboriginal legend connects the possum to the man in the moon. Once upon a time, so the story goes, Moonan, a warrior, and his sons went hunting for witchetty grubs. The moon shone bright in the night sky, and they soon came across a big tree where they hoped to find many grubs. Moonan climbed to the top of the tree, and to his great joy found that he could reach the moon.
As his sons rocked the tree in their excitement, he nearly fell out of the branches, and to save himself he clambered on to the moon. The sons quickly climbed the tree after him, but by the time they reached the top the moon had drifted away.
Ever since, when the moon is bright, the sons climb trees to find their father. Through the ages they have grown sharp claws on their fingers, and a long tail from their spine.They have become possums.
Perhaps the story needs a New Zealand ending, I wonder, as I walk along the forest track under my own moonlit sky, spotlight at the ready.
One where Moonan comes back to earth, finds out what his sons have done, and shoots every last one of them. Well, we can dream . . .