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Friday, February 24, 2012

#140 Common Brush-tailed Possum

Trichosurus vulpecula
IUCN Redlist: Least Concern
Location: Carlton Gardens, Melbourne, Australia
Time: 2115 AEDT, January 24 2012

Australia is well known for being ravaged by hordes of invasive species. The Invasive Species Specialist Group of the IUCN lists 326 invasive species found in Australia in their Global Invasive Species Database. Cane toads are spreading throughout the northern part of the country, eating and poisoning natives. Red fox and feral house cats are implicated in the decline of many small species. Rabbits have grazed large parts of the country to bare earth. Camels introduced in the late 19th century, and used in the construction of rail and telegraph lines, now roam the outback, and may number more than a million. Feral pigs, goats, deer and water buffalo inhabit the more well-watered parts of the continent.

But Australia is also the country of origin of several invasive species. Here in Australia the brushtail possum is fairly well regarded, kind of like a raccoon, but they don't get into your trash quite as much. They're primarily herbivores, feeding on leaves, flowers, seeds, and they're preyed upon by snakes, owls, carnivorous marsupials and goannas (a type of large lizard), among other species. However I was raised in New Zealand, so for me the possum is synonymous with environmental destruction.

Introduced to New Zealand in 1837 to form the basis of a fur industry, possums found themselves in paradise. In Australia the plants have evolved in the presence of numerous arboreal mammalian herbivores, and accordingly many have evolved toxic and distasteful substances to dissuade them from eating too much. In New Zealand there have been no terrestrial mammals, apart from three small species of bat (one of which is now extinct), for tens of millions of years, and the closest equivalent to the possum would be the kakapo, a large flightless parrot, so the tender leaves of New Zealand's trees were at the mercy of the possums.

As you might expect in a country lacking mammals, there was also a lack of species which might prey on a possum. The surviving birds of prey are too small to tackle them, no lizards are even close to being big enough, and there are no snakes at all in NZ (and I mean none - not even in zoos). Only introduced mammals like cats and mustelids would be able of catching a possum, but why tackle a climbing, jumping mammal with teeth and claws when there are plenty of nearly defenseless birds, reptiles and bugs to be had?

Freed from the constraints of their homeland possums began to spread throughout the country, reaching most of the country by the middle of the 20th century, and today occupying essentially every bit of suitable habitat, as can be seen in the diagram below from Landcare Research.

These rampaging marsupial hordes soon proved detrimental to the NZ forest. They can strip their preferred tree species to bare branches, killing the trees and leading to the collapse of the canopy. The skeletons of their victims can be seen on the hillsides all around New Zealand today. Possums will also opportunistically eat birds' eggs and chicks, and native land snails. To make matters worse they also spread bovine tuberculosis, a big issue in a country with a large dairy sector.

With that background it's not surprising that in the eyes of most New Zealanders the possum is an ecological villain to be hated, and ultimately eradicated. And we are certainly trying to eliminate them, or at least control their numbers, and our current solution can also be found in Australia. Sodium fluoroacetate, commonly known by the brand name 1080, is a compound found in the leaves of certain plants, including quite a few from south west Australia such as members of the genus Gastrolobium.

Picture from here, thanks to Greg O'Beirne

1080 is a potent poison, and New Zealand uses 80% of the worldwide supply to control possums, rats, mice, and rabbits, as well as stoats which feed on poisoned carcasses.

The use of 1080 in New Zealand remains controversial, especially when it is distributed aerially in areas too rugged or remote to bait on foot. Concerns include the possibility that the poison will enter water supplies, the effect on non-target introduced species such as deer, and the unintended poisoning of native species. It can be a fairly acrimonious debate, so I won't go into too much depth here, but I will say that I support the use of 1080 to control invasive species at this time.

While I can understand concern about 1080 drops in your local water catchment area, and support alternative methods in such areas, 1080 degrades very quickly in water and after the vast majority of operations no 1080 at all has been detected in water samples. Careful planning and management - as there should be in any activity of this nature - should ensure the safety and peace of mind for all involved.

As for by-kill of species like deer, forgive me for not being too upset. Deer are invasives too, and severely impact on native vegetation, as anyone who has stood next to a deer-proof fence, or seen the before-and-after in an area where they have been removed can attest. The protestations of the hunting community will surely prevent deer from ever being completely removed from New Zealand, but I won't shed a tear if their numbers are cut down a bit.

The final concern, that native species are being inadvertently killed by these drops, is the most pressing in my eyes. 1080 is toxic to birds, though less so than for mammals, and by-kill can kill not-insignificant numbers of birds, particularly insect-eating species which ingest prey that have been feeding on baits, as well as curious omnivores such as kea. Obviously the aim of these poisoning programs is to help the birds, not kill them, so any advances which reduce these deaths would be a good thing. But the far bigger impact on these birds is the predation and competition of invasive mammals.

Again, anyone who has been to an area of forest after a 1080 drop can attest to its effectiveness. If you walk through the forest immediately after a drop, you may find some dead fantails and tomtits, but if you return in the following months and years the increase in birdsong, and the recovery of the canopy, are testimony to how our forests can recover when they are not bearing the load of introduced species.

Brushtail possums are not the only invasive species to come to New Zealand from Australia. Several species of wallaby have established populations, rainbow skinks are endangering native lizards, and eastern and crimson corellas compete with local birds, while rainbow lorikeets are beginning to get a foothold in Auckland. Other species such as kookaburra and sulfur-crested cockatoos now have self-sustaining populations in New Zealand, but as yet aren't posing a major threat to local species.

So, the brushtail possum provides an excellent example of how invasive species aren't just things like rats, cats and foxes, but many species that seem harmless in their native land can wreak havoc when they are introduced to an environment that hasn't adapted to their presence. This story also highlights how despite being (relatively) close, Australia and New Zealand are very different places. One is full of mammals, snakes, poisonous spiders, and huge lizards, while the other has a grand total of 2 land mammals, no snakes, and really nothing that could pose a threat to a human.



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