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In 2005, Dr Hugh Tyndale-Biscoe, a Fellow of the Australian Academy of Science, said that « It is surely more realistic to accept that the brushtail possum is now a permanent component of the New Zealand biota, to which the native vegetation and wildlife will accommodate ». This sentence looks surprising coming from someone who chairs its Fund for Research on the Conservation of Endangered Native Animals. To discuss this statement we first need to investigate the effects of the brushtail possum on New Zealand biota. Then, we will study if it is realistic to believe that New Zealand native flora and fauna will accommodate to the possum. Finally, we will discuss the best way to consider the brushtail possum issue in New Zealand.

 

The brushtail possum (Trichosurus vulpecula) is a noctural arboreal marsupial native to Australia. It was introduced in New Zealand by European in 1837, in an unsuccessful attempt to establish a fur industry [5]. The possums escaped and, given the favourable conditions of their new habitat, they became an invasive species. The estimated population is now about 60 million individuals [4]. Possums are doing so many damage in New Zealand that they are considered as pests. They eat 7 million tonnes of vegetation per year, some of which are native endangered species (native red mistletoe, pohutukawa, kohekohe, canopies of rata, totara, kowhai, titoki ...). They also deprive other animals of this food and habitat. Although possums are predominantly herbivorous, they can prey on native land snails and on eggs and chicks of endangered birds’ species like the kiwi, kokako, and kereru [2]. So, as they are not only competitors but also direct predators, possums seem to be a plague for native fauna and flora. But we can wonder if, as Hugh Tyndale-Biscoe suggests it, this same wildlife will not accommodate to the presence of possums in New Zealand ecosystems.

New Zealand’s early separation from the Gondwana resulted in a biotum that evolved in the absence of any mammalian herbivory [1]. There were no native vertebrate having a significant report on plants and birds. Therefore, possums took up an almost unoccupied ecological niche. Possums acts as selective pressure on biota, and selective pressure is at the basis of natural selection. They behave like competitors and predators, and these functions are ecologically significant [7]: predation and competition have a sanitary action, eliminating the weakest individuals. They are responsible for the dynamics of prey populations and enable them to colonise several ecological niches. They enable a restructuration of the population pyramid, by preyind eldest and youngest individuals, and prevent overpopulation. They apply a genetic pressure and enable a better adaptation to the environment. Finaly, they increase the biodiversity by mainly preying the most numerous species and prevent by that way that one species strangle the others. However, if the predation is a driving force in the common case of evolution, possum in New Zealand biota is a special case. Native fauna and flora might need longer than 150 years to evolve and adapt to the recent settler. Local endangered species which are ever understaffed and in which the genetic variety is low don’t need predators. And surveys show that possums do not use all available food uniformly but that they have preferred species which they can consume to dieback. Moreover, the possum itself has no major predator in New Zealand [1]. One of the key questions in possums management is if an equilibrium can be reached : will the native wildlife grow fast enough to confront the possum, or develop specific mechanisms or behaviours to stand up to the pest? How long will it take? What will the damage be during this time lapse? Experience shows that the denouement is uncertain : for example, the Kerguelen cabbage disapeared in some islands because of the introduction of rabbits, before there could be any adaptation to this animal. So native fauna and flora may accommodate to the possum, but not at a human time scale and surely with a change in the composition and structure of the ecosystems and with the disappearance of many local endangered species.

 

So, if possum causes so much damage, shouldn’t it be eradicated of New Zealand? Why does Hugh Tyndale-Biscoe mean that it is more realistic to accept the possum as a component of the biotum? There were some experiences of eradication of the possum in some places of New Zealand, and particularly in islands. In Kapiti Island for example, possums were successfully eradicated after a sustained campain between 1980 and 1986, and the response of the forest was immediate : there was a marked improvement of the vegetation and the density of birds doubled in abundance during these six years [1]&[5]. But the question is now to know if it is possible to eradicate the possum from the whole country. And even if it is technically feasable, what will be the costs of such an operation? Will them worth the benefits? That is maybe why Hugh Tyndale-Biscoe says that it is more “realistic” to accept the brushtail possum as part of the ecosystems. But does accepting the possum mean no management of possum? Pests managers have to decide between eradication, control and doing nothing. Doing nothing in the case of the possum could be very damaging for New Zealand biota. Unlike Australia, New Zealand has few predators of the possums other than people or feral cats. Possums in New Zealand are infected with a smaller range of parasites and diseases. There are not any other animal competing with them for food and nesting. And native plants have few defences against browsers, such as thorns, tough leaves or poison like fluoroacetate. Therefore possums live in an environment without any control to limitate their population. The case of Kapiti Island before the eradication has shown the effect of the absence of control : the Government suspended possum control in order to determine whether or not it was efficient. The survey proved the devastating impact on the island’s forests [6]. So, accepting the possum without controling it does not seem to be a sustainable solution for the biota. The more realistic way to address the problem seems to be the control of possum populations : the goal of control is not necessarily to kill them but to conserve ecological diversity [2]. The traditional ways to manage this pest are trapping and poisoning but another approach has been developed for several years : the biological control. A sexually transmissible virus that makes the possums permantly sterile could be an ideal solution. The control solution seems to be a good costs-effectiveness compromise between the different methods.

 

Hugh Tyndale-Biscoe sentence is therefore not totally fitting the possum issue in New Zealand. If nothing is done to manage possum, biota will change, and certainly not in a good way, but will not “accommodate”. And it is surely not “realistic to accept the possum as a permanant component of the biota”, if this implies no control. But maybe the acceptance with control is more “realistic” than the eradication of New Zealand. Anyway, possum populations have to be managed, not only for the native fauna and flora but also for the primary production as they are a reservoir of bovine tuberculosis.