Possum Peril

Forest and Bird, Volume 21, Issue 3, 1 August 1990, Page 30

Possum Peril

by

Jan James

HAT A TRAGEDY for New Zealand when possums were first introduced from Australia in 1837. Now 150 years later it is hard not to feel a sense of despair when confronting the enormity of the changes they have wrought to our native plants and animals. Possums are most damaging to forests when their numbers reach a maximum following the colonisation of new habitat. The West Coast passed through this peak phase in the 1950s and 1960s when the central alpine valleys suffered extensive dieback of rata, kamahi, and Hall's totara. This is why such concern is being expressed at the present time for the dieback in the pohutukawa and kauri forests of Northland. When one considers that the trees in both areas have life cycles of several centuries it is obvious that the forests will take many years to stabilise. Because the funds available for controlling possums are limited it is important that we make the best choices in protecting conservation values. We must also remember that the commitment will be ongoing for decades, or longer, unless some effective biological control becomes available. There are only two strategies for wild animals: to eradicate or control numbers. Eradication, the most desirable and costeffective option over the longterm, is possible only for islands where there is no chance of recolonisation. The only realistic strategy for possums on mainland New Zealand is sustained control.

Determining Priorities

The Department of Conservation (DoC) is currently rethinking its strategies for possum control. In the past the Forest Service priorities were set primarily on the degree of rata

dieback in mountain valleys, and the downstream capital values believed to be at risk from induced erosion and flooding. DoC is taking a more holistic approach in that both the forest and wildlife values are considered equally. The first stage is to identify and rank the most important conservation values. For management purposes it is better to identify a single endangered species because the success of control can then be measured directly by the response of this species. The conservation values are classified according to how vulnerable they are to possums. Then comes the practical realities of control. What has been the history of control in the area? Is it possible to maintain possum numbers at the level required to protect the conservation value? What is the risk of recolonisation from adjacent areas? Are other damaging wild animals present and can they be controlled? Each proposal for possum control is ranked according to the above factors and considered on equal terms with every other proposal. It is a process involving difficult choices, especially where many deserving cases will miss out through lack of funds. Because there are many value judgements within the process, the priorities should not only be made by a few specialist people. Every interested person should have the chance to participate and are welcome to contribute their views. On the West Coast DoC has determined that its priorities should include the Paparoa National Park. The goal is to protect magnificent northern rata trees and the unique coastal forests, and to prevent possums interfering with the nesting sites of the Westland Black Petrel.