The Possum and Rata-Kamahi Dieback in New Zealand: A Review

Browsing by the introduced Australian brush-tailed possum (Trichosurus vulpecula) has been generally accepted in New Zealand during the past 30 yr as the principal cause of damage to hardwood forests dominated by rata (Metrosideros spp.) and kamahi (Weinmannia racemosa). Recently, this view has been challenged, and assertions have been made that the forests in a North Island tract were in poor order before possums invaded, that their natural collapse was only a matter of time, and that there is ample evidence relating the more dramatic changes in South Island forests to geological and meteorological events. In this paper, the evidence for repeated coincidence of increase of possum numbers and spectacular modification of the forests is reexamined. Such coincidences, the continued good health of montane hardwood forests where possums have not attained high numbers, and some experimental data, lead to the conclusion that the possum is responsible for dieback in rata-kamahi forests. Control of possums is therefore vital if the forests are to be maintained