Effects of opossum browsing on northern rata trees in the Orongorongo Valley, Wellington, New Zealand

Abstract

Browsing damage caused by opossums (Trickosurus vulpecula) was assessed visually, against a simple index, on 24 northern rata trees (Metrosideros robusta A. Cunn.) every 2 months from November 1969 to November 1974. All 24 trees showed some damage, and 3 died as a result of continued heavy browsing; 7 of 26 other rata trees in the study area have died similarly since 1969. When opossums were excluded from 5 previously browsed trees, all recovered. Stick insects (Phasmatidae) contributed about 3% to the total damage, but with other defoliating insects may be more important in other parts of New Zealand.

Effects of possum browsing on northern rata, Orongorongo Valley, Wellington, New Zealand

“…The extent of possum damage varied markedly between years, and from tree to tree, but overall damage levels increased progressively from 1990 to 1994 while possum density remained consistently above the long-term average. This study has reinforced Meads's (1976) findings that possums are the main cause of decline in numbers of northern rata, but does not confirm Meads's prediction of ongoing high mortality. The high level of mortality in 1970-74 and the increasing levels of damage seen in 1990-94 seem to result from a combination of periods of above-average possum numbers and other stress factors such as drought.…”

“…This mortality is attributed mainly to the extensive defoliation caused by browsing by the introduced brushtail possum, Trichosurus vulpecula (Brockie 1992;Meads 1976).…”

Diet of the opossumTrichosuvus vulpecula(Kerr) in the Orongorongo Valley, Wellington, New Zealand, in relation to food‐plant availability

“…Trees of this species, assessed at 10/ha, are heavily and consistently browsed; 4 of 24 trees regularly observed died between 1970and 1975(Meads 1976. Unless the opossum population declines or the animals' intake of M. robusta decreases, further trees will die at an accelerated rate.…”

“…An additional estimate of home range, as well as information on den sites and tree use, was obtained by monitoring the movements of 4 animals fitted with radio transmitters from September 1970 to August 1972 (Ward, in press). An intensive programme to measure the effects of opossum browsing on Metrosideros robusta, one of the large emergent trees in the area and an important food of opossums, was begun in 1969 (Meads 1976).…”

Suppression of fruit production of the endemic forest tree,Elaeocarpus dentatus, by introduced marsupial brushtail possums,Trichosurus vulpecula

“…Possum browsing is a major cause of mortality ofa variety of New Zealand's native trees (Fitzgerald 1981;Batcheler 1983). Meads (1976), Pay ton (1984), and Leutert (1988) recorded progressive defoliation and ultimate death of northern (Metrosideros robusta*) and southern (M. umbellata) rata. Fitzgerald (1976) and Campbell (1984Campbell ( , 1990 observed local extinction of palatable species such as five-finger (Pseudopanax arboreus), tutu (Coriaria arborea), toro (Myrsine salicina), and fuchsia (Fuchsia excorticata) in the Orongorongo Valley.…”