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Proceedings of the First Symposium on Marsupials in New Zealand

Some Consequences of Increased Numbers of Trichosurus Vulpecula

Some Consequences of Increased Numbers of Trichosurus Vulpecula

Zotov, in the early 1930s, noted damage done by possums and other browsing or grazing animals in the Tararua Ranges, though he did not publish this information until 1947. By this time the complacent attitude that had prevailed in the earlier part of the century was giving way to very great concern. Most of this concern stemmed from survey work done by the Wildlife Section of the page 44 Department of Internal Affairs and the New Zealand Forest Service. It was highlighted that in New Zealand most plant species have very little resistance to browsing or grazing animals, though there were some areas where the impact of the possum was minimised because the vegetation and geology differed significantly from that found in most areas occupied by the animal (Holloway 1959). It was Holloway's opinion that, particularly in the South Island, spectacular erosion had occurred even before the introduction of browsing or grazing animals, and that even now the removal of all animals from such areas would not prevent continuing instability.

Kean and Pracy (1949) and McKelvey (1959) thought that large possum populations could occur only in forests where the shrub tier and the forest floor cover had been reduced or practically eliminated by hoofed animals, principally deer (Cervus sp.). Possums apparently colonise an area with greater ease if under-cover is naturally sparse, or where sparseness is induced. If this theory is accepted as a generalisation, then the damage by possums must be superimposed on that of deer and goats Capra hircus, the possums defoliating the canopy above a depleted shrub layer. This type of damage is usually most severe in mixed hardwood-podocarp forest, but serious damage is sometimes confined to scrubby hardwoods that often dominate the valley bottoms and gullies. Heavy browsing on this type of vegetation can create areas of weak vegetation which run along main drainage patterns and culminate in zones of geological instability at sites which are most vulnerable to slipping (McKelvey 1959). Regeneration of the original scrub species is generally prevented by continued browsing, and those species which do regenerate successfully are the ones which are unpalatable to possums and deer. If any one species predominates, this species may in turn become vulnerable to insect pests that can then reach epidemic proportions sufficient to kill even the unpalatable species, e.g. geometrid moth larvae (Pseudocoremia rudisata) on Brachyglottis repanda and Olearia species (M.J. Meads pers. comm.). Any agent that defoliates and reduces vegetative cover minimises natural control of water runoff.

In the North Island there is a broken chain of steep, mainly forested ranges 1000–2000 m high, principally of readily fractured greywacke with frequent shatter zones and active fault lines. The central part is veneered with volcanic Taupo pumice. Protection forests in this part receive in excess of 2500 mm of rainfall per year, and the presence of dense forest prevents catastrophic floodwaters from reaching the fertile plains. However, in the Ureweras (for example) the grazing and browsing of ungulates and possums has page 45 destroyed much of the vegetation, and this and the presence of feral pigs Sus scrofa prevents regeneration of some species. In some places the ground becomes bare and compacted, while in others abnormal slips have been induced (McKelvey 1959), sometimes dislodging the more porous pumice mantle and leaving areas of more vulnerable greywacke. Though deer culling has reduced much of the deer damage, the impact of the possum in this and other areas is still great.

In general, however, it is difficult to differentiate between the effects of different browsing animals - even canopy damage may be caused by defoliating insects. The general forest deterioration is caused by the combined effects of deer, goats, cattle, pigs, and possums. Many plant species have been either eliminated or drastically reduced in abundance: kamahi Weinmannia racemosa, northern rata Metrosideros robusta, fuchsia Fuchsia excorticata, makomako Aristotelia serrata, pate Schefflera digitata, mahoe Melicytus ramiflorus, and some of the Pseudopanax species.

It appears that deer and possums cannot reach equilibrium with the environment without further undesirable results. Even if stability is reached, the end result will be a depleted forest which will contain fewer species and be much less efficient in controlling runoff. Besides introducing a number of undesirable mammals to New Zealand, man has converted vast areas of lowland forest to land suitable for grazing sheep and cattle. The topography and altitude of the remaining forest render it especially vulnerable to the pressure of browsing or grazing animals, aggravating the consequences of their feeding habits.