The Opossum

The Garden, especially the bush remnant, has been significantly affected by the possum invasion. While they have been significantly controlled in the Garden, ongoing surveillance is required.

From 1837 to 1924 Australian possums were released in New Zealand with the aim of establishing a fur trade, but it was not until the 1930s that it became clear that considerable damage was being done to native forests.

Possums are good climbers so can forage at all levels in the forest. They eat the foliage of a wide range of plants including kamani, northern and southern rata and pohutukawa, five-finger, tree fuchsia, mahoe and wineberry, vines include the lawyers and supplejack, and the native mistletoes. The result is that many of the plants die and some species have been greatly reduced. Possums also eat a wide range of flowers, fruits and seeds and so compete with native birds, lizards and insects.

It is estimated that there are about 70 million possums in New Zealand. At one time trapping for skins was widespread, but the international campaign against the use of animal fur in clothing has largely ended this. Poison bait has been used either in bait stations or aerial drops. Smooth metal bands around their trunks to prevent possums from reaching the crown have protected individual trees.

Brushtail possums were among the earliest animals introduced into New Zealand by European settlers. They were first brought from Australia in 1837 to establish a fur industry. By 1922, 36 batches of possums had been imported, mostly from Tasmania where possums were larger and most had the black fur preferred by furriers. These possums and their descendants were liberated at more than 450 places around New Zealand by 1930. At the time, these introductions and liberations were considered entirely beneficial, but after 1900 a number of reports of possum damage to crops, orchards, and forests prompted the Government to commission investigations by two of the country's leading botanists, Professor HB Kirk and Dr L Cockayne. They both concluded that damage to New Zealand's forests was negligible. Kirk went so far as to state "opossums may, in my opinion, with advantage be liberated in all forest districts except where the forest is fringed by orchards or has plantations of imported tree species in the neighbourhood".
However, from 1921 to 1947, the Government attempted to stop any further liberation of possums, which was prohibited by the Department of Internal Affairs. Both hunting and selling skins were regulated. The regulations merely provoked a flurry of illegal liberations. During the 1940s, evidence of damage by possums to New Zealand's forests increased, and in 1947 all restrictions on possum hunting were removed and penalties for releasing them were increased. The need for action against possums on a national scale was recognised, and in 1951 a bounty of 2/6d (25c) per head was introduced for animals whose skins were not sold. During the following 11 years, more than 8 million bounties were paid, but this did little to control the increasing and expending populations of possums.

In the late 1940s the first survey of possum distribution in New Zealand showed that possums had occupied about half of New Zealand. By 1961-63 they had spread to 84% of the country. Within the last five years Northland and the Coromandel Peninsula have been colonised, leaving few areas yet to be occupied.

The number of possums in New Zealand has been estimated at 70 million, about the same as the number of sheep. Because there are no predators or competitors here in New Zealand, and suitable habitats are readily available, possums occupy most habitats - all types of native and exotic forest, montane scrublands and tussock grasslands; swamps, farmland, orchard and cropping areas; and areas in and around towns and cities. Possums tolerate habitats with a wide range of climates - from arid plans to areas with high rainfall, and from temperate coastlands to harsh alpine slopes at more than 1800 m altitude in the North and South Island ranges.

Today possums are considered the major animal pest in New Zealand. In farming areas, they spread bovine tuberculosis to beef and dairy cattle, and to farmed deer, damage crops and orchards, kill poplars and willows planted to control hill-country erosion and stabilise riverbanks, and eat pasture. In exotic forest plantations they kill young trees and stunt the growth of older trees by ring-barking them or breaking the uppermost branches.

In conservation areas, possums cause severe damage by altering habitats important to native animals. Tree species that are palatable to both possums and native birds (e.g. rata, kamahi, and pohutukawa) become much reduced or locally extinct, and are replaced by plants that are less palatable such as tree ferns and pepperwood. As well as altering the composition of native forests and competing with native fauna, possums also prey directly on native insects and birds.

Possums are a serious conservation pest not just because they can decimate localised stands of trees such as the spectacular, red flowering pohutukawas trees on Rangiototo Island, but more importantly because they change the overall structure and composition of native forests and other ecosystems.

Overall impacts: Possums eat about 21,000 tonnes of vegetation per day (300 g wet weight per possum x 70 million possums). This oft-quoted figure is frequently used to depict possum as a rapacious consumer of all things green, but that implication ignores the daily foliage production of perhaps 300,000 tonnes for forests alone (7.5 million ha x 15 tonnes wet weight of foliage/ha/yr). These rough calculations are backed up by a study at Waihaha, West Taupo, that showed possums there ate only a small percentage (<15% by weight) of the annual foliage production of any of the 15 most common plants. Possums do not, therefore, threaten total deforestation on a national scale-in most forests, the process is one of compositional rather than structural change.

How do forests change? There are three broad processes at work:

Catastrophic dieback of forest: the change with the highest public profile is undoubtedly where possums cause all or most of a forest canopy to dieback over a short period. Although much effort has been directed at understanding and ameliorating this process, it only affects a relatively small proportion of our native forest -those dominated by just a few possum-preferred species such as the rata or kamahi. Historically, the relationship between dieback and the timing of possum invasion has been subject to much debate, but it is now widely accepted that catastrophic dieback is typically (but not always) caused by possums about 15-25 years after they colonise an area, which is about when their numbers reach an unsustainably high peak before dropping to lower levels. Susceptibility to catastrophic dieback varies between areas, depending on factors such as stand history, age, diversity, substrate type, and landform.
Gradual depletion: In more diverse communities that have a greater mix of palatable and unpalatable species the main initial impact process is one of gradual, possibly episodic, depletion. Possums selectively remove some species over many decades, resulting in compositional shifts. The greatest shifts are likely in mixed broadleaf forests in which species preferred by possums are abundant. However, even in the least susceptible forest types some minor species disappear (e.g.; mistletoes in beech forest). Gradual depletion is also likely to continue in areas where catastrophic dieback has occurred, but which still contain preferred species in canopy remnants.]

The key question in research is whether the process continues indefinitely until all possum-preferred species are completely eliminated, or whether some equilibrium is reached in which at least some possum-preferred species remain as a substantial component of the forest. Obviously possums can only persist if adequate forage is available. However possums do not use all available forage uniformly, even within species-an individual rata tree can be completely defoliated, but its neighbour left untouched until later.

The forest as a whole might withstand possum browsing if it was spread evenly across all species. In reality, depletion continues because individuals within species are selectively targeted. In addition, possums can rely heavily on fruit and flowers that may sustain a possum population even when the preferred-foliage species have been depleted.

Inhibition of regeneration: Ultimately, the recovery of forest from possum damage must depend on providing adequate protection of regenerating seedlings. The impacts of possums on regeneration are poorly understood-on Kapiti Island, possums killed seedlings of northern rata, tawa, and fuchsia seedlings, although typically a few specimens of each persisted. Researchers and managers have focused on the much more immediate and obvious changes in canopy condition, but also because possum effect on regeneration are not easily separated from the impacts of deer and goats. If, as is usual, deer and/or goats ungulates are present in moderate numbers regeneration of most possum preferred species is suppressed, but many such as kamahi can regenerate profusely after dieback if ungulates are absent. For the few possum-preferred species that are not palatable to deer or goats, such as totara, possums do not appear to affect the growth of young seedlings.

Susceptibility to damage by possums: Differences between communities: On the whole, the relative susceptibility of various forest types has become obvious over time. At one extreme, possums have little affect on simple beech forest with few preferred species. At the other extreme, possums cause catastrophic dieback or major compositional shifts in rata, pohutukawa, or kamahi dominated types. The only major uncertainty is perhaps the susceptibility of the diverse and unique forests in Northland (where possums have only recently colonised). For the most severely affected communities, the key issue is whether the change is reversible (i.e., whether possum removal would result in the forest returning to the original type or continue to develop into some alternative type).

Differences between species: The species most threatened by possums fall into two main groups:
(i) Common species that are major stand components and loss of which fundamentally changes the nature of the stand or community (e.g., kamahi, rata, pohutukawa, tree fuchsia, totara, kaikawaka); and
(ii) Species that are rare and could be driven to local or national extinction by possums, such as some mistletoe species and the wood rose.]

For some commonly browsed species, the impact of possums is not yet clear. Possums are implicated in the dieback of totara, but totara in areas without possums are often in poor health. Possums do not appear to affect totara seedlings.

Differences within species: Some species vary widely between areas in their ability to tolerate possum browse - tree fuchsia in the eastern South Island appears far less palatable to possums than tree fuchsia elsewhere. It is not clear whether this variation has a genetic basis, but if it does then concern for the long-term survival of tree fuchsia is lessened