Proceedings of the First Symposium on Marsupials in New Zealand
Some Effects of the Feeding Habits of the Possum Trichosurus Vulpecula
Possum foods have been examined since about 1920 though more scientific research did not begin until 1946-47. The results of both incidental observation and detailed research have many aspects in common and a few which show peculiarities restricted to certain localities or types of habitat. An overall picture of the food of possums and their feeding habits is covered in this paper. Food habits of Trichosurus vulpecula in New Zealand are compared with those in Australia.
Trichosurus Vulpecula in Australia
In Australia, the common brushtail possum Trichosurus vulpecula is not often regarded as a pest. It occurs in South Queensland, New South Wales, Victoria and Tasmania. The distribution of T. vulpecula overlaps narrowly with that of the mountain brushtail possum Trichosurus caninus, generally in dry sclerophyll forests which have an unusually dense under-storey. Broadly, T. vulpecula is restricted to dry sclerophyll forest but it is also a common suburban dweller, while T. caninus is largely restricted to wet sclerophyll, sub-tropical or rain forest (Owen and Thomson 1965). T. vulpecula feeds mainly on eucalypt leaves, which may form 66% of its diet (Freeland and Winter 1975), as well as on a variety of indigenous and exotic shrubs. It is primarily arboreal, while T. caninus feeds mainly on ground or under-storey plants; there is little or no evidence of seasonal variety in the diets of either species (Owen and Thomson 1965). Mature leaves are preferred by both species so that food availability is constant all the year round. Nest sites of the two species differ; T. vulpecula mostly up trees, T. caninus in fallen logs or hollow stumps. T. caninus is more tolerant of wet conditions than the mainland T. vulpecula, and is probably more terrestrial.
Trees in exotic plantations (mostly Pinus radiata) may be attacked by both species of Trichosurus but also by Rattus fuscipes 1, Wallabia bicolor, and occasionally by the common wombat Vombatus ursinus and the rabbit Oryctolagus cuniculus (McNally 1955). The bark of the trees is removed by all these species, with a typical pattern for each. T. vulpecula strips off the bark and eats the soft cambial flesh, and there is not much sign of bark in the stomachs. W. bicolor and R. fuscipes eat both stripped bark and apical portions. Both T. vulpecula and T. caninus may girdle the trees, which 1formerly Rattus assimilispage 42
causes either tree malformation or death. With T. vulpecula the damage is often much more difficult to detect because it occurs higher up in the trees. There may be repetitive attacks on particular trees. In four plantations sampled for possum damage, only 20% of the trees remained untouched (McNally 1955).
In Tasmania, Eucalytpus regnans (mountain ash) is browsed by T. vulpecula (Cremer 1969), as are Acacia and Pomaderris species. The main damage is done to trees up to about 1 metre, and it is uncertain what part of the damage is attributable to T. vulpecula and what is caused by other browsing species (e.g. wallabies). Petioles of E. regnans are eaten by T. vulpecula, with leaves appearing to be eaten mostly in autumn and winter. However, this may be an erroneous observation because it is then that browsed foliage would be least likely to be hidden quickly by new growth. Tasmanian plant species seem able to recuperate from browsing more readily and in more months of the year than most New Zealand species.
Some workers (e.g. Green 1974) consider that T. vulpecula is primarily a ground feeder in Tasmania and that it has a liking for a wide variety of food, including meat. Though formerly restricted to dense sclerophyll forest in Tasmania, it is now much more widely distributed, and despite its mortality from hunters it has increased and is now plentiful.
Troughton (1941) says that though T. vulpecula is specialised for an arboreal existence, it thrives in comparatively treeless areas, making dens in rabbit warrens, etc., as readily as in trees. He considers that in the wild state its food is variable, and includes many species of eucalypt. In Australia generally, T. vulpecula is thought to be rarely responsible for other than localised damage. The Tasmanian T. vulpecula, thought by Troughton to be a separate species (T. fuliginosus), is much heavier than its mainland grey-furred counterpart, and has much denser, longer fur which is more commonly brown.
Historical Background of Trichosurus Vulpecula in New Zealand
Trichosurus vulpecula was initially introduced into New Zealand from the Australian mainland and Tasmania in the mid-nineteenth century, but did not become established successfully until about the end of the century. Thomson (1922), quoting F. Hart of Southland, wrote that the food of the possum was chiefly seeds of broadleaf, kamahi, panax, maple (Pittosporum), fuchsia, and makomako, rata blossoms and supplejack berries, and practically all seeds page 43 that grew in the bush. The possum was not considered to be a grass-eating species, though it would eat red and white clover and sweet-briar shoots. The damage that the animals could do when running at large was thought by Thomson to be very little, seeing that they never came out into open country.
Perham (1924) said that in a search for damage by possums that might be construed as materially affecting the forest, results had not been very fruitful - in fact only one instance of extensive bark-biting had been found. Perham considered that their leaf-eating habit, if moderate (as had been the case up until then), would have no bearing on the welfare of the forest "as trees have been subjected to and withstood such from pigeons through many centuries". Where death of one or two specimens of some tree species (e.g. northern rata Metrosideros robusta) was evident, the theory was that the tree had reached its allotted span and was dying away. Other similar examples were quoted.
Cockayne (1928) thought that if the possum was doing noticeable damage, the forests would unmistakeably point to this in their dead trees. But the forest, according to Cockayne, was untouched by damage from possums: "If damage of any kind there be, it is so slight as to be negligible". Nor did he consider that the animals affected the all-important floor covering or reduced the seed crop to any great extent. "At any rate, if it is proved eventually, which is most unlikely, that opossums are a menace to forests, their skins are so valuable that at any time the animals can be reduced in numbers to the extent desired without any cost. Unlike deer, it would pay handsomely to keep the opossum under strict control".
However, Kirk and Bendall (1919) were much more concerned about the possums' effect on Kapiti Island. Damage to kohekohe Dysoxylum spectabile, mahoe Melicytus ramiflorus, passion-flower Tetrapathaea tetrandra, and fuchsia Fuchsia excorticata was noticeable, and these authors thought that the only safe aim was the absolute extinction of the possum on Kapiti Island.
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.
Feeding Habits of Trichosurus Vulpecula
Possums in particular show very pronounced preferences for some plant species, and this intensifies their impact. Kean and Pracy (1949) listed 70 or so species eaten by the possum, and ascribed to them an order of preference shown by the animals. Subsequent work on the food of possums has largely substantiated this list and quantified some aspects of the diet (Mason 1958, Gilmore 1967, Fitzgerald 1976), but the habitats and areas studied by the more intensive method of stomach or faecal analysis have been more restricted than those covered by Kean and Pracy.
From all the work done on the food of possums in New Zealand, the same species occur regularly and reliably in the diet: fuchsia, rata (Metrosideros robusta or M. umbellata), kamahi, fivefinger Pseudopanax arboreus, kohekohe, page 46 makomako, titoki Alectryon excelsus, and toro Myrsine salicina. Where bush borders farmland the choice of food is widened to include grasses and clover.
However, the order of preference by possums is not always consistent. For each species mentioned as highly preferred there may be exceptions where these species are virtually ignored, and others where species normally considered unpalatable are preferred despite the presence of so-called highly preferred plants. The reasons for these exceptions, and for that matter the reasons for preference of any species, are not clear. Information on species eaten in Australia suggests that the leaves of these plants are highly sclerophyllous; this probably indicates that a large amount of food must be eaten in order to extract the nutrients required. Lipid content in eucalypt species is quite high, as it is in Metrosideros spp. - particularly M. fulgens during the months that it occurs most frequently in the diet of possums in the Orongorongo Valley (Fitzgerald 1978). Just how much of this lipid is or can be used by possums is not known.
Other preferred species in New Zealand seem to have amounts of protein and carbohydrates that are comparable with the amounts in those species which are not generally eaten. Plant toxicity (or lack of it) is another feature which may determine the possums' choice of food, but it is well known that possums can and do eat - with apparent immunity - leaves of Coriaria species which are actively toxic to cattle. Fluctuations in the presence and abundance of fruit may also alter the possums' dependence on leaves and change their selection patterns.
Our feeding trials with captive possums have suggested reasons for both preferential selection of some plant species and seasonal selection of others, but we do not yet know the habitats or nutritional plane necessary for possums to breed twice a year. More work which will throw light on the nutritional requirements of possums may help to sort out some of the management problems of possum control.
A comparative study of some aspects of the ecology of the possum in two different-aged stands of Pinus radiata has recently been completed (M. Clout, pers. comm.). One stand was planted in 1960, the other in 1971. Stomach analyses showed that, in the older stand, the possums ate pine foliage mainly in the autumn, when it formed up to 50% of the diet and was supplemented with female strobili. From June to September large quantities of male strobili were eaten and formed about 70% of the diet by August. Bibionid larvae page 47 (Insecta: Diptera) were also eaten, often in considerable numbers, from April to October. During late spring and summer, stomach contents consisted mostly of leaves of lower-storey shrub species with some ferns. The switch from this mixed diet in spring/summer to one comprising predominantly pine was quite abrupt.
In the 1971 stand the diet was more varied, less predictable, and with less seasonal pattern. Reproductive parts of the pines were not available, but pine foliage was still eaten in small amounts and bark was eaten mainly in winter and spring. Grass was an important food throughout the year; though dicotyledons formed a small proportion of the vegetation, they often accounted for about 40% of the stomach contents. Fern was also eaten in this habitat.
Possums in the two ages of pine stands thus had strongly contrasting diets, particularly in winter, largely as a result of the presence or absence of strobili. This difference in diet, with other variables possibly associated with the differences in the age of the two pine stands, produced quite marked differences in body weight and fecundity between the two groups of possums. Other such studies (e.g. Bell this symposium) together with laboratory experiments may eventually allow us to predict with confidence the response of the possum to its habitat and food supply.
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COOK. During which months were rata trees eaten?
FITZGERALD. In the Orongorongo Valley northern rata Meterosideros robusta was eaten in all months of the year. Over the whole year this species comprised about 30% of the diet.
CHARLESTON. Do they more than accidentally eat insects? Having kept the old possum I found them willing to eat almost anything and they just loved meat.
FITZGERALD. Yes. that is right, they will eat practically anything. Dr Ben Bell has done some work on possums eating puriri moths. When these moths are in season he found them taken in large numbers by possums when attracted to a light. He had the moths analysed and they had a high lipid content. Mick Clout's example I cited was not just a case of a few odd larvae in the gut -the dipterous bibionid larvae were there in considerable numbers, something like 1,800 in one stomach. To a very large extent these larvae came through the gut intact.
PEKELHARING. Is there totara Podocarpus totara in the Orongorongo Valley and, if so, do possums eat it?
FITZGERALD. Yes, it is there and they eat it, but it is not in their first 14 choices of tree species.
BROCKIE. Mistletoes were once widespread through New Zealand but have been disappearing at an alarming rate. One species Trilepidea (Elytranthe) adamsii was confined to the Coromandel district and is now thought to be extinct because possums have eaten it out. This is the one and only New Zealand native species that seems to have become extinct on the mainland, and possums seem to be the culprit. It looks as though other mistletoe species are heading in the same direction and very rapidly.
FITZGERALD. This is an interesting point. Dr Given has produced a recent paper* on rare and endangered plant species and it would be worth checking - presumably you could make some intelligent guesses about other species likely to become extinct like the mistletoes.
ANONYMOUS. The reverse is reported from South Australia where mistletoe is alleged to be spreading through the lack of possums.
FITZGERALD. I think Sir Charles Fleming noted that pollen analyses indicate species such as Meterosideros were quite widespread in parts of Australia, but no longer occur there.
MIERS. Kamahi and fuchsia were also in a similar category in Australia.page 50
* GIVEN, D.R. 1976. A register of rare and endangered indigenous plants in New Zealand. N.Z. Journal of Botany, 14: 135–149.