The Effects of Mammals on New Zealand Forest


The only mammals New Zealand forest fed until about 150 years ago were a few bats, and the polynesian rat which did not thrive. Since then cats, deer, goats, pigs, possums, ship rat and a few other mammals have been introduced and have found sufficient food to flourish. The questions are 1) are the plants they are eating surviving and 2) what other animals are being fed less?

Some grass and bush-clad hectares of Uganda feed about 175kg of elephants, hippos and buffaloes. Forests of North American and Europe feed about 5kg of deer. New Zealand forests have supported from 17kg to peaks of 105kg of possums per hectare, but the average in the well studied Orongorongo Forest is believed to be about 25kg. Birds add about 0.6kg, Rats 0.2kg, Cats and Mice 0.03kg each, and stoats 0.002kg . This gives a total mammal weight of close to 26kg per hectare.

Harmony has the occasional visit from pig and more frequent visits from goats. What weight these mammals add, and whether they displace some of the possums, is not known. Either way in the last 150 years the New Zealand forest has gone from feeding a very small weight of mammals to feeding maybe as much as 50 times more.

In its native Australia the possum cannot find as much food as it does in New Zealand as many Australian plants have adjusted to their presence by producing toxins.

Two to three hundred possums were introduced in 37 separate introductions from Australia between 1837 and 1924 as a potential fur industry. In 1920 a report to the government found their value to the fur industry far outweighed their damage done to forests and orchards and more were introduced. In 1928, it was again reported that their effects on forest were negligible. In time people witnessing the damage realised that the forest was being damaged severely - it was just taking time to show; and in 1939 the Tararuas were saturated with deer and possum and the destruction was full evident. In 1946, W. Kodzicki conducted a national survey which concluded that the introduced mammals were doing damage that far outweighed their value to the fur industry. The possum was now a major pest, but still valued by fur trappers and traders.

Comparing Orongorongo (NZ) rata-rimu forest with a oak-hornbeam forest in Belgium: Reptiles and amphibians were 0.0:1.3, birds 0.6:1.3, mammals 25:7, and earthworms 333:600 kg per hectare. Orongorongo had also measured their litter arthropods at 145kg per hectare.

A study of possum diet in the Orongorongo Valley in 1946 - 1947 showed that leaves in the stomach were from the following sources: 32% fucshia, 29% northern rata, 26% titoki, 19% kamahi, 18% fiverfinger, 15% clover, 10% rangiora, 9% manuka or kanuka; and 8% was petioles of fivefinger and occasional leaves of Muehlenbeckia australis, bush lawyer, hinau, scarlet rata vine, and mahoe. Flowers of northern rata, clover, manuka or kanuka, and fivefinger were mainly eaten in the summer. Fruits and seeds of pigeonwood, tutu, hinau, kawakawa, and poroporo were eaten in the summer and autumn. As Brockie describes it "The animals were smorgasboard eaters, each stomach containing on average three kinds of food. Several plants known to be poisonous to other animals, such as tutu and ngaio, were often eaten. Possums sometimes severely damaged lacebark, pate, young astelias and mistletoe. The ate the flowers and buds of nikau, astelia, mahoe and mountain flax, and the fruit and seeds of astelia, bush lawyer, horopito, kiekie, native passionfruit, supplejack, tawa and totara. Eleven fern species were also eaten. Though growing near the traps, black, hard and silver beech, stinkwood, clematis, hangehange, heketara, horopito, kiekie, matai, mapou, rimu, and supplejack showed no signs of being eaten".

Brockie lists the species being eliminated by possums at Orongorongo in order of preference as: Mistletoe, Milktree, Melicope, Fuchsia, Toro, Raukawa, Tutu, Fivefinger, Titoki, Muehlenbeckia, Bush lawyer, Wineberry, Tawa, Kamahi, Northern rata, Mamaku tree fern, Scarlet rata vine, Supplejack, and Hinau. He lists the species being eliminated by goats and deer as: Lacebark, Ribbonwood, Hen-and-chicken Fern, Stinkwood, and Mountain five-finger.

At Harmony, fucshia showed signs of significant browsing in the late summer of 2002 before leaf drop, but all appear to be tolerating the level at which they are being eaten. It may also be that at Harmony there are more palatable foods. At Orongorongo it was the most eaten food and no trees have been left alive. Brockie reports that "Fuchsia seedlings seldom grow more than a metre in height before possums defoliate or smash them but seedlings out of the reach of possums, or growing amidst tall nettles, escape their attacks" (75).

In 1946 kamahi was the fourth most eaten food. Between 1969 and 1973 it was the favourite. It is probable that in the years between the three foods more favoured than kamahi were eaten out. Though kamahi is possibly New Zealand's most abundant tree, Brockie predicts that it may dissappear unnoticed as it has little value as timber, its flowers and seeds are not readily noticed, and it quickly decays after death. (72).

Rata trees have been widely decimated by possum, and scarlet rata vine had become the third most eaten food in Orongorongo, by 1973.

In 1946 mahoe was 26th most eaten food, but by 1969 it had shifted to fourth place, probably because unlike other trees it can grow new leaves , bud, and twigs in any season when eaten.

Supplejack has shifted from a rare food to fifth.

Fivefinger at Orongorongo remains a favourite but has slipped from fith to seventh rank owing to becoming an 'endangered' species there.

Tawa saplings at Orongorongo are also well eaten by deer and goats. This and their low numbers appears to be why it is only eaten about 1%.

Ferns continue to provide only 1% of the possum diet at Orongorongo. Brockie says that "the white mucilaginous pith of uncurling tree fern fronds contributed a small amount to their diet but the possums are very selective. Of the five species present on the Station Grid only black tree fern or mamaku were eaten. Usually the animals attacked the still-furled young fronds, particularly in early September when they are amoung the first plant species available to break the winder food shortage. Although it is not a major item in their diets, possums selectively eliminated about one third of mamaku from the Station Grid between 1969 and 1978.

Tutu though poisononus to most farm animals is relished by possums and at Orongorongo is now only found in places where the possums can't get at it.

Mistletoe, muelenbeckia, bush lawyer, toro and titoki are no longer eaten at Orongorongo as they have been eaten out.

Ripe fruit and seed of trees is essential for the succession and regeneration of a forest, and it is crucial that some saplings are allowed to become mature, fruiting trees in each generation.

Brockie lists the species which are currently withstanding the presence of these animals, and which will probably be the main species in the Orongorongo Forest in the future as: Rimu, Miro, Matai, Totara, Rewarewa, three Beech species, Lemonwood, Pukatea, Pigeonwood, two Horopito species, Nikau, Pate, Heketara, four Tree Ferns, Mingimingi, Kiekie, Collospermum, Astelia, Ongaonga nettle, Hookgrass, Handfern, Kaikomako, and Putaputaweta. He also states that Broadleaf (g. littoralis), coprosmas, hebes, hinau, karaka, lancewood, leatherwood, lemonwood, maidenhair and most other ferns, mapou, rangiora, raweara, and tauhinu are rarely eaten. (76). Plants like coprosmas, kahikatea and mingimingi have such small leaves they are not usually eaten. Plants like coprosma and marbleleaf also branch (bivurcate) so extensively that it is difficult for browsing animals to eat all the leaves, and to damage all the stems.

It must be noted that possums can make do with a wide variety of foods, and so their diet varies from area to area, from season to season, and from tree to tree. Even when other trees of the same species can be extensively eaten and killed by possums, some often continue to thrive. Possums at Harmony eat a diverse diet and self limit their numbers rather than eating out all of any particular plant or species. Patches of wild blackberry and about 2.7 hectares of sweet tasting clovers and grasses in pastures and scattered clearings also provide them fast regenerating food in summer and autumn. Harmony possums are also encouraged to eat the bush lawyer and other plants growing on the many walking trails, and to keep the muehlenbeckia from smothering its hosts.

Brockie says a "Most grasses and pasture plants lack chemical defenses and taste sweet or bland. These plants are so sought after that possums may travel up to a kilometre a night from the depths of the forest to feed in grassy paddocks. This predilection for pasture plants partly explains why fucshia, tutu, fivefinger, and kamahi, eaten to death in forests, are often left untouched on farmland" (77).

Possums are also very efficient and thrifty eaters. They have the ability to keep high fibre foods in their digestive tract several days longer to extract their nutrition. Their gut allows them to eat considerable quantities of low quality, high bulk material when that is all that is avaialable. For protein they may prefer the higher concentrations in titoki, hangehange and mahoe, and the 25% or so found in young curled mamaku fronds may be why those are often eaten. They also eat fungi spores and insects year-round for protein.

An average possum weighing in at 2.3 kg consumes about 0.8 - 1kg of leaves a night, or may spend the whole night gorging on ripe berries. If subsisting on fresh leaves without flowers, fruits or berries, an average possum would eat about 300 - 400kg a year.

They have been reported to be able to obtain as much as 30% of their diet from grasses and clovers; and to subsist on native plants and grasses growing beneath pines in plantations. In the radiate pine forest at Ashley, Canterbury, leaves and flowers of European broom are reported as providing the staple diet, supplemented with pollen cones and pine needles in winter and spring. (76).

George Batzali found that terpenes, which taste like pepper and turpentine, made up more than 12% of rimu leaves, more than 9% of horopitos (pepperwoods), 8% of kawakawa (pepper tree), and more than 5% of tauhinu and the filmy fern, Hymenophyllum demissum. Brockie says "The plants very rarely feature in possum's diet and were amoung the last chosen in feeding trials. The plants most favoured by possums - fuchsia, kamahi, northern rata, tawa, toro, tutu, titoki and wineberry - contained less than 2% terpene. It therefore seems that terpene acts as a deterrent to possums if present in large enough quantities.

Very high levels of astringent tasting tannins, such as the more than 14% by weight found in young rimu leaves and the young fronds of piupiu, crown fern also appear to put off possums and goats.

Though phenols protect plants against bacteria, fungi and insects they seem to be appetising to possums. Plants containing the most phenols: bush lawyer, fucshia, kamahi, mamaku - young fronds, northern rata, scarlet rata vine, and wineberry are all extensively eaten. (77)

Leaves eaten by possums has a ragged, ripped look. Caterpillars give their leaves a cut wave, deer eat the leaf straight across, and goats usually take of the whole leaf.

Trunks of trees are often marked by their teeth with a series of several horizontal strips taken out of the bark, and you can see this in several places at Harmony Farm.

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