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The pressures on our biodiversity

Left to itself, New Zealand's bird-dominated world would have continued to depart from the evolutionary mainstream. But the arrival of people changed all that. In one sense, species diversity increased after humans arrived, because of the influx of exotic plants and animals. But the invaders also wiped out several unique endemic species and endangered many others. The gains were local but the losses were global. Today, Earth's biodiversity is dozens of species poorer because of the effect of humans and their exotic plants and animals in New Zealand.

The arrival of people brought three main pressures to bear on New Zealand's endemic species: human predation (hunting, fishing and gathering); habitat destruction (deforestation, wetland drainage, fragmentation and degradation of ecosystems); andpests and weeds (alien organisms which prey on or compete with indigenous species or degrade their habitat). These pressures arrived in two historically distinct waves; the process started by the first wave (Māori) was accelerated by the second wave (European).

The First Human Invasion

New Zealand's long period of ecological quarantine ended when Polynesian mariners, with their families, dogs and rats, settled here 700800 years ago (Anderson, 1991; McFadgen et al., 1994). The first settlers also brought half a dozen tropical plants for food, fibre and gourds. This seemingly small ecological invasion of New Zealand was the last leg in our species' colonisation of the non-polar world. As with human intrusions elsewhere, the native species felt the effect almost immediately (Anderson and McGlone, 1992; McGlone et al., 1994).

Forests were repeatedly set alight over wide areas (see Chapter 8). Seals and birds were hunted, with seals initially providing the main meat supply (Davidson, 1984). The birds were also preyed on by rats, and probably dogs, which ate the eggs and chicks of ground-nesting species. First to go were the native swan, giant goshawk, coot, crow, two geese and three ducks, followed by the giant rail, the native pelican, the giant Haast's eagle - the largest eagle that has ever lived - four smaller rails, the owlet-nightjar, a snipe and at least two wrens. The largest birds of all were given the Polynesian name for chicken, moa, and were avidly hunted. All eleven species went into rapid decline (King, 1984; McGlone, 1989; Anderson, 1989; Anderson and McGlone, 1992).

The number of moa skeletons in known Māori food sites is estimated to be between 100,000 and 500,000 - many more than the total moa population of 60,000 - 70,000 estimated to have been alive at any one time (Anderson, 1989; Diamond, 1991). One of the earliest archaeological sites, at Shag Mouth in the South Island, was assumed to have been occupied for a long time because it contained the bones of about 6,000 moas. New radiocarbon dates, however, indicate that it was settled sometime after a.d 1300. and was occupied for a comparatively brief time, possibly little more than 20 years (Anderson and Smith, 1996).

Pressure on the environment seems to have intensified as the human population expanded and prey species declined. By about 1400, fires were raging in many parts of the country, destroying both lowland and upland forest and replacing them with bracken and tussock grassland (Anderson and McGlone, 1992; McGlone et al., 1994; McGlone et al., 1995). The fires continued to devastate the forests for about two centuries, finally dying out between 1550 and 1600. As the deforestation came to an end, fortified villages began to appear, suggesting that resident tribes were having to defend their territories and dwindling resources against each other (McFadgen, 1994). Regenerating forest continued to be cleared by fire, but the destruction of mature forest had almost ended by 1600 and would not resume again until the early 1800s (Cameron, 1961, 1964).

By then, a third or more of the forest had gone and, with it, a quarter of the endemic land-based birds (some 34 species and 1 subspecies) and a fifth of the endemic seabirds (4 out of 22 species). The extinct seabirds and all but 9 of the extinct land birds have been found in Māori middens. One of the species thought to be absent from middens, the owlet-nightjar (Megaegotheles novaesealandiae), has recently been identified with a midden (McCulloch, 1994). The non-midden species were probably exterminated by rats, fires and the effects of deforestation.

Among the survivors, the flightless takahe (Porphyrio mantelli), kakapo (Strigops habroptilus), huia (Heterolocha acutirostris), southern crested grebe (Podiceps australis) and little spotted kiwi (Apteryx oweni), were reduced to localised populations, and the merganser duck (Mergus australis), which once occurred throughout New Zealand, was eliminated from all but the remote Auckland Islands (Anderson and McGlone, 1992).

The extinctions also included three native frogs and an unknown number of invertebrates, few of whom left fossils. Tuataras became extinct on the mainland, with at least one species vanishing altogether and the two remaining species limited to rat-free islands (Daugherty et al., 1990). Some lizard species also vanished except on rat-free islands. Fur seal colonies, which once existed the length of New Zealand, were eliminated from the northern North Island by 1500 and from all but the far south by 1800 (Davidson, 1984; Smith, 1989; Anderson and McGlone, 1992). Sea lion and elephant seal remains are found throughout New Zealand in early Māori middens, but had become rare by 1500 and absent by 1800.

The elimination of seals, sea lions and large birds forced the Māori population to concentrate more heavily on a limited range of plant foods (e.g. fern root, kumara, cabbage tree root) and on fish, crustaceans and molluscs. None of these became extinct, but midden remains show that some coastal shellfish populations disappeared completely (Davidson, 1984; Anderson and McGlone, 1992). On the other hand, freshwater fish appear to have benefited from the increased attention, occasionally having their populations enhanced by the transfer of some species (e.g. eels and smelt) to waters where they were rare or depleted (Strickland, 1993). Similarly, flax snails from Northland seem to have been transferred to the Poor Knights Islands as a food delicacy (Hayward and Brook, 1981).

As meat and animal fat sources declined and competition for territory increased, a small number of Māori left the South Island and settled in the Chatham Islands. Archaeological evidence suggests that this occurred between 1550 and 1600 (McFadgen, 1994). Surrounded by sea lions, fish and seabirds, these people, the Moriori, were able to abolish warfare and establish a pacifist society which remained isolated from the rest of New Zealand for more than two centuries. During this short time, however, the sea lions disappeared and about 20 bird taxa, including half the endemic species, became locally extinct while another four species were heavily reduced in range (Anderson and McGlone, 1992). The lost endemic species included several seabirds and the Chathams duck (Holdaway, 1989; Bell and Robertson, 1994; McFadgen, 1994). Of the 21 petrels that once bred in the Chathams, only 6 remain today (McGlone et al., 1994).

As with human societies elsewhere, it took some time for the Māori communities to learn from their environmental mistakes. By the time they had developed customs and practices to limit access to, and abuse of, treasured parts of the environment, they were living in a depleted landscape where competition for food resources had become a fact of life (McGlone, 1989). The same process has been recorded from Hawaii, Easter Island, Henderson and Pitcairn Islands and other parts of the Pacific, as well as Madagascar, the Mediterranean (e.g. Cyprus and Crete), mainland Europe, North and South America and Australia (Culotta, 1995a; Diamond, 1991; Dodson, 1992; Flannery, 1994; Martin and Klein, 1984; Pimm, 1995a Pimmet al., 1995; Steadman, 1995; Timson, 1993).

The Second Human Invasion

The pressures on the remaining indigenous species intensified with the arrival of the Europeans, beginning with the release of rats, pigs and goats by Captain James Cook in 1769. Cook was followed several decades later by sealers and whalers who quickly devastated the remaining seal and sea lion populations and also drove the Right whale to the brink of extinction. After the Treaty of Waitangi was signed in 1840 by the British Government and Māori chiefs, the door opened to a torrent of land- and gold-hungry immigrants, mostly from Britain and Ireland. By 1920, the new settlers had converted half the remaining forests into farms and towns and had introduced legions of plants and animals, some of which displaced or preyed upon the indigenous plants and animals. Wetlands, dunes and estuaries were also invaded and converted into pasture or urban settlements.

The destruction of native habitat continued into the 1980s and still occurs at lower levels today. Although most native forest clearance has ended, wetlands are still being drained in some areas and dunes are still being built on. In fact, New Zealand is still in the throes of the second invasion as both our native species and our society struggle to respond to the enormous pressures we have unleashed on the nation's biodiversity. The rest of this chapter is largely about the effects of the second invasion.

Since European settlement, 16 land birds (9 species, and 7 subspecies) have been driven to extinction, together with a native bat, 1 fish, at least a dozen invertebrates and possibly as many plants. The lost birds include: the New Zealand quail (probably exterminated by early collectors), the North Island thrush (probably eliminated by feral cats), the huia (see Box 9.6), the North Island laughing owl and the North Island bush wren (both probably eradicated by rats). The stitchbird and the North Island saddleback (respective victims of dogs and rats) were eliminated from the mainland and only survive on a few islands. The South Island kokako is another probable extinction victim. Although possible sightings are occasionally reported, none has been confirmed to date.

Most significantly, the new pressures generated by the colonists and their agricultural economy greatly increased the number of threatenedspecies. Many are still coping with the fallout from habitat destruction and predation. One example is a native parrot, the kea (Nestor notablis). Once widespread in the South Island, its original range was first reduced by Māori deforestation, then by the conversion of the high country into sheep pasture.

Late last century, when it became known that some keas attack sheep, a bounty was paid for their destruction and continued to be paid well into this century. At 10 shillings a beak in the 1920s (equivalent to $65 today) keas provided a lucrative living for bounty hunters. Extrapolating from Ministry of Agriculture and Fisheries records, it is estimated that about 150,000 keas were killed between 1870 and 1970, bringing the population down from about 50,000 to 5,000-15,000 today (Temple, 1996).

The kea only received formal protection in 1986 after an agreement was reached between the Department of Conservation and high country farmers, in which the Department undertook to control 'rogue' birds. Keas were also shot in the early days of ski-field development but today most ski-fields are kea-proofed. Wildlife smuggling and random shootings are the main threats now (Temple, 1996). The keas' natural intelligence and adaptability allowed them to cling on until the new invaders had an eleventh hour change of heart. Unfortunately, not all species have been so favoured and many may still succumb to the second invasion.

Box 9.5: The hairy invaders

A group of 34 introduced mammals, headed by our own species, now dominates the New Zealand environment. Of the 54 mammalian species that were originally introduced, some died out, 18 have restricted distributions, and only 14 are widespread (King, 1990). Five of these species are clearly dominant - humans, rats, sheep, cattle, and possums. Most of the land's productivity, from both farm and forest, goes into feeding and pampering this mammalian elite. As their collective biomass (aggregate weight) has grown, that of the indigenous plants and animals has shrunk to a fraction of its original size. Landcare Research is developing and collecting information for a national database on wild populations of goats, pigs, tahr, chamois, the five species of wallabies, and the seven species of deer (red, sika, fallow, whitetail, rusa, sambar, and wapiti) that have been introduced here.

Because mammals are bigger, hungrier, and more active than most other animals their effect has been immense. The grazing mammals (sheep, cattle, and rabbits) and the mammals who eat them (humans and domestic cats and dogs) have had the greatest impact. Together, they dominate the land surface, which has been largely deforested and drained to accommodate them. Even where deforestation has not occurred, browsing mammals (e.g. goats and deer) have invaded the forests, eating young plants, preventing regrowth, and competing with birds for food. The predators (e.g. stoats, ferrets, feral cats, sojourning dogs) have reduced bird and reptile populations. And the omnivores (humans, rodents, possums and pigs) have done all these things.

Among the grazing animals, sheep, through their numbers, have had the greatest effect on forest and tussock ecosystems, while cattle have had the greatest impact on wetland ecosystems. In some places, sheep and cattle have become feral and degraded native forest (e.g. in the Chatham Islands where their removal is now allowing vegetation and native pigeons to recover.) Domestichorses are a minor presence. Their main effect on biodiversity was before 1920 when they provided the main labour source for converting forest to farmland. About 2,000 feral horses, in Northland and the Kaimanawas, are culled periodically to limit their eating and trampling of native plants.Rabbits are a serious problem in the South Island high country where they compete with sheep for pasture. Occasional population explosions have led to over-grazing, soil degradation and the spread of hawkweed. They pose a threat to the tussock ecosystem and are also a moderate risk to crops and pasture in dry eastern areas of both islands. Elsewhere their numbers are controlled by predators (stoats, ferrets and feral cats) and by rain-drowning, which together kill 80 percent or more of each litter.

Among the browsing animals, goats are worse than deer because they have wider food preferences and are more agile (Parkes, 1993). They were less widespread than deer until the early 1990s when the downturn in goat farming caused many to be released in native forest and scrub. Now they are spreading, and occupy about 25 percent of Conservation Department land. Deernumbers were reduced in the 1970s by heavy culling operations, particularly aerial shooting and capture for deer farming. In areas of Fiordland where deer numbers have been reduced, the vegetation is recovering. Sika deer, however, which were formerly confined to the Kawekas and Kaimanawas, have recently appeared in unconnected forests in the Tararuas and Coromandel, apparently transferred by recreational hunters. Himalayan wild goats, ortahr, have lived in the South Island high country since 1904. Their population peaked at about 60,000 in the 1970s when many rare alpine herbs were almost wiped out . Helicopter shooters reduced the tahr population to about 6,000, but, since 1983, tahr numbers have climbed back up to about 12,000, spread over 700,000 hectares of high country. The ecologically tolerable population size is estimated to be about 10,000 (Cuddihy, 1994).Chamois pose similar problems in these areas.

Of the predators, cats, stoats and ferrets are the most destructive. The mustelids ( stoats, ferrets andweasels ) were introduced to control rabbit numbers, and have done so in many areas. They may also have played a part in removing Norway and Pacific rats (kiore) from our forests, although they have had little effect on the tree-climbing ship rat. Their biggest impact on indigenous species appears to have been on ground-nesting birds, such as black stilts in the Mackenzie Basin. Ferrets are probably the main predators of albatrosses and possibly yellow-eyed penguins on the Otago coast. Stoats are major predators of hole-nesting species, especially yellowheads, parakeets and kaka. Stoats are probably the most difficult predator to control. They are bait-shy and trap-wary, and very fertile.

Dogs are meat consumers, vital labourers in the pastoral economy, a social problem in some urban areas, and, occasionally, wildlife predators. Nearly 29 percent of homes surveyed by Statistics New Zealand in 1991/92 had pet dogs, whose total population came to nearly 398,000. In addition, the country's 80,000 farms had an unknown number of working dogs, perhaps totalling 240,000 (assuming, on average, one 3-dog-team per farm). In some areas unrestrained dogs have decimated ground-living bird populations, such as kiwis in Northland. Last century, packs offeral dogs preyed on sheep but were soon eradicated and have not become re-established. In the early centuries of human settlement, Polynesian dogs may have preyed on flightless birds and reptiles, but there is no direct evidence of this.

Unlike dogs, many cats (perhaps a million or more) live entirely in the wild. A further 770,000 were living as pets in 48 percent of New Zealand homes during 1991/92. The impacts offeral cats are similar to those of the mustelids. They prey mostly on rabbits, other mammals and introduced birds, but also kill many indigenous species. Feral cats on islands have devastated some seabird populations, and on the mainland they pose a threat to ground-nesting birds. Dotterels on Stewart Island, black stilts in the Mackenzie Basin, and yellow-eyed penguins on the Otago Peninsula, are among their worst-pressed victims.

Omnivorous mammals are particularly dangerous because they eat both plants and animals. Pigs, for example, can wipe out a snail population in a day, and are very adept at rooting out petrel burrows. However, except for Northland, parts of Nelson/Marlborough, and the Chatham and Auckland islands, feral pigs are generally not numerous enough to pose a serious problem. A far bigger threat comes from the ship rat, one of four feral rodents in New Zealand. They exist in podocarp-hardwood forests at densities of 510 per hectare, but are less common in beech forests because of the lack of palatable fruit. They eat eggs, chicks and lizards, and compete with birds for food. The threat from other rats has diminished this century, partly because of predation by stoats. Mice may pose a threat to lizards and some invertebrates. Since stoats arrived, Pacific ratsdisappeared from all areas other than Fiordland and about 50 offshore islands. The once abundant Norway rats retreated to towns, farms and waterways.

Public enemy number one, however, is the possum. Possums were introduced to make money but now cost millions of dollars a year to control. Between 1837 and 1922, several hundred possums were brought here from Australia to establish a fur industry. In the 1890s, enthusiasm was so strong that acclimatisation societies bred and released their own strains and the government also imported and released animals. They were protected until 1921 and, for the next 25 years, were harvested under a restricted licensing system. In 1947, possums were reclassified as pests because of the damage they were inflicting on native trees and shrubs. Once thought to be exclusively vegetarian, the possum is now known to be an opportunistic omnivore. In captivity it will eat meat. In the wild, it is known to eat invertebrates, snails, mice and small birds. It raids bird nests, including those of the endangered kokako, killing the parent bird and eating its chicks and eggs (Brown et al., 1993).

The possum's main food, however, is vegetation. The huge population consumes about 20,000 tonnes of plant matter every night. Their impact is heightened by the fact that they are choosy eaters which maim a wide range of plants by feeding only on the most palatable bits. They mostly eat leaves but also take buds, flowers, fruits, ferns, bark and fungi. They are known to eat from 70 native tree species, 20 ferns, a few vines and epiphytes, grasses, herbs, sedges, cultivated crops and flowers. The less palatable plants they leave untouched are attacked by the less choosy species, goats and red deer. Between them, possums, red deer and goats have thinned out forest understoreys, prevented regeneration and caused the collapse of tens of thousands of hectares of forest (Rogers, 1995). Some forests are more susceptible than others. Those whose canopies feature rata, kamahi, fuchsia and Hall's totara are particularly vulnerable, as are pohutukawa trees (Rose et al., 1993).

Box 9.6: The huia's royal send-off

The huia was one of the most magnificent birds to grace the forests of the North Island. By the time Europeans arrived, it was confined to parts of the Kaimanawa, Ruahine, Tararua and Rimutaka ranges (Falla et al., 1979). Quiet and curious, with its glossy green-black plumage and prized white-tipped tail feathers it was an easy target. Huia couples foraged together, the male's sharp stout beak dismembering rotten logs for insects and the female's curved slender beak reaching into the wreckage for those which escaped her partner. They were often shot together. Among the killers was Sir Walter Buller, New Zealand's foremost ornithologist. In the late 1800s collectors avidly hunted the rarest New Zealand birds to sell to foreign museums. In 1874, one expedition to the Tararua Ranges brought back 600 huia skins. The local chiefs had banned huia hunting for the previous seven years to prevent their extinction, but the collectors, both European and some Māori, were motivated by money and, in some cases, by the belief that the birds were bound to become extinct anyway so the best thing to do was get good specimens to as many museums as possible.

By the 1890s the huia had become rare enough to raise the concern of the Governor of New Zealand, the Earl of Onslow. As a result, in 1892, the Government announced a national ban on the killing of huia. As late as 1895, however, Buller was still urging collectors to get complete representative series of specimens of the rarest native birds "before it is too late." (King, 1984). The final blow came during the royal visit of 1901 when a huia feather was presented to the Duke of York (later King George V). This triggered an instant demand for huia feathers and the price of a single feather soared from one pound to five pounds (Morris and Smith, 1988). Brought to the brink of extinction by introduced predators, forest destruction, and previous hunting and collecting, the huia, like many species since, was finally unable to withstand the market forces of the fashion industry. Faltering plans to preserve huia populations on what would have been our first island sanctuaries were silenced by the collectors' guns. The last accepted sighting of a huia was on 28 December 1907. After that it was pronounced extinct.

Current pressures on biodiversity

Today about 1,000 of our known taxa are considered threatened (about 800 species and 200 subspecies). This excludes many species still unknown to science which may be threatened and also excludes known species not yet threatened but known to be declining as the pressures on them mount. These pressures are the same ones that arrived here with the first people, though their relative importance has changed over the centuries: human predation; habitat destruction; and the effects of pests and weeds.

Human predation

These days human predation is not the threat it once was, but it can still put pressure on species already threatened or declining. Examples are: the poaching and stripping of shellfish beds in some areas (e.g. around Auckland and Wellington harbours) (Weatherley, 1996); the illegal harvesting of sphagnum moss; the impacts of fishing, both on target species (e.g. orange roughy, snapper) and non-target species (e.g. sea lions, seabirds, Hector's dolphin, corals and other marine invertebrates); the potential effects of bioprospecting on valuable rare species; and the potential effect of Māori cultural harvesting on threatened species (e.g. the native pigeon, whose population is declining faster in Northland than elsewhere, having halved since 1979). This last issue has been the subject of considerable discussion and debate (Barrington, 1995; Geden and Ryan, 1995; New Zealand Conservation Authority, 1994; Moller, 1996; New Zealand Ecological Society, 1995; Taylor, 1994; Wright et al., 1995).

The fishing industry, both recreational and commercial, is the only remaining form of large-scale predation on wild animal populations. Fishing affects non-target species as well as commercial species, but little is known of these effects. Commercial fishing in New Zealand is based on a quota system. Catch levels are set by the Minister on the basis of scientific information and submissions made by industry and other interest groups.

A new form of human predation is the collection of plants and marine organisms in the search for genes and natural chemicals that may have medicinal, cosmetic or industrial uses. Most anti-cancer and antibiotic drugs are poisons, and medical researchers are constantly seeking new ones which may be more effective or more selective in their action. Plants and marine invertebrates are in particular demand because most are stationary (either rooted in soil or attached to a rock or reef) and so tend to rely heavily on natural poisons for self-defence.

Although biomedical companies have a strong interest in biodiversity preservation, their activities can pose a risk to some species, especially as most of the chemically useful ones also happen to be rare. Bioprospecting is not known to have endangered any New Zealand species but it has caused problems in other countries through the over-harvesting of rare plants and marine organisms (Anderson, 1995; Pain, 1996b). Bioprospecting in New Zealand is funded primarily by North American companies and is carried out by scientists at various research facilities, including Crown Research Institutes, such as Landcare Research and the National Institute of Water and Atmospheric Research, and several universities. The scientists are aware of the dangers to rare species and are generally careful to harvest only small samples. Back at the lab the samples are ground to a paste which is then subjected to about 60 different chemical tests.

About 40 percent of tested species yield potentially useful compounds, and about 2 percent advance to trials in the United States. A recent example is the sponge Lissendoryx (see Box 9.7). Where valuable compounds are discovered and the species that produce them are too rare to harvest commercially, production can only proceed through farming or through laboratory manufacture of the compounds (e.g. in genetically modified bacteria), allowing the natural populations to remain unmolested.

Another form of human predation, which mostly affects birds and reptiles, is collecting for the international wildlife trade. Between 20 and 50 wildlife smugglers are believed to operate here, mostly as intermediaries for the trade in foreign species, for which New Zealand is alleged to be a popular alleged to be a popular conduit (Anderson, 1997; Ansley, 1995). The wildlife trade monitoring agency, TRAFFIC Oceania, says that at least 600 birds from species listed under the Convention on Trade in Endangered Species are exported from New Zealand each year, most of them parrots and cockatoos in transit from Australia (Anderson, 1997). Worldwide the illegal bird trade has contributed to the decline of about 40 parrot species (Wildlife Conservation International, 1992). The trade poses a potential threat to New Zealand's native parrots (principally the kea, but also possibly the kaka, kakariki and nearextinct kakapo) as well as our rarer reptiles, particularly the tuatara.

Box 9.7: Sponging for drugs

Sponges of the genus Lissendoryx are known overseas as cancer fighters. They produce a substance called Halichondrin B (HB) which kills cancerous cells in laboratory mice at doses of just a millionth of a gram. Several years ago, scientists discovered a new species of Lissendoryx on rocky outcrops at depths of 100300 on the edge of the Kaikoura Canyon off the South Island's east coast. This species, nicknamed 'yellow slimy', was found to produce HB and also a new compound called Isohomo HB which is thought to have similar powers (Statistics New Zealand, 1994; Pain, 1996b). 'Yellow slimy' is one of more then 1,500 marine species that scientists have been investigating for more than a decade in the search for potentially useful drugs.

A 50 microgram extract of Isohomo HB (i.e. 50 millionths of a gram) was sent to the National Cancer Institute in Washington D.C. where it is being tested on laboratory animals before proceeding to clinical trials costing about $200 million. If successful, the discovery could put considerable pressure on our Lissendoryx species . It takes one tonne of these sponges to produce one gram of the extract, and about 5 grams would be needed to complete large clinical trials. Scientists estimate that there are only about 300 tonnes in existence. With such a low yield and 200 kg of the sponges already harvested, researchers have been investigating ways to protect the species while also ensuring a commercially useful supply. Laboratory production (e.g. through chemical means or genetically-modified bacteria) has been ruled out because the process is too costly and time-consuming. Although sponges are notoriously tricky to grow, the most promising solution has turned out to be aquaculture (marine farming). The National Institute for Water and Atmospheric Research (NIWA) has experimented successfully with growing Lissendoryx in several locations around the coast, and even has a thriving colony in Wellington Harbour, at a depth of only three metres (Statistics New Zealand, 1994; Pain, 1996b).

Habitat destruction

For most indigenous species, habitat destruction is a graver threat than human predation. Although predation targets only a few species, habitat loss indiscriminately hits all the species in an area. Recent overseas research has shown that the effect of habitat loss on biodiversity increases markedly once a critical threshold is crossed. According to American scientist, David Tilman: "The 'good news' is that if you destroyed half the Earth, you'd lose only about 10 percent of the species. But the bad news is . . . as you near 60 percent-70 percent-80 percent habitat loss, a very slight increase in destruction leads to a sharp increase in the number of species lost" (Culotta, 1994; Tilmanet al. ,1994).

The magnitude of these extinctions is masked by the fact that they do not all happen at once - some species can hang on grimly for decades, or even centuries, before finally succumbing. To date, Earth is just barely on the 'good news' side of the ledger with about 52 percent of its land ecosystems replaced or disturbed by farmland, settlements, logging, mining and roads (World Resources Institute, 1994). However, New Zealand is on the 'bad news' side of the habitat balance sheet, with our area of domesticated land now standing at 63 percent (see Chapter 8, Table 8.3) and the percentage of significantly disturbed habitat estimated at 73 percent (World Resources Institute, 1994). Habitat loss in New Zealand has occurred at three levels:

  • wholesale removal of ecosystems for conversion to farmland, exotic forests and settlements;
  • partial removal or fragmentation of ecosystems into 'islands' surrounded by farmland;
  • degradation of ecosystems through loss of species and disruption of ecological processes.

In areas where large amounts of rich lowland habitat have been removed, many of our threatened species are locally extinct. In less heavily modified areas, however, many still hang on. As a result, the more mountainous regions are where the greatest number of threatened species are found (see Figure 9.2).

Figure 9.2: Threatened species in the Department of Conservation's conservancy districts
Textual description of figure 9.2

Numbers of threatened species in Department of Conservation's conservancy districts:

  • Northland: 43 plants; 58 animals
  • Auckland: 28 pants; 42 animals
  • Waikato: 22 plants; 35 animals
  • Bay of Plenty: 16 plants; 25 animals
  • East Coast: 17 plants; 21 animals
  • Tongariro-Taupo: 13 plants; 14 animals
  • Wanganui: 28 plants; 25 animals
  • Hawkes Bay: 13 plants; 18 animals
  • Wellington: 19 plants; 34 animals
  • Nelson-Marlborough: 49 plants; 67 animals
  • West Coast: 18 plants; 48 animals
  • Canterbury: 50 plants; 57 animals
  • Otago: 42 plants; 37 animals
  • Southland: 35 plants; 79 animals

Source: Department of Conservation (1994b)

Removal of ecosystems

Habitat destruction has been extensive in New Zealand. Lowland forests, wetlands, dunelands and tussock grasslands have been largely converted to farmland (see Chapter 8). Many rivers, lakes and streams have been heavily modified by dams and drainage and irrigation schemes and by pollution from farms and towns (see Chapter 7). The widespread removal of riparian vegetation is a dramatic example of how, in building our agricultural economy, we have turned much of New Zealand into a biodiversity desert (see Chapter 7, Box 7.4).

A survey by the former Wildlife Service showed that in the 5 years between 1978 and 1983, the total area of wildlife habitat in Northland declined by 5.7 percent, or 1 percent per year (Anderson et al., 1984). Losses varied in extent with different types of habitat. Freshwater wetlands had declined by 14.4 percent, forest and scrub by 7.5 percent, and coastal estuarine habitat by 1.8 percent.

Today, most of New Zealand's surviving, relatively undisturbed habitat is either at high altitudes in the mountains, or in small lowland forest stands, shrunken wetlands and other ecological 'islands'. Some of this habitat is on real, offshore, islands. Many island ecosystems are being restored by the removal of pests and weeds and the rescue of depleted indigenous species (Towns et al., 1990). An increasing number of our most endangered species are being relocated to these islands to protect them from predation and further habitat loss (see Box 9.23).

Fragmentation of ecoystems

With so much of the lowland indigenous habitat gone and little connection between the remaining fragments, New Zealand's ecosystems offer little protection to the endemic species within themeven where formal protection exists. An increasing body of research, both here and overseas, shows that fragmented ecosystems are death traps for most species, including those that survive the actual period of habitat destruction (Askins, 1995; Burkey, 1989, 1995; Diamond, 1984; East and Williams, 1984; Kreuss and Tscharntke, 1994; Leach and Givnish, 1996; Lord and Norton, 1990; Pimmet al., 1993; Robinson et al., 1995; Wahlberg et al.,1996).

The extinctions occur for the following reasons:

  1. Fragmented habitats can only sustain small populations and these are vulnerable to what statisticians call 'stochastic processes', or chance events, such as disease outbreaks, a run of poor breeding seasons, slight increases in predation, prolonged bad weather or inbreeding.
  2. The isolation of many fragmented habitats prevents their declining populations from being replenished by new immigrants.
  3. Where fragments are small in area the species within them can never get far away from the dangerous boundary zone, which is often where the threatening predators and invasive species are most abundant.
  4. The small areas also limit resources and ecological niches so that only a limited number of species can share the habitat before crowding and competition occur.

These factors make extinction inevitable for many species, though a considerable time lag can occur before some species finally succumb. Because of their adaptability or high initial numbers, dominant species may persist in fragmented habitats for 50 to 400 years. This leads people to underestimate the full ecological costs, or 'extinction debt', incurred by habitat destruction (Tilmanet al., 1994).

The high percentage of threatened birds in New Zealand suggests they are still in the lag period following the massive habitat destruction of the past century. In their shrunken habitats the declining species are still paying off the 'extinction debt' arising from the development of our extensive farming system. Reversing this situation would require the re-establishment of wetlands, riparian vegetation and native forests on at least 10-20 percent of our farmland.

The fragmentation research has shown that, at least for forest birds, large areas of habitat are needed to cushion the effects of chance events, predation, species crowding and genetic isolation. The most viable fragments and island reserves seem to be those which are large or within migrating distance of other reserves (Hackwell and Dawson, 1980; Diamond, 1984; Burkey, 1995; Robinsonet al., 1995).

Degradation of ecosystems

Habitat size is not enough on its own, however. Even within relatively extensive areas of alpine forest and marine habitat, biodiversity is threatened by ecological degradation. This occurs when changes in the physical environment or in the composition of key species trigger a sequence of ecological changes that can lead to the loss of species.

Before humans, ecological degradation was caused solely by climate change and natural catastrophes. For example, 1,800 years after the Taupo eruption, some freshwater fish species are still almost absent from the 'ash zone' east of the volcano, even though vegetation and land animals have long since recovered (McDowall, 1995). Study of ancient river sediments has identified several periods of significant climate change in the past few thousand years where temperatures rose or fell by up to 2°C with some forests being devastated by natural fires during dry periods and some by wind-throw during wet stormy periods (Grant, 1994).

These days humans are the main agents of ecological degradation. Even where apparently intact habitat remains, we exert a range of insidious pressures on ecosystems, whose cumulative effects may be more damaging than the single impact of each pressure on its own. These pressures include:

  • the construction of roads and tracks, which provide access for people, pests and weeds;
  • industries such as farming, forestry, mining and prospecting, and hydro dam construction and smaller river obstruction work, which can disturb or even remove forest and freshwater habitat;
  • recreational activities, such as beach houses, buggies and trail bikes (which can damage duneland vegetation), tramping (which can bring alien seeds, human wastes and other forms of disturbance to heavily visited areas), sport fishing (which maintains trout in our freshwater ecosystems) and even gardening (which has introduced many of the weeds which cause problems in natural habitats);
  • drainage or hydrological disruption of wetlands, rivers and lakes;
  • dredging and bottom trawling (which can reduce the species on the sea floor by physically disturbing the bottom,and by causing high levels of suspended sediment);
  • sedimentation and pollution of rivers, streams or coastal waters from point and non-point discharges; and
  • pollution from other sources, such as air pollution, which can kill sensitive lichens.

Although they allow nature-loving humans into natural areas, roads and tracks are also a danger to natural ecosystems. They provide extensive edge zones where possums, other mammals and invasive weeds can gain entry to new habitats. Even well-intentioned human visitors can disturb sensitive micro-habitats through trampling and waste disposal.

The pressure from recreation and tourism has grown rapidly in recent years and almost amounts to a 'third human invasion' of our more popular natural parks and reserves (see Box 9.8). With a population of 3.5 million, New Zealand played host to more than 1.3 million overseas visitors in 1994-95 - 10 percent more than the previous year. This is well above the 3 percent growth rate in world tourism. Five years ago, the New Zealand Tourism Board was aiming to attract 3 million visitors by the year 2000. It has since backed off this target because of the potential impacts on visitor facilities, the tourism experience and the environment.

Figure 9.3: Tourist activities in New Zealand
Textual description of figure 9.3

Tourist activities in New Zealand

  • Shopping 75%
  • Gardens / botanical 46%
  • Museum / art gallery 46%
  • Geothermal 45%
  • Māori cultural performance 36%
  • Historic site 36%
  • Glow worm caves 35%
  • Bushwalk (half hour to half day) 29%
  • Wildlife park / zoo 31%
  • Scenic boat cruise 30%
  • Milford sound 29%
  • Other wildlife in natural setting 29%
  • Short bushwalk (less than half hour) 29%
  • Gondola ride 27%
  • Mt Cook 25%
  • Farm show 25%
  • Other farm attraction 25%
  • Bar / nightclub 25%
  • Fox / Franz Joseph Glacier Valleys 25%
  • Jetboating 19%
  • Other Māori experience 18%
  • Experience centre 17%
  • Scenic flight 16%
  • Cinema 15%
  • Dolphin watching 14%
  • Live theatre / concert 13%
  • Wine tasting 13%
  • Long bushwalk (half to whole day) 12%
  • Mountain climbing / caving 9%
  • Freshwater fishing 8%
  • Whale watching 8%
  • Water sports other than fishing, rafting, jetboating 8%
  • Casino 8%
  • Cape Reinga 7%
  • Bungy jumping 7%
  • Rafting 7%
  • Snow sliding 7%
  • Trekking / tramping 7%
  • Horse riding 7%
  • Sailing 6%
  • Golf 6%
  • Sea fishing 6%
  • Cycling 5%
  • Air sports 4%
  • Off road mountain biking 3%
  • Heli-skiing 1%

Source: New Zealand Tourism Board (1996b)

Box 9.8: Tourism growth and the environment

World tourism has boomed in recent decades as international travel has become easier and global income has become more unevenly distributed among the world's rich and poor. New Zealand's tourism growth has been much greater than the world average, partly because of a favourable exchange rate which made us a relatively cheap destination, and partly because of the aggressive marketing of our 'clean, green' image. The first time more than 1 million overseas visitors arrived was in 1992. By June 1996, the number had reached nearly 1.5 million. More than half these (823,178) listed 'holiday or vacation' as the reason for their visit (Statistics New Zealand, 1996). Just over 2 million visitors are expected in the year 2000 (New Zealand Tourism Board, 1996a).

Since the 1980s there has been a switch from pre-arranged package tours, in which visitor impacts were confined to major routes, to what is known as 'free and independent travel' (FIT) holidays where people drive campervans and rental cars wherever the spirit moves them (O'Neill and Kearsley, 1993). Even more recent developments include adventure tourism (bungy jumping, white water rafting), and 'eco' or 'green' tourism (Warren and Taylor, 1994). In 1993, more than half our overseas visitors went to at least one of our national parks. The most popular attractions were Milford Sound in the Fiordland National Park, the Whakapapa Skifield, Rotorua's Whakarewarewa, Taupo's Huka Falls, the Waitomo Caves and Mt Cook. Each of these gets 250-500,000 visitors per year, with 6070 percent from overseas (Ministry of Commerce, 1994).

The rapid increase in visitors during the past decade has raised concerns about environmental effects and sustainability (Hall, 1994; Lincoln Outlook, 1995; Sage, 1995; Sowman, 1994). In 1995, the New Zealand Conservation Authority asked its regional boards to identify sites where visitor activities were thought to be having a detrimental effect (see Table 9.2). The resulting list of 60 sites is not necessarily representative, or even accurate, being based on perceptions rather than systematic monitoring, but it identifies overcrowding as a recurrent seasonal problem each summer. Overcrowding lowers the quality of visitors' experiences and intensifies pressure on facilities such as parking, space in huts, and toilets. Fouling of ground and streams, including water supplies, by human waste was seen as a nationwide problem at huts, bivouacs, camping areas, reserves and (especially) roadside areas (New Zealand Conservation Authority, 1995). Other visitor effects can include: habitat destruction and wildlife disturbance (particularly at nesting sites) by off-road vehicles, jetskis, horses, dogs and guided tours; increased risk of fires and new weed introductions ; vandalism and souveniring at historic sites; vegetation clearance for campsite firewood; track deterioration ; noise; and visual pollution (Department of Conservation, 1996; New Zealand Conservation Authority, 1995). Visitor impacts are greatest in fragile landscapes such as sand dunes and subalpine areas.

Although problems with visitor numbers have started to show at some tourist destinations, the effects are still relatively limited when compared with the widespread devastation caused by introduced animal pests.

At current visitor levels, the pressures can be controlled by careful planning and management. For example, several popular walking tracks (e.g. the Milford and Routeburn tracks in Fiordland) have booking systems to limit visitor pressure. Responsibility for tourism planning and impact management falls to regional councils and the Department of Conservation. The latter's Conservation Management Strategies and Plans try to balance the public's free access to protected areas (as allowed in the Conservation Act, National Parks Act and Reserves Act) against efforts to protect indigenous species, ecosystems and historic sites (as required by various laws, including the Wildlife Act). At a broader level, the Tourism Policy Group of the Ministry of Commerce is responsible for developing our national policy on tourism, while the New Zealand Tourism Board is responsible for the international marketing of New Zealand as a tourist destination.

Table 9.2: Some environmental pressures at 60 important visitor sites
Pressures Sites affected
Overcrowding 27
  • general
(14)
  • campgrounds
(1)
  • waterways (boat use)
(2)
  • huts
(3)
  • hot springs
(1)
  • parking facilities
(4)
  • fresh water supplies
(1)
  • cooking facilities
(1)
Waste Disposal 17
  • toilet facilities
(6)
  • waste disposal facilities
(6)
  • sewage pollution
(4)
  • contaminated fresh water
(1)
Facility Degradation 11
  • campground degradation
(1)
  • access road deterioration
(1)
  • track deterioration
(7)
  • forest depletion for firewood
(2)
Wildlife Disturbance 4
  • birds
(1)
  • wildlife (general)
(3)
  • dogs
(1)
Environmental Degradation 13
  • damage to vegetation
(5)
  • sand dune damage
(1)
  • rock damage
(1)
  • water scouring
(2)
  • pugging
(3)
  • fire risk
(1)
Recreational Vehicles 5
  • offroad vehicles
(1)
  • jetboats
(1)
  • jetskis
(1)
  • mountain bikes
(2)

Source: New Zealand Conservation Authority (1995)

About 55 percent of overseas visitors went to at least one national park, with the average park visitor making three such trips. The two most popular activities are bush walks and scenic boat cruises. In addition to the overseas tourists, many domestic New Zealand tourists also visit national parks. In all, about 2 million people are recorded at Department of Conservation visitor centres each year. The number has increased yearly by tens of thousands. These visitors can place considerable pressure on some areas. They also place enormous pressure on the limited resources of the Department of Conservation, a quarter of whose budget is devoted to tourist servicing. More than 600 private recreation and tourism operators have concessions to operate on conservation land, some on a trial basis, and each year the Department receives more than 300 new applications for concessions.

More visitors means more roadworks, more tracks and track maintenance, and more and bigger camp sites, accommodation and service facilities. It also means more crowding, trampling, sewage and waste, and weed invasions, particularly in small reserves, near roads and along the most popular walking tracks (e.g. Rakiura on Stewart Island, Milford, Kepler, Routeburn, Heaphy, Abel Tasman, Waikaremoana and Tongariro). When one area becomes degraded, visitors tend to seek more pristine areas, thus widening the impact zone.

Visitor impacts also occur at marine reserves. New Zealand's oldest marine reserve, the Cape Rodney-Okakari Point Marine Reserve at Leigh north of Auckland, gets over 100,000 visitors a year. Recent research indicates that the trampling feet of visitors as they walk over a nearby reef platform to observe the inter-tidal marine life can alter the structure of ecological communities (Brown and Creese, in press). Similarly, heavy visitor numbers at seal rookeries can drive the seals to more distant hauling out areas, such as offshore rocks.

Until recently, logging for timber in native forests has caused severe habitat disturbance. From mid-1996 the indigenous provisions (Part IIIA) of the Forests Act 1949 requires most timber logging in native forests to use low-impact techniques (such as helicopter extraction rather than heavy-duty ground-based equipment). Concern has been expressed, however, that even coupe felling and selective single-tree logging of podocarps, such as rimu, could affect some threatened species, such as the pigeon, bellbird and tui (Spurr et al., 1992; Warburton et al., 1992). Coupe and selective logging of podocarps generally target the largest old trees, which also happen to be the prime habitat for these species. Mining and prospecting in natural areas can also disturb vegetation and wildlife. Several hundred licences are current on conservation land. Although efforts are made to minimise site impacts, their cumulative effect is difficult to gauge.

The effects of hydro development range from flooding of land-based ecosystems to disturbing the flow of aquatic ecosystems. Across the country, the construction of smaller floodgates, weirs, tidegates and dams, generally associated with flood control, drainage and irrigation, have had significant cumulative effects on indigenous freshwater fish stocks causing local extinction and population fragmentation. Flow disturbances caused by heavy river floods are known to dramatically reduce the number and abundance of fish species in a river - at least in the short-term (McDowall, 1993). Flow variations can reduce habitat and feeding areas, increase water temperatures and silt up gravel spawning areas.

Fish are also affected by the physical barrier which a dam presents to their movement (Irvine and Jowett, 1987). This can have a fatal impact on upstream populations - and not necessarily the migratory ones. A case in point was reported by McDowall and Allibone (1994). They found that a species of non-migratory native fish, the common river galaxia (Galaxia vulgaris), has been eliminated in the waters above the Lake Mahinerangi dam in Otago. This species apparently succumbed to competition from a larger migratory galaxiid, the koaro (Galaxia brevipinnis ), which became trapped by the dam in 1923 and was apparently thrown into ecological competition with its smaller cousins.

In marine habitats, pressures from trawling and dredging also reduce species numbers. A recent review of research on trawling impacts found that repeated dragging of heavy fishing gear across the sea floor can cause permanent changes to sea-floor biodiversity by scraping and ploughing the sea-bed, creating sediment clouds, killing sea-floor organisms (e.g. crustaceans, molluscs, sponges, corals, kina) and dumping processing waste (Jones, 1992).

The Department of Conservation commissioned a three-month study by the National Institute of Water and Atmosphere Research (NIWA) to investigate the effect of scallop dredging on marine mud-dwelling species (Thrushet al., 1993). The researchers found that, in general, after only one dredging episode, the number and variety of bottom dwelling animals was reduced and remained low three months later. They concluded that repeated dredging over large areas is likely to result in reduced diversity in sea-floor communities.

In deeper water (below 1,000 m), where the animals are less adapted to sediment clouds and storm disturbance, recovery of sea floor communities probably takes decades once trawling has stopped (Jones, 1992). On seamounts, recovery of coral thickets and their associated ecosystems will probably take centuries (see Chapter 7, Box 7.3).

The commercial alternative to shellfish dredging is aquaculture. Oyster and mussel farms are well established in many bays in New Zealand and are visible as rows of floating oyster racks or rows of buoys supporting mussel long-lines. While most of the environmental objections to fish farms are based on the water space they occupy and their impact on the scenery, they can also have less visible impacts. Large amounts of faecal and other wastes accumulate on the seafloor beneath them and these can pollute water or deplete oxygen levels in the sediment (Forrest, 1994).

The few New Zealand studies to date have detected oxygen depletion but no significant water impacts. The oxygen is depleted by micro-organisms as they break down the wastes. In severe cases, this can make the sea floor uninhabitable to many animals, resulting in a 'black zone' that exudes the 'rotten egg' smell of hydrogen sulphide. Fortunately, the effect is limited to the farmed area and the few metres beyond its perimeter. Some of the ecological changes caused by fish farms are more pleasant. They provide areas for bird roosting, artificial reefs for other marine organisms, and food for opportunistic sea floor scavengers. They can also attract fish from outside the farm area, such as flounder around oyster farms and snapper around mussel farms (Forrest, 1994).

Another source of pressure on the sea floor ecosystems is the dumping of fish processing waste. For example, about 50,000 tonnes of hoki offal are dumped into the sea each year by boats fishing on the continental slope off the West Coast of the South Island, raising concerns that the decomposing waste could locally deplete oxygen levels (Livingston and Rutherford, 1988). A preliminary assessment confirmed that enough waste reaches the sea floor to alter the species composition (Grange, 1993).

Pests and weeds

Even when human disturbance of an ecosystem seems slight at face value, its impact often comes on top of far more serious disturbances by other species. Alien plants and animals have turned many of our protected areas into war zones. When not being smothered or overshadowed by exotic weeds, our native plants are eaten by browsing and grazing animals, such as goats, deer, tahr, cattle, sheep, wallabies, rabbits and, above all, possums. Without pest control programmes, an estimated third of our protected forests (1.8 million hectares) would suffer significant biodiversity losses from browsing animals (see Chapter 8). At present, the Department is holding the browsers at bay over about 1.3 million hectares.

Meanwhile birds, reptiles, frogs and the larger invertebrates fall prey to mammalian predators (e.g. stoats, rats and cats) and several aquatic species appear to have been displaced by trout (see Box 9.9). Among the introduced birds, Australian magpies and Asian mynas have acquired reputations as aggressive competitors with native birds, attacking them and preventing them from nesting within their territories. A recent study of the impact of mynas found that, after their removal from an area, the number of tuis and other native birds increased (Drent, 1996; Tindall, forthcoming). Introduced mallard ducks compete and sometimes breed with native grey ducks. Blackbirds, song thrushes and little owls prey on native invertebrates, including the threatened Cromwell chafer beetle.

The spread of introduced wasps in recent decades coincides with the rapid disappearance of a threatened bird, the yellowhead, from most of the northern South Island. The wasps may be compounding the intense predatory pressure from stoats (Davidson, 1992; Elliott, 1992). Wasps can kill nesting birds and are known to compete with them for insects and honeydew (Moller et al., 1993). The removal of honeydew may also be excluding the threatened kaka parrot from honeydew beech forests (Beggs and Wilson, 1991; Molleret al., 1993). Other wasp victims include native moths and butterflies, which are preyed on heavily. The impact on their populations is unknown but may be significant (Thomas et al., 1990).

Table 9.3: Key mammals which have reduced New Zealand's biodiversity
Species Population Status
Humans (Homo sapiens) 3.6 million (1996) Population growing at 1.5% per year (1990-95).
Economy (GDP) growing at 2.2% per year (1990-95)
Energy use growing at 2% per year (1990-95)
Sheep (Ovis aries) 48.8 million (1995) Peaked at 70.3 million (1982). Steadily declining.
Cattle (Bos taurus, B. indicus) 9.3 million (1995) At record level. (Averaged 8 million through the 1980s)
Horses (Equus caballus) Domestic: 40,000 (1981) Peaked at 400,000 (1921).
Feral: Kaimanawa ca. 1,800 Northland ca. 500

Feral populations can damage native plants so are periodically culled in conservation areas.

Rabbits (Oryctolagus cuniculus) Tens of millions (1995). Occupy 56% (15 million hectares) of the land area. Pose a high to extreme risk to pasture over 1 million hectares of South Island high country where they are prone to dramatic population explosions.
Goats (Capra hircus) Farm: 337,000 (1995) Farm goats peaked at 1.3 million (1988).
Feral: 300,000-1 million Feral goats were reduced by helicopter shooting in the 197080s, but increased during the farming downturn. They occupy 3 million hectares, two-thirds of it DOC land.
Tahr (Hemitragus jemlahicus) 10,000-14,000 (1994) Peaked at 60,000 (1970s).Were reduced by helicopter shooting to about 6,000 (1983).
Deer (Cervus spp. andDama dama) Farm: 1.8 million (1995) Farm deer still increasing.
Feral: 250,000 (1993) Wild (mostly Red) deer peaked in 197075, and are now controlled by hunting.
Pigs (Sus scrofa) Farm: 431,000 (1995) Farm pigs peaked at 771,000 (1964).
Feral: at least 300,000 About 100,000 feral pigs are killed annually. Problem areas are Northland, Nelson/Marlborough, the Chatham Islands and Auckland Island.
Possums (Trichosurus vulpecula) 70 million (1993) Occupy more than 90% of the country, still spreading, and subject to widespread control operations.
Mustelids (Mustela spp.)   Absent from Stewart and Chatham Islands.
Stoats (M. erminea) Possibly millions Stoats are common in forests, including Fiordland beech forests.
Ferrets (M. putorius) Possibly millions Ferrets are common in open country where rabbits, their main prey, are abundant.
Weasels (M. nivalis) Probably thousands Weasels are uncommon
Rats (Rattus spp.)
Ship rats (R. rattus) Tens of millions Ship rats are common in forests, especially podocarp-hardwoods.
Norway rats (R. norvegicus) Tens of millions Norway rats peaked before stoats arrived and are now limited to towns, farms, water margins and islands.
Pacific rats (kiore) (R. exulans) Tens of thousands Pacific rats are now limited to Fiordland and about 50 islands.
Cats (Felis catus) Pets: ca 770,000 (1991-92) Almost half the nation's homes have pet cats.
Feral: Possibly millions Feral cats are widespread. Population trends unknown.
Dogs (Canis familiaris) Pets: ca 398,000 (1991-92) Around 29% of homes have pet dogs and at least a third of the nation's farms have 1 or more teams of working dogs.
Farm: 150-300,000 (1992)
Feral: Insignificant

Dog population trends are unknown.

Because pest and weed control is such a large problem, much of New Zealand's species preservation effort is concentrated on island sanctuaries where it is easier to maintain pest-free habitats. Despite the problems of limited habitat size, some heroic rescues have been made (see Box 9.23). Unfortunately, the small size of most islands limits the number and variety of threatened species that can be sustained on them. In effect, the islands are similar to fragmented reserves on the mainland in which small isolated populations are vulnerable to chance events. The one threat that most island sanctuaries do not have, however, is alien species - and that can make the difference between a successful recovery programme and an unsuccessful one (Towns et al., 1990).

On some islands, however, the threat is not from alien species, but from a transplanted endemic one. The weka or woodhen (Gallirallus australis) is a large flightless rail whose nearest relatives are on Lord Howe Island and New Caledonia. Wekas run quickly, swim readily, and are capable killers of rats and stoats. They are inquisitive, aggressive and omnivorous. Their fearlessness made them easy prey for Māori hunters. By the time Cook arrived, wekas had become rare in many areas and only remained abundant in isolated regions, such as the South Island's West Coast and the North Island's East Cape (Falla et al., 1979; King, 1984).

Museum collectors, cats and ferrets reduced the remnant populations in less isolated areas.

They had disappeared from Taranaki by 1920, from Canterbury by 1925 and from large parts of Northland by 1940. However, some wekas had an unlikely saviour. Mutton-birders (seabird hunters) took them to several offshore islands for food supplies. The wekas prospered on the predator-free islands by preying on endemic island birds and invertebrates to the point of wiping some out. As a result, the weka now bears the unusual distinction of being listed by the Department of Conservation as both a threatened species and a pest.

More than 25,000 plant species have been introduced to New Zealand. Of these, nearly 2,000 are now established in the wild (Halloy, 1995; Timmins and Williams, 1991). In fact, exotic species of vascular plants may now outnumber natives in the wild (see Table 9.1). Every year several hundred more arrive and half a dozen or so take root in forest margins, waterways and road verges. In urban Auckland, four new species go wild each year. More than 200 of these exotics now have the potential to displace native plant ecosystems (see Table 9.4).

Pine trees and pasture grasses, the mainstays of our primary economy, are probably the most widespread introduced species, having altered ecosystems so drastically that, in some places, we have lost any vestige of native New Zealand. Other abundant intruders include willows, which can clog rivers, ginger, which has invaded forest edges, Hieracium (or hawkweed), which has over-run large areas of South Island high country, and various weeds which have invaded lakes and estuaries. At Pukepuke lagoon near Levin, for instance, two-thirds (120) of the 176 identified plants are introduced species (Ogden and Caithness, 1982).

Only a few Northland lakes, some isolated South Island lakes, and the Chatham Islands may have no introduced aquatic plants (Howard-Williams and Davies, 1988). One lake invader is the waterweed Lagarosiphon major, which was introduced from southern Africa about 1950. By 1957 it had reached nuisance proportions in Lake Rotorua and by 1966 had reached Lake Taupo. By 1980 it had colonised all potential sites in Lake Taupo and had displaced native water plants over large areas of the shoreline (Howard-Williams and Davies, 1988).

Other introduced plants, such as old man's beard, pose an increasing threat to native forests. A study of 234 small to medium lowland forest reserves (500 hectares or less) found that invasive plants are more common in reserves that are close to towns, rubbish dumps, roads and farms. The alien plants are also more common in reserves which have a high number of human visitors or a large perimeter-to-area ratio (Timmins and Williams, 1991).

Box 9.9: Routed by trout

Brown trout (Salmo trutta) were introduced from their native Europe in 1867 and rainbow trout (Oncorhynchus mykiss) were brought in from North America in 1883. The sports anglers and acclimatisation societies that imported them gave little thought to the effects on native fish, 10 of which are now threatened and one of which is extinct. They thought even less about the effects on other species such as the threatened blue duck (Hymenolaimus malacorhynchos) and the freshwater crayfish ( Paranephrops zealandicusand P. planifrons). Trout now form the basis of a thriving recreational industry involving several hundred thousand people and millions of tourist dollars a year. An estimated 45,000 tourists come here each year to catch trout, and the number is expected to grow (Mortimer, 1994). With their close relatives, European and American salmon ( Salmospp., and Oncorhynchusspp.), trout are the only introduced animals singled out for habitat protection in the Resource Management Act 1991 (Part II, Section 7 (h)).

Trout are highly aggressive, particularly brown trout, both as predators and as competitors for food and territory. Our native fish evolved without the need to defend themselves from such able enemies. Ill-equipped to resist the invaders, many were eaten, while others were robbed of their food and forced out of their territories. Several studies have shown that brown trout compete with some native species, such as various galaxiids ( Galaxiasspp.) and bullies (Gobiomorphus spp.), for the same foods, particularly mayfly nymphs (Hopkins, 1965; Cadwallader, 1975a, 1975b, 1975c; Edge et al., 1992). Galaxiids are hardest hit because they and the trout appear to eat the same food at the same time. In contrast, eels ( Anguillaspp.) seem to coexist with trout, despite their similar diets, because their feeding habits differ in manner, location and timing (Burnet, 1969).

Trout may also compete with blue ducks for food, perhaps influencing the ducks' current distribution and preventing their re-establishment in many areas. Experiments have shown that trout and blue ducks eat the same insect species, and that insects and blue ducks are less abundant in troutinhabited waters (Towers, 1996). However, the two species have different feeding behaviour which makes it difficult to assess how much competition, if any, is actually occurring. Blue ducks prefer smaller insects eaten from the stream bottom while trout prefer larger ones caught in-stream or on the surface. While competition is difficult to observe directly, predation is much more straightforward. Trout kill a variety of native species. Two of these are crustaceansthe freshwater crayfish. Though not listed as threatened, crayfish have been reduced to such an extent that it is difficult to find them in areas where trout are present (McDowall, 1968). One species has been eliminated by trout from at least one North Island lake (Waingata) (Fish, 1966). The crayfish are highly vulnerable because they cannot detect trout easily. Whereas they can sense chemicals in the mucus of approaching eels, crayfish are 'chemically blind' to trout (Shave et al., 1994).

Trout predation has had an even greater effect on some native fish, including at least three threatened species: the koaro (Galaxias brevipinnis), the dwarf inanga (Galaxias gracilis) and the Canterbury mudfish (Neochanna burrowsius). Very young galaxiids are especially vulnerable. Up to 135 may be eaten by one juvenile trout in a day (Moller et al., 1993). Vast quantities of koaro were eaten when trout were introduced to our lakes (McDowall, 1987). They were so reduced in some North Island lakes that, by the 1920s, the trout themselves were declining too. Native smelt were quickly introduced to some lakes as substitute trout food, but they also preyed on the koaro, hastening their decline (Rowe, 1993). Trout impacts on native fish seem greater in lakes than in rivers and streams. This could be because upstream populations of native fish can take refuge behind waterfalls and dams, or it could be because streams are harder to survey accurately than lakes (Moller et al., 1993).

Today, trout rule our waterways while blue ducks, crayfish and several native fish are confined to backwaters and marginal areas. Trout cannot be blamed entirely for this, but their presence has often been decisive. This was shown in a study which directly compared the effects of land use and trout on one galaxiid species,Galaxias vulgaris, whose distribution in the Taieri River is extremely fragmented. The researchers concluded that, although land use had affected the fish populations, the only factor which explained their extreme patchiness was the presence or absence of brown trout (Townsend and Crowl, 1991; Moller et al., 1993).

Box 9.10: The spineless invaders

About 2,000 alien invertebrates have reached New Zealand by a variety of routes. Many came buried in the hair or guts of our domestic animals. Many came hidden in plants, containers, trouser cuffs or hats. Some were deliberately introduced. Total numbers are unknown but the invaders include more than 1,000 insects, perhaps 600 nematode worms (about half of them parasitic roundworms), about 75 platyhelminths (e.g. terretrial flatworms and parasitic tape worms), about 60 arachnids (spiders and mites) and about 40 molluscs, including about 13 land snails and 13 land slugs. No systematic assessment of their effect on biodiversity has been made, though their impact on agricultural production has been closely studied. Many appear to have had no effect. Some of the introduced earthworms, for example, have been beneficial for the soil, filling a niche left vacant by the 178 native earthworm species which retreated when their habitat was deforested. About 25 introduced earthworms may be living here, though only 17 have been identified (Yeates, 1991).

Other immigrants have been less benign. Some snails and slugs are suspected of competing with, and perhaps even preying on, native snails and slugs. A recently arrived South African spider (Steatoda capensis) is systematically decimating one of New Zealand's few feared wild animalsthe katipo spider (Lactrodectus katipo). In the past 10 years the katipo, which is an endemic species, has disappeared from many North Island beaches. Although it is a superior fighter, the katipo does not breed fast enough to keep pace with its alien competitor. Nor is it so adaptable and so quick at recolonising disturbed territory. The South African spider is able to live in vegetation above the beach and quickly colonise former katipo territory after natural or human disturbance (Faulls, 1991; Daugherty et al., 1993).

Insects provide examples of both good and bad aliens. A large number are agricultural pests, while a smaller number are biocontrol agents introduced deliberately to prey on or parasitise the pests (P.J. Cameron et al., 1989, 1993; Harman et al., 1996). Of the 321 biocontrol insects introduced since 1874, about 70 have become established, but only one, a tachinid fly (Trigonospila brevifacies) is known to parasitise harmless native species - in this case, moths (Atkinson and Cameron, 1993; Cameron, 1994). In general, alien invertebrates have tended not to invade native forest and so have not come into direct conflict with most native species. An example of this may be the Australian passion vine hopper (Scolypopa australis) which has recently been suspected of infecting cabbage trees on farms and roadsides while those in forests remain healthy (Brockie, 1995).

However, some insects do invade forests. The German and common European wasps (Vespula germanicus and V. vulgaris) became established here in 1944 and 1978 respectively and are now widespread in New Zealand forests (Moller et al., 1993). In southern beech forests their biomass in infested areas can exceed the combined biomass of birds, rats, mice, stoats and ferrets, and in podocarp forests, nests as large as 14 cubic metres have been found. Apart from posing a hazard to people visiting the forests, the wasps appear to compete with native wasps and bees, prey on the larvae of native moths, butterflies and other insects, compete with birds for honeydew and insects, and may be accelerating the decline of the yellowhead and kaka (Elliott, 1992; Moller et al., 1993; Thomas et al., 1990). An even more recent wasp invader is the Asian paper wasp (Polistes chinensis) a close relative of the Australian paper wasp (P. humilis) which was the first foreign wasp to arrive in New Zealand .An attempt is being made to control the alien wasps with an introduced biocontrol species - fittingly enough, a tiny parasitoid wasp called Sphecophaga vesparum burra (Beggs et al., 1992).

Box 9.11: Stemming the plant invaders

To combat the plant invasion, both legal and voluntary measures have been adopted. Under the Biosecurity Act 1993, the Ministry of Agriculture is drawing up a list of plants whose sale or importation will be prohibited from June 1996. In addition, the Royal Forest and Bird Protection Society, the New Zealand Institute of Noxious Plants Officers and the New Zealand Nurserymens Association have agreed on a list of plants which should not be sold by garden centres because of their harmful effects on native forests (Craw, 1994). Participating garden centres receive a 'Forest Friendly' award.

The Department of Conservation also maintains a database of introduced plant species the Department considers as weeds on the land it administers (see Table 9.4). The list of harmful plants includes African club moss, periwinkle, wandering willie and Chinese ladder fern (also known as Boston fern, ladder fern and tuber sword fern), all of which cover the forest floor, preventing seedling regrowth. It also includes climbing asparagus and its close relative, smilax, which climb into the sub-canopy layers, ringbarking trees and killing them. Shade-tolerant trees and shrubs are also listed - cotoneaster, the privets, evergreen (or Italian ) buckthorn and the Japanese spindle tree. Others on the list are: boneseed(an aggressive invader of the coast and offshore islands); convulvulusorbindweed(a white-flowered vine which infests roadside verges and invades hedges, trees and even gardens); lantana (which is not only a problem around the upper North Island, but also one of the worst weeds of the entire Pacific region, invading coastlines, islands and forest edges); pampas grass(which replaces regenerating forest and native dune plants); and Mexicandaisy (which is one of the most popular garden plants but invades bluffs, cliffs, stream edges, scrub, and forest margins, replacing rare small shrubs).

A further list maintained by the Department, identifies introduced plant species still to be added to the first database, or those identified by botanists as potential ecological weeds. Among the plants on this list are Chinese lantern ,banksia, Port Jackson fig, blue gum and various species of plectranthus. A third list maintained by the Department names species which are singled out in various operational plans, but which botanists consider as not warranting inclusion on the 'weeds' database. Some of the plants on this list are fennel, buttercup, ice plant,privet, g ladiolus and kiwifruit .

Vines are also prominent on the list, though most of these are a problem only north of Taupo. They include: moth (kapok) plant, banana passionfruit, eleagnus ,jasmine, mignonette (ormadiera vine),blue morning glory, German ivy, Japanese honeysuckleand mile-a-minute. South of Taupo the following additional species are listed: cathedral bells ,buddleia, heather, Cape ivy, Himalayan honeysuckle, aluminium plant, and, in the lower South Island, flowering currant. A recent addition to the list isHouttuynia cordata, a colourful ground cover plant with attractive heart-shaped leaves which has recently been sold in some garden centres. It spreads even more vigourously than wandering willie, is self-seeding, and can grow in conditions ranging from full sunlight to dark shade. It covers the ground with a dense carpet of leaves which smother seedlings and other forms of vegetation.

Table 9.4: Alien plants which are considered weeds on conservation lands in New Zealand
Common name Taxonomic name
African club moss (see selaginella)
African feather grass Pennisetum macrourum
African fountain grass Pennisetum setaceum
African olive Olea europeae subsp. cuspidata
agapanthus Agapanthus praecox
alder Alnus glutinosa
alligator weed Alternanthera philoxeroides
American spartina Spartina alterniflora
apple of Sodom Solanum linnaeanum
aristea Aristea ecklonii
arum lily Zantedeschia aethiopica
Australian sedge Carex longebrachiata
barberry Berberis glaucocarpa
barberry, Darwin's Berberis darwinii
bindweed / field bindweed (see convolvulus)
blackberry Rubus fruticosus agg.
blueberry / highbush blueberry Vaccinium corymbosa
bone seed Chrysanthemoides monilifera
boxthorn Lycium ferocissimum
broom Cytisus scoparius
browntop Agrostis capillaris
brush cherry Syzygium australe
buddleia Buddleja davidii
buttercup bush Senna septemtrionalis
Cape honey flower Melianthus major
Cape ivy Senecio angulatus
cathedral bells Cobaea scandens
cheatgrass Bromus tectorum
Chilean flame creeper Trapaeolum speciosum
Chilean needlegrass Stipa neesiana
Chinese ladder fern / tuber sword fern Nephrolepis cordifolia
clematis Clematis flammula
climbing asparagus Asparagus scandens
climbing dock Rumex sagittatus
climbing spindleberry Celastrus orbiculatus
cocksfoot Dactylis glomerata
coltsfoot Tussilago farfara
convolvulus / bindweed / field bindweed Convolvulus arvensis
cotoneaster Cotoneaster glaucophyllus
crack willow Salix fragilis
Douglas fir Pseudotsuga menziesii
egeria / oxygen weed Egeria densa
elaeagnus Elaeagnus x reflexa
elder / elderberry Sambucus nigra
elephant's ear Alocasia brisbanensis
evergreen buckthorn/ Italian buckthorn Rhamnus alaternus
floating sweetgrass Glyceria fluitans
German ivy Senecio mikanioides
giant reed Arundo donax
ginger (kahili) Hedychium gardnerianum
ginger (yellow) Hedychium flavescens
gorse Ulex europaeus
grey willow Salix cinerea
hakea, downy Hakea, gibbosa
hakea, prickly Hakea, sericea
hakea, willow-leaved Hakea, salicifolia
hawkweed Hieracium spp.
hawthorn Crataegus monogyna
heath, Spanish Erica lusitanica
heather Calluna vulgaris
honeysuckle - Himalayan Leycesteria formosa
honeysuckle - Japanese Lonicera japonica
hops Humulus lupulus
horsetail Equisetum arvense
hydrilla Hydrilla verticillata
inkweed Phytolacca octandra
Italian buckthorn (see evergreen buckthorn)
Italian lily / Italian arum Arum italicum
ivy Hedera helix
Japanese walnut Juglans ailantifolia
jasmine Jasminium polyanthum
jasmine, yellow Jasminum humile
Jerusalem cherry Solanum pseudocapsicum
kangaroo acacia Racosperma paradoxum
Khasia berry Cotoneaster simonsii
Kikuyu grass Pennisetum clandestinum
knotweed, giant Reynoutria sachalinensis
knotweed, Japanese Reynoutria japonica
lagarosiphon Lagarosiphon major
lantana Lantana camara var. aculeata
loquat Eriobotrya japonica
lotus Lotus pedunculatus
lupin, Russell Lupinus polyphyllus
lupin, tree Lupinus arboreus
madeira vine Anredera cordifolia
Manchurian rice grass Zizania latifolia
marram grass Ammophila arenaria
Mercer grass Paspalum distichum
Mexican daisy Erigeron karvinskianus
Mexican devil Ageratina adenophora
mile-a-minute Dipogon lignosus
mist flower Ageratina riparia
monkey apple Acmena smithii
monkey musk Mimulus guttatus
montbretia Crocosmia x crocosmiiflora
Montpellier broom Teline monspessulana
moth (kapok) plant Araujia sericifera
Mysore thorn Caesalpinia decapetala
nassella tussock Stipa trichotoma
nasturtium Tropaeolum majus
old man's beard Clematis vitalba
onion weed Allium triquetrum
orange cestrum Cestrum aurantiacum
orange firethorn Pyracantha angustifolia
oxygen weed (see erigea)
oxylobium Oxylobium lanceolatum
palmgrass Setaria palmifolia
pampas grass

Cortaderia selloana

pampas, purple Cortaderia jubata
parrot's feather Myriophyllum aquaticum
passionfruit - banana Passiflora mollissima
passionfruit - black

Passiflora edulis

passionfruit - northern banana

Passiflora mixta

perennial ryegrass Lolium perenne
periwinkle Vinca major
pine, cluster / maritime Pinus pinaster
pine, dally Psoralea pinnata
pine, lodgepole Pinus contorta
pine, wilding Pinus spp.
privet, Chinese Ligustrum sinense
privet, tree

Ligustrum lucidum

poplar, white Populus alba
potato vine Solanum jasminoides
pyp grass Ehrharta villosa
ragwort Senecio jacobaea
red cestrum

Cestrum elegans

rowan Sorbus aucuparia
royal fern Osmunda regalis
rush, bulbous Juncus, bulbosus
rush, heath Juncus squarrosus
rush, jointed Juncus articulatus
rush, sharp Juncus acutus
rush, soft Juncus effusus
salvinia / water fern Salvinia molesta
selaginella Selaginella kraussiana
Senegal tea Gymnocoronis spilanthoides
smilax

Asparagus asparagoides

spartina Spartina x townsendii
spartina hybrid Spartina anglica
spindleberry

Euonymus europaeus

spindleberry, Japanese Euonymus japonicus
St John's wort

Hypericum perforatum

stinking iris Iris foetidissima
stonecrop Sedum acre
sweet briar Rosa rubiginosa
sweet cherry / wild cherry Prunus avium
sweet pea bush Polygala myrtifolia
sword fern (see Chinese ladder fern)
sycamore Acer pseudoplatanus
tall fescue Festuca arundinacea
tall oatgrass Arrhenatherum elatius
thistles

Cirsium spp.

thistle nodding Carduus nutans
tuber sword fern (see Chinese ladder fern)
tutsan

Hypericum androsaemum

veld grass

Ehrharta erecta

viper's bugloss Echium vulgare
wandering willie Tradescantia fluminensis
water fern (see salvinia)
water net

Hydrodictyon reticulatum

watsonia Watsonia bulbillifera
wattle, brush Paraserianthes lophantha
wattle, silver Racosperma dealbatum
wattle, Sydney golden Racosperma longifolium
wild cherry (see sweet cherry)
wonga wonga vine Pandorea pandorana
woolly nightshade

Solanum mauritianum

yellow flag iris Iris pseudacorus

Source: Department of Conservation (1997)