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Human effects on the environment

by  Janet Wilmshurst

Isolated for millions of years, New Zealand’s plants and animals were very vulnerable to the impact of humans. When the ancestors of Māori arrived around 1250–1300 AD, bringing rats and dogs, they started a wave of extinctions that continues today.


Early human impact

Like many groups of islands in the Pacific, New Zealand was very sensitive to human settlement. Its unique plants and animals had been isolated for millions of years, evolving in the absence of people and mammalian predators.

Almost as soon as the first humans (Polynesian peoples, later known as Māori) first arrived in New Zealand, around 1250–1300 AD, they began to have an impact on the environment. Their arrival, and that of the two mammals they brought with them – the kiore (Pacific rat) and kurī (dog) – marked the start of an extinction cascade.

Food remains

Material from early archaeological sites, particularly the Wairau Bar in Marlborough, reveals that Māori first exploited the larger game animals (over 2–3 kilograms). Middens contain many bones from all the moa species, geese, swans, adzebills, takahē, shags, large penguins, New Zealand sea lions and fur seals.

Changes over time

In later archaeological sites (from the 14th century on), the larger vertebrates are absent or uncommon. These sites show a growing dependence on shellfish, fish, eels and plants. Some later middens, such as at the mouth of the Shag River (Waihemo) in Otago, show that smaller shellfish were taken as the larger ones became locally depleted.

Decline and extinction of large animals

The difference between early and late middens shows that intensive hunting caused most of the larger, slower-breeding birds to become extinct within a few hundred years. Māori also hunted fur seals and New Zealand sea lions, greatly reducing their natural range and causing them to become locally extinct. These animals once occurred up to the far north of New Zealand before becoming extinct there.

Kiore

At the same time, many smaller animals were preyed on by kiore. Radiocarbon-dated fossil bones indicate that this caused the rapid extinction of Scarlett’s shearwater, the South Island snipe, the stout-legged wren, Hodgen’s rail, the New Zealand owlet-nightjar and the greater short-tailed bat.

Kiore had an impact on tuatara, lizards, frogs and invertebrates (animals without backbones). They also ate seeds. Rat-gnawed seeds of native trees such as miro, mataī, pōkākā and hīnau have been found, preserved in sediment. The oldest radiocarbon dates for rat-gnawed seeds are from the 13th century, around the time that Māori arrived in New Zealand, so it seems likely that kiore came to New Zealand with the earliest settlers. Whether they stowed away or were brought deliberately is not known.

Kurī

Kurī (dogs) were domesticated, and were an important source of food and skins for capes. They were probably kept close to camp, and did not run wild in packs or contribute much to early extinctions.

Early introduced plants

Early Māori settlers brought tropical crop plants from their Polynesian homelands. While many species did not survive because of the cooler climate, others were grown successfully, providing carbohydrate or useful resources. They probably include kūmara (sweet potato, Ipomoea batatas), yams (Dioscorea species), taro (Colocasia antiquorum), gourd (Lagenaria siceraria), paper mulberry (Broussonetia papyrifera) and perhaps the Pacific Island cabbage tree (Cordyline fruticosa).

Horticulture

Archaeological evidence suggests that these plants were being cultivated soon after human arrival. There are stone garden walls, large terrace gardens, and pits for storing seasonally abundant tubers. There are also signs of soils that have been disturbed by deep cultivation, and ditches for taro cultivation in swampy areas. In some places the soil structure was deliberately improved by adding sand, gravel and pebbles, taken from quarry pits that are still visible.

As Māori grew crops, they changed the landscape by altering soils and wetlands, and by reducing forest areas.


Pre-European deforestation

Continuous forest cover

Before people arrived, more than 80% of New Zealand was covered in dense forest. Pollen and charcoal records from over 150 lake, swamp or peat bog sediment core samples give a clear picture of the country’s vegetation and fire history since the end of the last ice age, about 14,000 years ago.

Early fires

Only small, occasional fires occurred in the forests. These are shown by minor, temporary declines in pollen from tall forest and shrub species, along with increases in charcoal, and in spores and pollen from bracken, grasses and other pioneer species (the first plants to grow in a cleared area).

Some scientists believe that these minor bracken and charcoal peaks from 100 AD are evidence of human-lit fires, and that people were in New Zealand before the 13th century. However there are equally convincing natural explanations, including lightning strike after droughts, and volcanic eruptions. There is no other supporting archaeological evidence for human occupation.

Deforestation by fire

Sediment records show a huge increase in charcoal and bracken spores around the 13th and 14th centuries. At the same time, there was a massive decline in pollen from forest trees, marking a striking and devastating change: up to 40% of the forest was burnt within 200 years of Māori settling in New Zealand. Radiocarbon dating shows that deforestation began at the same time throughout the country. There are also many archaeological sites dating from this time.

The forests in the drier eastern regions burnt rapidly, and were cleared quickly and completely. In wetter or mountainous areas, clearance occurred later and was more piecemeal.

In most sites, charcoal continues to appear after the first forest clearance, suggesting that Māori used fire to stop tall forest and scrub from regenerating.

Purpose of burning

Reasons for clearing the forest included opening up the landscape to make it more habitable. Crops could be grown, and bracken fern (Pteridium esculentum) was encouraged for its edible starchy rhizomes. Burning also kept tracks clear and made travel easier. The first forested areas to be cleared must have been easy to burn, particularly during or after a drought.

Swamp and lake sediments

Pollen records from swamp and lake sediments show major changes in the plants that grew around wetland edges. Raupō (bulrush, Typha orientalis) became more abundant around swamps, because of increased nutrients and water flow. Māori used the pollen from raupō flower spikes to make cakes, so they may have encouraged its growth by burning lake-edge vegetation.

Little erosion

Erosion did not always follow deforestation, even in areas that are now very erosion-prone. The dense bracken with its network of rhizomes, and the tree stumps that remained after burning, would have protected against the landslides and soil erosion caused by heavy rain.


European impact

First visits

The first European visitors to New Zealand created a new wave of major environmental impact. New crop plants such as potatoes were introduced from the time of James Cook’s voyages in the late 18th century. Māori grew potatoes widely by the early 1800s, clearing more forest and bracken to make way for gardens.

European settlement

As Europeans settled in New Zealand, they brought more changes to the remaining forests, animal diversity and landscape stability. Along with immigrants came new animals, crop plants, parasites and diseases. The remaining lowland forests and scrubland were burnt, drained, logged and cleared for farms and cropping. By 2005, forest cover was reduced to 24.8% of the total land area.

Logging

Forest seen as unsuitable for farming or other development was selectively logged. Large conifer species such as rimu (Dacrydium cupressinum), mataī (Prumnopitys taxifolia), tōtara (Podocarpus totara), kahikatea (Dacrycarpus dacrydiodes) and kauri (Agathis australis) were taken, significantly changing the forests. Later, beech forest (Nothofagus species) was felled and exported for wood chips. In the 1970s and 1980s, large areas of conifer–broadleaf forest, particularly on the West Coast, were clearfelled for timber and replanted with introduced pine (Pinus radiata).

Draining wetlands

The settlers drained large wetland and swamp areas, converting them for farming or towns. By the early 2000s, only 10% of New Zealand’s original wetlands remained.

Erosion

When the settlers cleared the bush for farming, they removed tree stumps and the protective cover of ferns and scrub – unlike the 13th-century deforestation, which left these in place. The loose soils in hilly areas became very vulnerable to erosion, especially during heavy rain. This can trigger massive slips, and the runoff to rivers carries high sediment loads. Over time, river mouths and estuarine systems have become silted up, and mangroves have spread extensively in some silted areas of northern New Zealand.

Introduced plants

By the 2000s, more than 30,000 plant species had been introduced to New Zealand. At least 2,166 had become naturalised – they can survive and reproduce without human help. Some early introductions rapidly became irrepressible weeds – such as gorse (Ulex europaeus) which spread from shelter belts and hedges. Others, such as Scotch broom (Cytisus scoparius), wattles (Acacia species) and hawthorn (Crataegus monogyna), have spread more slowly, but are now major weeds.

Some woody species such as contorta pine (Pinus contorta) have spread into alpine areas, even growing above the treeline (the natural limit for native trees) because of their greater tolerance for cold. Coastal sand dune habitats have been altered dramatically, with native plants like pīngao (Desmoschoenus spiralis) being replaced by marram grass (Ammophila arenaria) and tree lupin (Lupinus arboreus). Herbaceous hawkweeds (Hieracium species) have taken over some grasslands, reducing them to wasteland, and are spreading into forests and along creeks.


Impact on animals

From their first arrival in New Zealand in 1769, Europeans brought animals deliberately (such as pigs, for food) or accidentally (such as stowaway rats). This was the beginning of a new period of extinctions and range reductions for native animals.

Loss of marine mammals

From 1792, sealers arrived to hunt marine mammals, mostly in the southern South Island and subantarctic islands. Whalers followed from about 1800. By about 1850, this brief but devastating exploitation was over, leaving New Zealand sea lions and fur seals exterminated from most of the areas they had occupied before the hunters came. The total number of seals killed is unknown, but more were taken from the Bounty and Antipodes islands alone than the entire fur seal population in New Zealand today.

Effect of introduced mammals

Since the late 1700s, 54 more mammal species have been brought to New Zealand. Nineteen of these, including two devastating predators (the Indian grey mongoose and the North American raccoon) did not become established in the wild. However, 11 predators successfully became naturalised: three more rodents; three mustelids (ferrets, stoats and weasels); pigs, hedgehogs, possums, dogs and cats. These have had a massive impact on the remaining native birds, invertebrates and reptiles. Forty out of a total 91 land birds are now extinct, with many others in serious decline.

Loss of seabirds

Fewer seabird species have become totally extinct, but only because small numbers have survived on islands free of rats, mustelids or cats. The many millions of seabirds that once bred on the main islands have been reduced to just a few small colonies. When kiore (Pacific rats) arrived in New Zealand, they killed the smallest birds such as storm petrels and prions. Larger predators introduced by Europeans attacked larger seabirds – particularly eggs and chicks in burrows. This has also affected soil fertility, as seabirds transport key nutrients such as nitrogen and phosphorus from the sea to the land.

Tally of loss

Sixty-four endemic species of birds, lizards, frogs and a bat have become extinct since humans settled in New Zealand. They succumbed to hunting by people, and predation by kiore (Pacific rats) and 11 other mammals. Over 30 more species survive only on small predator-free offshore islands.

Impact of grazing animals

New Zealand’s forests are now home to 14 wild introduced grazing animals, such as goats, deer and the brushtail possum. These have transformed forest understoreys and regeneration patterns. Some forest trees are very long-lived, so it may be hundreds of years before the effects of browsing by mammals shows in the forest canopy.


Hononga, rauemi nō waho

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How to cite this page: Janet Wilmshurst, 'Human effects on the environment', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/human-effects-on-the-environment/print (accessed 14 November 2019)

Story by Janet Wilmshurst, published 24 Sep 2007