From: Flight of the Huia, by Kerry-Jayne Wilson

Chapter 1: New Zealand: archipelago and mini-continent

In the bottom left-hand corner of the world’s largest ocean lies a land once home to some most unusual animals and plants. These species were so unlike those elsewhere that Jared Diamond, the well-known American biologist and author of the book Guns, Germs and Steel, suggested that studying life in New Zealand was the closest he could come to researching on another planet.1 Imagine how our understanding of what is biologically possible would be enhanced by an experiment in evolution independent of life as we know it. We are unlikely to get any such opportunity, and certainly not in our lifetimes, so the best experiments in the independent evolution of life accessible to today’s biologists are to be found on isolated islands such as New Zealand.

Most oceanic islands are geologically young or insufficiently isolated to prevent plants and animals colonising from larger land masses. Diamond identified four island groups that have been isolated for long enough to evolve dramatically different life forms, and are large enough not to be plagued with high natural extinction rates. These are Hawaii, New Caledonia, Madagascar and New Zealand. Of these, he suggests New Zealand is the most interesting. Hawaii is the smallest and youngest; New Caledonia is ancient but small; and Madagascar, while both ancient and large, is too close to Africa to prevent the influx of that continent’s mammals.

Once part of the enormous southern continent known as Gondwana, New Zealand is large, remote, has been isolated from other land masses for 80 million years and lacks the mammals that are dominant elsewhere. At least 80 per cent of the species belonging to most non-marine animal and plant groups are endemic. Some of these species belong to families or orders that occur nowhere else, which illustrates how long they have been separated and how distant their relationships have become. Only other remote islands like Hawaii have a similar proportion of endemic species (Table 1.1).

New Zealand consists of two large islands surrounded by loosely clustered small to moderately large islands. Effectively it is both an archi-pelago and a very small continent. As an archipelago New Zealand is of particular interest because it includes smaller islands of two distinct types. There are numerous land-bridge islands lying close inshore that were connected to the main islands during glacial advances (such as Stewart and Kapiti Islands). Further offshore there are truly oceanic islands that have never had a mainland connection. They range from the subtropical Kermadec Islands to subantarctic Campbell Island, and have been colonised by a subset of species found on the mainland.

Figure 1.1 Native and endemic non-marine vertebrate animals breeding in New Zealand and in the British Isles

Since humans discovered New Zealand about 2000 years ago, and permanently colonised this Gondwanan liferaft almost 1200 years later, the natural environment has been greatly altered. Almost a half of the native non-marine birds are extinct, and about half of those that remain are threatened or endangered. Of the endangered or threatened birds listed by Birdlife International in 2000, 5.3 per cent are endemic to New Zealand.2 Considering the country does not have a long list to start with, this is a disproportionately high figure: there are 62 New Zealand species on the list, placing New Zealand eighth equal when counties are ranked this way. All the other countries in the top 25 are larger, and most are tropical nations with far greater species diversity. As well as losing so many species, New Zealand’s ecosystems have been much altered in other ways, with many foreign species introduced and habitats fragmented. Other catastrophic changes are less obvious; for example, the loss of many pollinators and seed dispersers. The integrity of our unique ecosystems is under severe threat.

We can appreciate how different New Zealand is from the intensely studied northern hemisphere ecosystems by comparing our numbers of vertebrate animals with those of the British Isles, a similar-sized temperate archipelago (Figure 1.1). Not only are many New Zealand species endemic, but many belong to families or orders that are restricted to this country (see Chapter 3).

The foundations of ecology were developed in the European and North American temperate zones, and even today most of the world’s ecologists and the headquarters of most environmental organisations are based in that part of the world. Conversely, most endemic birds, most endangered species and the ecosystems under most immediate threat are in the southern hemisphere or the tropics.3

A foreign land

The first Polynesians who came to New Zealand found landscapes, plants and animals very different from those they had previously known. They must have been thrilled to discover a land mass much larger and with a far greater variety of animals than the small, scattered islands whence they came. Not only were there more kinds of animals, but a special bonus was the large, easily hunted, meaty birds. European settlers also found New Zealand to be a foreign land, but for different reasons. They probably expected something resembling their temperate island homeland, and indeed there were superficial similarities, but the ancient evergreen Antipodean forests and their animals were fundamentally different from the open, deciduous forests of Europe.

Initially, neither Polynesian nor European settlers had time to contemplate the strange land, plants and animals, because they were faced with the pressing needs of food and shelter. Just like colonists elsewhere in the world, Maori and Pakeha alike tried to adapt the land to their previous lifestyles, rather than they themselves adapting to the new land. The European settlers were especially assiduous, and it would take several generations for either group to evolve a conservation ethic reflecting the needs of the native animals of this land.

It was long believed that the Maori settlers were the first people to visit these islands. However, during the 1990s it was discovered that kiore (Polynesian rat) had been in New Zealand for about 2000 years, more than a thousand years longer than the earliest likely date of Maori settlement.* They could not have arrived without human assistance, and appear in the subfossil record at about the same time on both main islands. We will probably never know just who those people were that brought the rodents here, as at that time kiore were present on most other South Pacific islands. In a land bereft of mammals, the small ground-dwelling birds and reptiles were defenceless against even this small rat, and we can picture a plague of kiore sweeping across the land with no pied piper to come to the rescue. By the time the Polynesians settled New Zealand around AD1200 (see Chapter 5), many of the small ground-dwelling birds, reptiles, frogs and invertebrates were already extinct.

When Europeans encountered New Zealand, the bird fauna, in particular the large meaty species such as the famous moa, had become further depleted by Maori. By the time of Captain Cook’s brief visit in 1769, at least 40 species of bird had become extinct, along with three species of native frog. Many other birds and reptiles had also disappeared from large parts of their original range.4 Almost a third of the forests, mostly in the drier eastern parts of the main islands, had been destroyed (Figure 1.2). Many New Zealand birds had evolved to become giants of their kind, with reduced powers of flight, long lifespans and small clutches. Such an existence had served them well for millions of years but left them ill-prepared for the threats posed by rats and humans.

Figure 1.2 New Zealand vegetation cover

Sources: Ministry for the Environment 1997, McGlone and Wilmshurst 1999

The first European visitors did not venture far from their ships but, like those unknown first people, they brought their own, larger rat that posed additional threats to native life forms. By 1800, explorers, sealers, whalers, missionaries and traders were regular visitors. Settlers began arriving early in the nineteenth century, and in 1839 the first of the planned British settlements was founded at what is today Wellington. Other settlers quickly followed. While the Polynesians and first Europeans had been largely hunters, the new colonists brought foreign seeds, animals and technology – and the will to carve out new British settlements in the Antipodes. The European arrival heralded an era of extra-ordinarily rapid and extensive habitat change. Between 1800 and 1950, 90 per cent of all wetlands and more than half the remaining forests were lost, and about 16 more bird species become extinct. Indigenous habitats were fragmented into small patches that subsequently became highly modified by introduced mammals. Numerous species of animals and some plants now survive only on predator-free offshore islands.

Figure 1.3 Geological time scale spanning that part of the world’s history relevant to this book

Why were so many of New Zealand’s species so vulnerable to hunting and habitat change? To answer this question we need to understand their ancient origins, and that means going back into the depths of time, before New Zealand existed as such.

Break-up of Gondwana

During most of the Mesozoic the lands that make up the bulk of the southern hemisphere land masses, plus additional lands now located in the northern hemisphere, were united in the supercontinent Gondwana. On the eastern fringe of Gondwana, flanked by west Antarctica on one side and Australia on the other, lay the ancient proto-New Zealand land mass.5 During the Cretaceous, Gondwana gradually separated into smaller continents, eventually leaving what is today Antarctica centred on the South Pole, while offshoots became Africa, India, Madagascar, South America, Australia and New Zealand. The processes of plate tectonics and drifting continents are not well understood by many people, and an excellent explanation of this seemingly improbable process, with particular reference to New Zealand, is presented by Graeme Stevens in his 1985 and 1988 books Lands in Collision and Prehistoric New Zealand.

There has been land in the New Zealand region since mid-Jurassic times, about 160 million years ago, and at its peak size, 135 million years ago, the ancient land mass was almost half the size of present-day Australia and completely different in shape. It extended north to New Caledonia, west to the Lord Howe Rise, east to the Chatham Islands and south to the edge of the Campbell Plateau.6 At that time warm, temperate conditions prevailed over much of Gondwana, and the New Zealand region was virtually contiguous with what was to later become Australia and Antarctica.

Late in the Jurassic, Gondwana began to rift into the sectors that would eventually become today’s continents. The initial split separated a larger western part (South America, Africa, Arabia, Madagascar and India) from a smaller eastern part, which included Antarctica, Australia, New Guinea, New Caledonia and New Zealand. The fragmentation of West Gondwana has little bearing on our story and can be summarised briefly. A rift opened between India/Madagascar and Africa, then in the late Cretaceous India separated from Antarctica and began to move northward, eventually to collide with what is now Asia. Next, Africa and South America pulled apart, then Africa separated from the remaining Gondwanan lands.

The history of East Gondwana is of much greater relevance to New Zealand. It remained a single continent long after the western part had split up. New Zealand and South America were still connected via Antarctica until about 85 million years ago, which explains the close affinities of some New Zealand and South American plants and animals; for example, southern beech trees and some freshwater insects and fishes. The presence of marsupials in both Australia and South America suggests that land connections between these two continents and Antarctica were not broken until the Eocene period, around 50 million years ago.

A rift between Australia and the greater New Zealand region formed much earlier, about 120 million years ago in the early Cretaceous, so the Tasman Sea began to form while both lands were still attached to Antarctica. During the mid- to late Cretaceous, the climate over much of Gondwana (including the New Zealand region) cooled but remained temperate as the Australasian part of Gondwana rotated southwards.7 The land links between the greater New Zealand region (including New Caledonia) and Gondwana eroded, and the land connections with Antarctica were finally severed 80–85 million years ago.8 At the end of the Cretaceous, the land mass destined to become New Zealand still lay close to the eastern edge of Australia, and it appears that the ancestors of the kiwi colonised New Zealand about this time or shortly there-after.9 The Tasman Sea continued to widen until it reached its present size about 60 million years ago.

New Zealand has drifted northwards throughout the Tertiary period. By the end of the Cretaceous, the greater New Zealand region, which still incorporated New Caledonia and the Chatham Islands, was smaller than it had been earlier in the Cretaceous, low-lying and partly covered by ocean. The last major land link to be severed was between eastern Antarctica and Australia, about 55 million years ago. Since this was one of the key events in shaping the New Zealand fauna, some knowledge of the flora and fauna of the Cretaceous age is necessary to understand subsequent events in New Zealand’s ecological history.

Figure 1.4 The break-up of Gondwana, showing the position of New Zealand in the mid-Cretaceous and late Eocene

Based on New Zealand Historical Atlas, plate 4

New Zealand’s Cretaceous flora and fauna

Reptiles were the dominant terrestrial vertebrates at the time New Zealand severed its land connections with Gondwana. The Cretaceous reptiles were a diverse and magnificent lot, including terrestrial dinosaurs, flying pterosaurs, marine ichthyosaurs and plesiosaurs. The Cretaceous was an especially interesting period for New Zealand to begin this experiment in evolutionary isolation. Soon the reptiles were to suffer mass extinction. Mammals had appeared on the scene a hundred million years before the demise of the dinosaurs, but despite many of the lineages having evolved, they were of little consequence and their major radiation took place after the demise of the ruling reptiles.

Unfortunately, our knowledge of New Zealand’s Cretaceous vertebrates is fragmentary and few land animals of that time appear in the fossil record. Since that period New Zealand has undergone tectonic and volcanic upheavals that have destroyed or distorted most rocks that could contain Cretaceous fossils of terrestrial animals.

When New Zealand separated from Gondwana, it took along a selection of the animals and plants that lived during the reign of the dinosaurs. No New Zealand dinosaurs, pterosaurs or marine reptiles appear to have survived the Cretaceous/Tertiary mass extinction,10 but other species did and their descendants comprise an important element in the present-day biota. Modern plant and animal groups derived from ancestors that lived on the proto-New Zealand land mass during the Cretaceous include moa, New Zealand wrens, tuatara and native frogs; probably geckos, kauri, podocarps, weta, peripatus, the giant land snails and, possibly, skinks.11 Some of these species, including tuatara and native frogs, belong to primitive groups that have since become extinct in other parts of the world but soldiered on in New Zealand, isolated from other evolutionary pressures. New Zealand’s beech, podocarp and kauri forests date back to the Cretaceous when these forest types covered much of Gond-wana. Today, southern beeches (Nothofagus) occur in New Zealand, the wetter, cooler parts of southern South America and eastern Australia, the montane forests of New Guinea and much of New Caledonia – all lands formerly part of Gondwana.12 Nothofagus fossils have also been found in Antarctica.

One peculiarity of the New Zealand biota is the absence of terrestrial mammals apart from three species of bats. During the Cretaceous, mammals were present over much of Gondwana, and it has always been a puzzle that there were not more in New Zealand. However, in 2002 as-yet-unidentified fossils were discovered in Otago of mammals that became extinct sometime during the last 20 million years.13

Africa and South America collided with northern continents in later epochs, so their Gondwanan heritage is mixed with species from the north; however, the ostriches of Africa and rheas in South America remind us of these continents’ Gondwanan origins. Australia and South America separated after the extinction of the dinosaurs and after the monotremes and marsupials had dispersed across parts of Gondwana.

New Zealand during the Tertiary age

At the beginning of the Tertiary, the sea flooded large areas of land, breaking the New Zealand region into a series of islands.14 The Tasman Sea had almost attained its full width and the oceans south of New Zealand had formed, creating a barrier to the dispersal of animals from Antarctica, which still had a temperate climate. New Zealand and New Caledonia, for so long parts of the same land mass, now became separated as the ocean flooded the intervening land. Islands along the Norfolk Ridge, which still runs under the sea between New Zealand and New Caledonia, appeared and disappeared over the course of time, facilitating the dispersal of certain animals and plants between the two archipelagos. The ancestors of the short-tailed bats,15 piopio and wattlebirds may have arrived early in the Tertiary before the ocean crossing became too wide. By the late Eocene, Antarctica had cooled, glaciers were forming on it and, 30 million years ago, ice sheets began to spread out and blanket the continent. Gradually the beech and podocarp forests of Antarctica and the species inhabiting them were extinguished.16

The New Zealand land mass continued to erode until, by the Oligocene (35 million years ago), all that remained was a chain of small islands surmounting a drowned plateau. The area of dry land was possibly just a fifth that of present-day New Zealand,17 with the largest island about the size of Canterbury. The land was not only small in area but also low-lying, so it offered a reduced range of habitats. Although fossil evidence is lacking, we can imagine the extinction of many species that could not adapt to changing habitats. Conversely, snails, insects and perhaps small vertebrates such as geckos may have evolved into many more species during this time as isolated island populations diverged genetically from one another. The populations of kiwi, moa and New Zealand wrens were presumably reduced to low numbers and much of the previously present genetic variability may have been lost.18 It is interesting that recently discovered reptile and mammal fossils found in Otago date from the Miocene (15–22 million years ago), showing that some animals survived the Oligocene drowning, only to become extinct at a later date.

Volcanoes and earthquakes

A period of volcanism and mountain building began about 20 million years ago, just in time to save this Gondwanan liferaft from sinking beneath the waves. This geological activity was caused by the collision of the Pacific and Indian-Australian tectonic plates, the boundary of which New Zealand straddles. It has increased in tempo and continues today. During the last 20 million years, new habitats have been created that have greatly influenced the biogeography of invertebrates and plants but have had little influence on the vertebrates. The environment and conditions during the Pleistocene (the last two to three million years) have been vastly different from the previous 60 million years, with a cooler and less even climate, and a more mountainous landscape than at any previous time during the Tertiary.

It may seem surprising that such dramatic events had so little impact upon the vertebrate species, but it is not so when you consider the volcanic activity of the last century, during which White Island, Ngauru-hoe and Ruapehu have erupted from time to time. The 1889 eruption of Tarawera ejected heavy falls of scoria for 16 kilometres and ash for 24 kilometres. While some forests close to the mountain were totally destroyed, just a few kilometres further away there was, within a few years after the eruption, little lingering evidence of damage. Bigger still was the Taupo eruption of 1800 years ago, the impact of which was recorded as far away as China. The pumice, ash and rock ejected almost totally destroyed vegetation over an area of 20,000 square kilometres, yet just 300 years later, tall forest once again covered the devastated area.19 The ash and mud from volcanic eruptions actually enrich the soil, so the damage is short term and soon obliterated by regeneration. Volcanoes can build up quickly; for example, Ngauruhoe is just 2500 years old and was probably just a small hill when those Pacific explorers visited these shores and left their rats. Rangitoto emerged from the Hauraki Gulf after Maori settled the Auckland isthmus.

Similarly, earthquakes are seldom catastrophic to whole species or populations. New Zealanders can expect to experience at least one major quake during their lifetime. Some of the most significant have been at Wellington (1855), Murchison (1929), Napier (1931) and Inangahua (1968). Earthquakes and volcanic eruptions are memorable events, but actual mortality is low compared with, say, the arrival of new predators.

The mountains that are so much a feature of New Zealand are only a few million years old. Most uplift of the major mountain ranges has occurred during the last five million years, and the present rate of uplift is probably as rapid as it ever was in the past. The main ranges continue to grow in height at an average rate of about a centimetre a year, and New Zealand continues to move north about six centimetres a year.

The West Wind Drift and Australian immigrants

When Australia moved away from Antarctica it completed the circumpolar ring of water that is the great Southern Ocean, thus during the Eocene a westerly current circling Antarctica began to develop.20 Since the Miocene, prevailing westerly winds have encircled the globe between 40 and 60 degrees south. This has been, and still is, the main factor influencing the biogeography of marine birds and mammals throughout southern latitudes, helping to transport animals and plants across the seas and enabling circumpolar movement of marine life21 – the importance of which will later be seen. Dispersal across the Tasman has been the main source of land vertebrates for the past 20 million years, and most birds with Australian affinities have probably colonised New Zealand some time during that period. Several species have even colonised more than once, the first arrivals evolving into new species before the second influx took place. For example, the takahe, weka and black stilt evolved from earlier colonisations of New Zealand by the pukeko, banded rail and pied stilt, all of which now co-occur in New Zealand with their daughter species.

Compared with many oceanic islands, New Zealand is not especially isolated, yet it has surprisingly few terrestrial and freshwater vertebrates. New Zealand is only 1800 kilometres downwind from the biologically rich Australian continent, and must have been receiving a steady flow of wind-blown animals since the Oligocene. The paucity of Australian-derived species in the New Zealand fauna has never been satisfactorily explained. Getting here was one thing, but finding suitable habitat and a mate were also necessary in order to leave descendants. In prehuman times the habitats that greeted the Australian immigrants were mostly dense forests with very different plants and habitats to those in Australia. Even though Australia and New Zealand have some closely related trees, such as their southern beeches (Nothofagus), these mostly occur in quite different associations in the two countries. Even the open habitats available in New Zealand were very different to those the animals were adapted for in Australia. Just how effectively this excluded Australian immigrants is illustrated by the fact that 14 bird species have successfully colonised New Zealand since human settlement (most during the last 150 years) – far more than in previous millennia.22

Table 1.2 Relative abundance of native birds, self-introduced silvereyes and intro-duced birds in unmodified indigenous forests on Little Barrier Island and in mainland indigenous forests modified by browsing mammals and other factors

Adapted from Diamond & Veitch 1981

Human-induced changes have enabled natural colonisation by these species as habitats became modified so they more closely resembled those in Australia. In 1886, the silvereye was first recorded in New Zealand and since then they have become one of our most common birds. On Little Barrier, one of the few large islands where forests have not been modified by browsing mammals or logging, silvereyes and introduced birds are very rare in the forests, yet they are numerous in the modified habitats around the ranger’s house. These birds are common in indigenous mainland forests that have been altered by browsing mammals and other factors (see Table 1.2). Most foreign species arriving in New Zealand before humans came on the scene were unable to adapt to the ancient Gondwanan forests, so the unique biota persisted until humans modified those habitats.

The Pleistocene ice ages

About two million years ago, the Earth’s climate cooled and glaciers covered much of the temperate lands in both hemispheres. We can picture the following period as one of fluctuating temperatures, causing the glaciers to vary in size. When they advanced, sea levels fell by up to 150 metres, so that the North, South, Stewart and most land-bridge islands were all joined in a single land mass. The most recent major advance reached its fullest extent 20,000 years ago and finished as the climate once again warmed some 14,000 years ago.23 The ice ages may not be over: the climate could again cool and glaciers advance once more.

With each glacial advance, vegetation changed as forests gave way to montane grasslands and herbfields, which were then re-invaded by forest when the temperatures rose again. On the continents, habitats and species would have shifted north or south as the glaciers advanced or retreated, but in New Zealand northward movement was limited and during glacial advances forest animals and plants were restricted to ice-free refugia. During major glacial advances, podocarp forest south of the Waikato would have been restricted to small, isolated fragments.24 Beech forest would have been more extensive, but large parts of the country would have supported only grasslands or shrublands. As the glaciers retreated and the climate warmed after the last glaciation, forest again covered most of the country long before humans arrived. Although the vegetation changes were dramatic, no bird species is known to have become extinct during the Pleistocene. The alpine habitats that first appeared during the Pliocene and the Pleistocene allowed the alpine kea and rock wren to diverge from their forest-dwelling ancestors, the kaka and bush wren.

New Zealand is thus a place where ancient animals and plants, survivors of past geological ages, live in a youthful landscape created during the last few million years by tectonic uplift and volcanic eruptions, and subsequently shaped by glaciers and erosion.

Rats, humans and other aliens

The arrival of rats, humans and other introduced species was the most momentous event in the history of New Zealand since the break with Gondwana, 80 million years earlier. The rate of ecological change during the last 2000 years is orders of magnitude greater than during any earlier era. Passengers on the Gondwanan liferaft were completely unprepared for the massive disruptions, and New Zealand’s ecological communities have been greatly altered by the loss of many species, including the major browsers, predators, insectivores and frugivores.

Even our national parks, reserves and offshore island reserves contain only a subset of the endemic species once present. Today, many of the species most familiar to New Zealanders are those that occur naturally in Europe or Australia. Only a few of the most widespread and hardy native birds still survive in the urban or rural habitats where most of us live, which bear little resemblance to those of 150 years ago. The most common city-dwelling native birds, such as silvereyes and welcome swallows, actually established here after the Europeans arrived.

The change from a mammal-free sanctuary to the heavily modified landscape of today took place over an amazingly short time, and the intensity and pace of change is unrecognised by most people. New Zealand was the last habitable land to be colonised by people. When the Polynesians discovered these islands and later settled here, they had already discovered almost every other island scattered across the world’s largest ocean. Australia had been inhabited by aboriginal people for at least 40,000 years (probably much longer), and the Americas for over 10,000 years. In Europe, the Greek and Roman civilisations had flourished then fallen; while in the Americas the great Mayan civilisation was in decay and the Inca civilisation was on the ascendant. Literate societies had existed in Egypt, Mesopotamia, India and China for at least 3000 years. At the time Polynesians settled New Zealand, Hadrian’s Wall and the Great Wall of China were already at least a thousand years old and castles were under construction in Britain.