Threatened New Zealand Biodiversity. The Fate of the Possum

Table of Contents

1. A Journey to New Zealand

2. Biological Variety – Essential for Ecosystems

3. Aotearoa’s Unique Biodiversity
3.1. Development of New Zealand’s Unique Biota
3.2. New Zealand’s Native Species Today
3.3. New Zealand in an Invasive World
3.3.1. Human Impacts
3.3.2. Alien Invasions – A One-Sided Battle

4. Trichosurus Vulpecula – ‘Keystone Aliens’
4.1. The Australian Brushtail Possum Invading New Zealand
4.2. The Multiple Impacts of Brushtail Possums

5. Pest Control – Preservation of Endemic Species
5.1. “Just a Dead Possum Is a Good Possum” – Methods of Pest Control
5.2. Restoration Methods used in New Zealand
5.3. Advantages and Risks of Pest Control and Restoration Management Methods

6. Constant Dripping Wears the Stone – New Zealand in the 21st Century

7. Appendix
7.1. The Brushtail Possum – A Brief Description
7.2. Glossary
7.3. Endemic New Zealand Biota Mentioned

8. Works Cited

9. List of Figures

1. A Journey to New Zealand

When you ask people in Germany what they know about New Zealand, its beautiful and impressive landscape and nature will definitely be among the first things named. This unique beauty and purity of nature is probably one of the main reasons why so many students from all over the world dream of spending some time in Aotearoa, the land of the long white cloud, as it is called in Te Reo Maori.

So I felt very lucky when I got the chance to spend five months in Auckland on the North Island from January to July 2007, not only because I could experience a different way of life on the other side of the world, but also because it offered me the opportunity to explore ‘godzone’, God’s own country, with its unique flora and fauna. The preservation of the country’s beauty is one of the main topics of New Zealand’s government policy and also a major issue in society.

As a visitor, you, most probably, will fairly soon get in touch with the strict rules that anyone entering the country has to obey, already before leaving the airplane. On my journey, all passengers on the plane from Sydney to Auckland were given a ‘New Zealand Passenger Arrival Card’ on which they had to answer questions about countries that they had visited within the last few years, goods that they were bringing in or whether they had recently stayed on a farm or had made hiking tours.

That way, the Department of Conservation* wants to make sure that no animal products and no soil, which could be left on the soles of hiking boots or stick to any other outdoor equipment and which could possibly contain bacteria or parasites that would endanger New Zealand’s endemic biota, are brought to the country. Therefore, all suitcases and bags are scanned twice for products like honey, eggs, feathers or meat. That is also why it takes much longer to get through customs at Auckland International Airport than it does e.g. in Frankfurt, Germany.

Now what is the reason why the Kiwis are so afraid of foreign animals and plants? Why are they doing everything to protect their endemic species by employing such a complex and expensive system of pest control, which, of course, is not limited to airports only, while at the same time being confronted with lots of crucial social problems to be solved for the solution of which the funds invested would also be badly required? May we even say the latter ones should be given priority? And what is it that makes nature on the green isles in the Pacific Ocean that unique and worth to be protected with such huge effort?

2. Biological Variety – Essential for Ecosystems

Taking a closer look at the importance and role of a healthy biodiversity, it becomes clear that the beauty which has to be preserved is just a small aspect compared to the multitude of natural services it provides^[1] from which the most important are named below:

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These processes and, moreover, the survival of a species is dependent if not mainly based on its interaction with others, which requires a broad range of species, i.e. a healthy biodiversity. Without the honey bee, the primary species that fertilises fruit-producing plants, “one-third of all our food – fruits and vegetables – would not exist”^[2], which would have fatal consequences for all herbivores* and omnivores*. Being linked to carnivores by food web* connections, the decline of herbivores would result in a decline of carnivores. Summarised, that would mean, that the decline of honey bees would indirectly result in the decline of carnivores. A further example is the rising numbers of species suffering from predation if predator species are reduced, leading to rising numbers of predators with food resources becoming more and more abundant. Such relations exist all throughout the world’s biota, making especially species which seem to be inconsiderable or unimportant at first glance the integral factor of ecosystem stability.

Considering all that, it is quite obvious, that the loss of single a species results in the decline and, at the same time, to the growth of other species, changing, in total, whole ecosystem compositions. An important point we should think about when deforesting huge parts of rainforest every day resulting in the decline of many species caused by the loss of habitat.

3. Aotearoa’s Unique Biodiversity

An archipelago lying some 2,000 kilometres southeast of Australia in the southern Pacific Ocean, the New Zealand hotspot covers 270,197 km² on three main islands (North Island, South Island and Stewart Island) and several surrounding islands^[3].

As it has been separated from other land masses for over 40 million years – when it split away from New Caledonia – it was possible for plants which were typical around the globe millions of years ago to survive, making the hotspot, nowadays, an “ancient life-raft”^[4] which has, in addition, evolved a unique flora and fauna.

This chapter will show how this biota – which is one of a kind in the world – could develop, what it is like nowadays and how endemic species have been and still are suffering from globalisation in an invasive world.

3.1. Development of New Zealand’s Unique Biota

As a land of varied landscapes – with rugged mountains, rolling hills and wide plains – and a wide range in latitude from subtropical to subantarctic^[5], the country offers a huge variety of living conditions throughout the islands, thus allowing a multitude of animals and plants characterized by utmost different requirements to develop pretty well.

A key role in biodiversity distribution, however, plays the highly variable climate which ranges from subtropical, with warm, moist conditions throughout the year, for example on the Kermadec Islands, to cloudy, humid climate, with cool, wet winters and warm, usually dry summers on the Chatham Islands. Together with an annual rainfall varying from 12,000 millimetres on the western slopes of the Southern Alps, to less than 300 millimetres in their rainshadow areas on the east, the landscape of the South Island is divided by the Southern Alps into rainforest on the west coast and wide plains on the east coast.

Not only the South Island, but also the North Island features many different ecosystems appealing to a variety of species. Going from north to south we will first encounter the huge Kauri forests in Northland which have been the reason why the island was once used by sealers and whalers as a base to restore their ships on their trips to the Antarctic.

The central North Island, however, is characterised by tectonic activities with mud pools reaching temperatures up to 800 °C at their ground, huge geysers and volcanoes like Mt. Ruhapehu. In contrast to the rich green rainforest and subtropical climate around Auckland, the barren region around Rotorua and especially the Tongariro National Park with its rocks and wide grasslands of yellow tussock seems to be bleak.

In total, that means that New Zealand’s variety of ecosystems have made it possible for such a multitude of species with all kinds of requirements to evolve, resulting in the formation of today’s impressive and often threatened biota.

3.2. New Zealand’s Native Species Today

Just like the other fragments of ancient Godwanaland – Madagascar, Australia and New Caledonia – New Zealand features remarkable levels of endemism among plants, birds and reptiles.

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Fig. 2 Endemic Species Overview

As shown in the chart above, nearly 1,900 of about 2,300 species of plants are endemic and even 35 of New Zealand’s plant genera are found nowhere else in the world. An example is the monotypic* genus Desmoschoenus spiralis or Pingao golden sand sedge*, a coastal plant which was used by the Maori in traditional building construction^[6].

The fern Loxoma cunninghamii is one of the hotspot’s ‘living fossils’ which constitutes, together with three species from Central America, the family Loxoma-taceae, whose closest relatives existed 60 million years ago. The hotspot has also one endemic family, the Ixerbaceae, which is represented by a single species (Ixerba brexiodes)^[7].

In addition, 90 of 200 bird species which occur regularly in New Zealand are endemic, which is why five Endemic Bird Areas identified by BirdLife International occupy nearly the entire area of the country in order to protect them. The country also has 17 endemic bird genera and three endemic bird families (Acanthisittidae, Aptery-gidae and Callaeidae) and is, moreover, the only hotspot in the world to have an endemic bird order, represented by the flightless kiwi (Apterygiformes), which is also the national bird of New Zealand^[8].

Although the country had a wide range of species in general, both land mammal species native to New Zealand were endemic bats (Mysticina tuberculata and Mystacina robusta), whereas endemic terrestrial mammals were completely non-existent before the arrival of humans. Their introduction following human settlement should proof to have fatal consequences on the hotspot’s native biota.

Overall, we can say that New Zealand still is an ‘ancient life-raft’ with an impressive biodiversity including lots of species which exist nowhere else in the world, like e.g. the tuatara*. Unfortunately, lots of these species have become very rare or even endangered if not extinct, which is why people have to act in order to protect their country’s beauty.

3.3. New Zealand in an Invasive World

3.3.1. Human Impacts

Although people came to New Zealand relatively late, i.e. about 600-800 years ago, human impact on the land and natural ecosystems has always been extensive. The first great impact was from hunting, fishing and gathering, which caused the extinction of native bird species such as the giant moa and the Haast’s Eagle^[9], “the largest eagle to have ever lived”^[10].

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Fig. 3 Model of the flightless Moa with Haast’s Eagle on top at Te Papa Tongarewa, Wellington

An even greater threat to the native biodiversity of New Zealand, however, was the introduction of invasive alien species^[11]. Apart from seals and bats, all other terrestrially-breeding mammals in New Zealand are immigrants to this land which were introduced by humans. The first mammals to be introduced to the country were kuri, a dog (Canis familiaris), and kiore, the Polynesian rat (Rattus exulans) which Polynesian immigrants brought with them on their ships. Nowadays, the kuri is gone and the kiore, itself the victim of competition from subsequent introductions of other species, is restricted mainly to offshore islands^[12].

Although the impact of those first alien mammalian species had already been fatal as it had resulted in the decline of numbers of native species, the most lethal invasion of mammals occurred with the arrival of Europeans at the end of the 17th century, who liberated everything from alpacas to zebras. Almost every one of the species that became established has left its destructive mark on the native New Zealand environment. Among the most important species liberated by Europeans were pigs (Sus scrofa), goats (Capra hircus) and several species of bovids, which were, like the goats, introduced during the 19th century as a food source for the young country’s growing society.

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Fig. 4 Traditional New Zealand sheep farming

Not only the bare introduction but also the treatment of alien species contributed to their success. Introduced sheep (Ovis aries), for example, were often put on unfenced range, which is the reason why the population of feral sheep, a danger to native flora, quickly became established and widespread^[13].

Nevertheless, it were the mammals that Captain James Cook, the early sealers and settlers at the beginning of the 19th century introduced unintentionally that have had some of the most catastrophic effects on New Zealand’s ecosystems. The Norway rat (Rattus norvegicus), the common ship rat (Rattus rattus), the house mouse (Mus musculus) – which is even able to kill lizards and has had major impact on New Zealand’s native invertebrate fauna^[14] – and also the ships’ cats (Felis cattus) are probably the species that have to be mentioned, as they have managed to establish huge populations throughout the country^[15].

Many cats were released by farmers from the mid to the late nineteenth century in an attempt to control the growing populations of the European rabbit (Oryctolagus cunilungus), which had been responsible for serious devastations to grazing land. For the same reason farmers demanded to import the rabbits’ natural predators, like e.g. ferrets (Mustela putorius), stoats (Mustela erminea) and weasels (Mustela nivalis). Hence, from 1879 to 1886, thousands of ferrets, 592 weasels and 214 stoats were released only on the South Island at sites including Lake Wanaka, the Makarora and Wilkin Valleys, Lake Wakitipu and parts of Southland. Unfortunately, these mustelids* had little effect on the rabbit population, but by 1892 the damage weasels and stoats were causing to native birds, as they moved into the forests of Fiordland and the like, was becoming apparent^[16].

The rabbit itself had been introduced to the country by James Cook in 1777, although the main reason for its spread certainly was the subsequent importation by the so-called Acclimatisation Societies, which organised releases on the North as well as on the South Island^[17].

Founded from the early 1860s onwards, the main goal of the Acclimatisation Societies was the “introduction, acclimatisation and domestication of all innoxious animals, birds, fishes, insects, and vegetables whether useful or ornamental”^[18] as is fixed in the Rules of the Acclimatisation Society of Auckland, stated at a meeting on 30 July, 1862. By stocking the country systematically with familiar animals and plants which had not existed there before, their purpose was to create a ‘second Britain’. The only problem was that most of the species that they considered ‘innoxious’, like e.g. rabbits, deer, cats, pigs or brushtail possums turned out to be a serious threat to the native biota of their new country.

3.3.2. Alien Invasions – A One-Sided Battle

Overall, we can summarise that basically all introductions of foreign species have had far-reaching fatal effects on New Zealand’s native flora and fauna. Although they occur all over the world, there are huge regional differences in the consequences biological invasions have on a country’s biota. But why have, in New Zealand’s case, those consequences been that disastrous?

The main reason for the devastating effects of alien biota on endemic species has been the “poor representation or absence of certain functional groups”^[19], i.e. the absence of indigenous snakes, predatory terrestrial* mammals, colonial bees, wasps and ants during its formation. This circumstance has resulted in a “one-sided battle following human settlement between ecologically naive islanders and sophisticated continentals”^[20]. One typical example is the absence of mammalian herbivores and the abundance of herbivorous birds like moas which had resulted in the development of avian-specific defences of plants against natural enemies. That is why large numbers of plants were sensitive to mammalian browsing following human settlement as they had not been used to it before and, therefore, had no defence against it. A similar problem occurred with a lot of native birds, like e.g. the kiwi. In absence of mammalian and snake predators, large flightless or weakly flighted birds could develop pretty well. Being confronted with exactly those predators now, they had no chance to survive^[21]. These are just two examples out of a lot more, but they give us an idea of what was the main reason of the “one-sided battle”^[22].

Secondly, the ephemeral nature of certain habitats in New Zealand over a glacial-interglacial cycle (e.g. lowland grasslands, cold tundra, warm-temperate forest) may have limited the degree to which indigenous species could adapt, which might have given an advantage to continental species that are specialists for such habitats. For instance, a number of cold-adapted continental trees that could grow well above the New Zealand tree line may have the potential to exclude the indigenous tussocks and low shrubs currently growing there^[23].

Finally, the total conversion of much of the lowland and montane New Zealand landscape into an open exotic grassland, different in basic ways to any existing in the past, has strongly affected the balance between islander and exotic. Burning and felling of dark, moist forests and their replacement with human-managed grass-lands have effectively created a drier, warmer and more fire-prone prairie or steppe environment with greatly elevated soil fertility which was essential for the upcoming agricultural society of early New Zealand^[24]. Even nowadays, the difference between what New Zealand was like before the arrival of humans and what huge parts of it are like now is still visible if you look at the biota of one of the many national parks that cover most of the country, where the ancient rainforest is still prevailing.

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Fig. 5 Tussock grassland near Mt Ngauruhoe

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Fig. 6 Rainforest at Lake Tarawera

Those three aspects combined, in the case of many indigenous birds, reptiles and giant invertebrates, have meant rapid extinction or reduction to small, protected enclaves. Although mammalian herbivore pressure has reduced many indigenous plants over wide areas, the overall influence has not been as fatal as on New Zealand’s endemic fauna^[25].

Summarizing all aspects mentioned before, we can say that New Zealand’s discovery by humans was probably the worst thing that could possibly happen to its endemic biota. Not only habitat destruction, through deforestation, wetland drainage and ecosystem degradation, but rather the introduction of invasive alien species to the country has led to the extinction of 16 land birds, one endemic bat, one fish, at least a dozen invertebrates, and ten plants within a period of about 200 years^[26]

[...]

^[1] Shah

^[2] Shah

^[3] McGinley

^[4] McGinley

^[5] McGinley

^[6] McGinley

^[7] McGinley

^[8] McGinley

^[9] Conservation International

^[10] Haast’s Eagle

^[11] Conservation International

^[12] Darby 142

^[13] Darby 142

^[14] Darby 143

^[15] Darby 144

^[16] Darby 145

^[17] Darby 144

^[18] McDowall 19

^[19] Allen 26

^[20] Allen 26

^[21] Allen 28

^[22] Allen 26

^[23] Allen 28

^[24] Allen 28

^[25] Allen 26

^[26] McGinley