Ecological restoration
Why New Zealand islands are important
Why restoration is possible
The three “R’s” of island restoration
The first NZ restoration project: Cuvier Island
Who undertakes restoration?
How restoration is planned
Who benefits from restoration?
Summary: why ecological restoration on islands is important
When Polynesians arrived in Aotearoa/New Zealand many centuries ago, the three large islands (North Island, South Island and Stewart Island) and hundreds of smaller islands, were inhabited by an abundance of birds, tuatara, lizards, and many large flightless insects. The first people also brought the first wave of pest species and their fires began the process of habitat modification. A second, larger number of pests were introduced by Europeans about 200 years ago. These pests (plants, insects, birds and mammals), combined with forest clearance and modification of wetlands and rivers, have caused the extinction of 40% of our native birds, 43% of our native frogs, and probably many species of large insects. The losses were most severe on the North and South Islands, but many species of pests were also either deliberately or accidentally introduced to offshore islands.
Fortunately, introduced pest species were unable to reach a few islands where some birds, tuatara, many lizards and insects and a few plants survived. Birds once common on the North and South Island, but now either confined to islands or only breeding on islands, include saddleback, stitchbird, little spotted kiwi and kākāpō; lizards include the large Duvaucel’s gecko, chevron skink and robust skink; insects include several species of giant weta and flightless beetles; plants include milktree and Cook’s scurvy grass.
Introduced species can have serious effects on native species and habitats. Introduced plants may make habitats unsuitable for native animals either by modification of nesting areas or by excluding native plants that are important to insects and birds. Introduced browsers, such as goats, deer, possums and rabbits modify vegetation and this may eventually either affect habitats through increased erosion or promote invasions by introduced weeds. Predators such as rats, cats, stoats, ferrets and some insects, use native species for food, and have wiped out some species that are unable to escape them.
On the North Island and South Island, many problem animal species (such as rabbits, possums and goats) are now controlled or have been eradicated from fenced areas. But continuous control of predators and browsers may be difficult and expensive to continue for long periods as animals eventually move back from surrounding areas. Fences also require constant maintenance and intensive surveillance for damage or breaches.
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A solution to problems with control is possible on many offshore islands where pest species unable to cross water barriers can now be removed permanently (eradicated). The first attempts at eradication were against large animals such as wild cattle and goats.
Predators such as cats have also been removed. Over the last 15 years the smaller and more difficult species have been removed from islands. There have now been many successful eradications of rats and even of mice from islands around New Zealand.
These successes provide new opportunities for conservation on islands:
- They reduce the chance that pests will reach islands where they have never previously been.
- They provide islands where threatened species of plants and animals can return to their former abundance.
- They enable ecological restoration on islands.
Restoration
Restoration aims to reconstruct interacting groups of native plants and animals, and usually requires the return of native species (relocation) after removal of introduced pests (rehabilitation). Restoration therefore involves rebuilding communities of plants and animals on island ecosystems.
Many species of birds and lizards, tuatara and a few species of invertebrates have been returned to islands from which predators have been removed. For example, in the Mercury Islands birds include the saddleback, lizards the threatened Whitaker’s skink and robust skink, and invertebrates the endangered tusked weta.
North Island saddleback being liberated
on Cuvier Island
Relocation
Relocation (also called translocation) has often been used to remove species under threat from introduced predators. The first attempted relocations were made by Richard Henry in the late nineteenth century when newly introduced predators (especially stoats) spread through the South Island. Henry caught hundreds of kiwi and kākāpō and shifted them to Resolution Island in Fiordland. Unfortunately, stoats swam to the island, so these first attempted relocations failed. More recently, kākāpō threatened by wild cats were removed from Stewart Island and relocated to Maud, Codfish and Little Barrier Islands. The relocation of threatened species to islands should not be regarded as restoration if those species had not been on the islands in the past.
Rehabilitation
For many species, island rehabilitation through removal of pests has removed the need for relocation. Rehabilitation provides a way for some species to return to their former abundance on islands where they were almost extinct. In the Mercury Islands, removal of rats and rabbits should help tuatara, one species of threatened sea bird, at least two species of large beetle and many species of plants (including native cucumber and milktree). Many other Mercury Island species are more abundant now that rats have gone. These species are not threatened, but are rarely seen or absent on the mainland. They include at least three species of lizards, six species of sea birds and two species of forest birds. Rehabilitation is therefore one step in the process of restoration.
When naturalists visited Cuvier Island in the nineteenth century they found a forested island somewhat modified by the presence of Pacific rat (kiore) and partial forest clearance, but still with unusual trees (later called milktree), abundant sea birds and numerous forest birds including saddleback, red-crowned kākāriki, pied tit, bellbird and tūī.
In the late nineteenth century, Cuvier was made a lighthouse station. Farm animals, including goats, were introduced as food for the lighthouse keepers, and cats escaped and became wild. By the later 1950’s, the forest had been reduced to an open parkland and there were few sea birds. Saddleback, red-crowned kākāriki, pied tit, tūī and milktrees were extinct on the island and the tuatara population was reduced to seven known animals.
Rehabilitation began in the late 1960’s: first farm stock were fenced out of the forest (then removed from the island in 1981), goats were removed in 1961, cats in 1964 and finally, Pacific rats were removed in 1993.
Restoration began in 1968 with the release of saddleback from Hen Island, red-crowned kākāriki were released in 1974, and tūī have returned naturally. Soon it will be possible to return the tuatara removed from the island in 1991 to protect them from the rats and in time, milktree and other rare plant species may be returned to the island. This restoration project began almost 30 years ago and may continue for many more years. What Cuvier Island will be like in 30 years time is difficult to predict, because there are few islands of its size without introduced predators and a heavily modified flora and fauna. The final composition of the biological communities on Cuvier Island will be determined by the natural processes of change in vegetation (succession) and this in turn will respond to climate (storms and drought). These are the natural interactions between species and their environment that restoration aims to reactivate.
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Who undertakes restoration?
At least 70 island restoration projects are being undertaken by the Department of Conservation and community groups. On some islands school groups are involved (eg Mokoia Island in Lake Rotorua and Limestone Island in Whangarei Harbour), and some are run as partnerships with local support groups (e.g. supporters of Tiritiri Matangi Island and Friends of Mana Island). Some, such as Motutapu Island, include involvement of school groups, voluntary organisations (such as the Royal Forest and Bird Protection Society) and other organisations (World Wide Fund for Nature). A few restoration projects are carried out on islands that are privately owned. One example is extensive plantings of native trees on Great Mercury Island by the Richwhite family.
For restoration to be successful, it needs to be carefully planned. The least complex restoration projects are on small islands that have not been greatly modified and require the return of only a small number of species. The larger and more modified islands become, the more difficult and complex the project becomes. Potential problems may include competition by aggressive species that take all available food or nest sites. Stitchbird releases on Hen and Cuvier Islands were unsuccessful, possibly because of the abundant and more aggressive bellbirds and tūī. Stitchbirds might have established had they been released before the more aggressive species became abundant. Therefore it is often necessary to have detailed information about the ecology of species being returned to an island.
Where islands have been farmed or have had wild browsing animals, other problems may appear. For example, farm animals may have suppressed weed species which spread rapidly once the stock is removed. Some weed species may slow or stop the natural establishment of native vegetation. Predators may also have suppressed other pest species – garden snails have greatly increased in abundance on some islands where they were previously kept at low numbers by rats or mice.
It is therefore also necessary to understand the ecology of potential problem species and how they might respond once browsers or predators are removed.
The most important part of the plan is to ensure that pest species removed from the islands do not find their way back. This may require restrictions on the numbers of people visiting islands, but certainly requires care about how equipment is landed. On many islands, rodent-proof stores are now being built so that all equipment is unpacked in special rooms from which rats and mice cannot escape. Introduced garden snails, ants and cockroaches can cause problems for native species, so groceries landed on these islands also have to be checked carefully. Clean gear and clothing packed in airtight plastic containers helps ensure that insect, animal and weed pests are not accidentally introduced.
As we become better at removing pests and restoring islands we will need to ensure that everyone has a chance to see how the islands improve. This is especially important to many Maori people who have strong links with the islands and in many cases have given them to the people of New Zealand as reserves for native plants and animals.
Some islands that are now being restored can receive visitors. One example is Tiritiri Matangi Island near Auckland, where the public has been involved in planting the island with native trees and shrubs and the release of many species of native birds. Islands such as Tiritiri Matangi will enable New Zealanders to see many spectacular and unusual plants and animals no longer found on the mainland. Public involvement in island restoration therefore helps New Zealanders learn about their natural heritage. Also, by providing some places that people can visit, other islands that cannot withstand public visits can be given a greater level of protection.
Restoration of islands also benefits threatened species. For some species, especially those that live in alpine areas or lakes and rivers, islands do not provide suitable habitats. But for many species of plants, insects, reptiles and birds, islands will not only be their refuge, they will become their stronghold. Birds, such as saddlebacks and the Chatham Islands black robin, have already benefited from restoration or rehabilitation projects on islands.
Importance to conservation
The Department of Conservation aims to restore species within their ecosystems as part of its biodiversity strategy. One way to achieve this is through island restoration. Often restoration on one island will provide solutions for several threatened species at once.
Importance to science
Our islands are unique in many ways. For example, they have many burrowing sea birds and reptiles not found elsewhere. Study of how they recover when browsers or predators are removed can tell us much about island systems. Island restoration can also provide opportunities to study how introduced species affect native ones because islands often have fewer complicating factors than the mainland. For example, removal of Pacific rats from some islands is providing the first detailed information on how rats affect birds, lizards, insects and plants.
Most ecological theories used in New Zealand conservation were developed overseas. Some ideas have never been tested in the wild. Restoration provides a rare opportunity to test theories such as whether small populations of animals or plants can survive for long periods in isolation.
Importance to island’s native biological inhabitants
The rehabilitation and restoration of islands is not just of benefit to threatened species. Many others, common on islands, but now absent or rare on the mainland, benefit from the absence of introduced predators and the wide range of habitats and food species that restoration can make available.
Importance of the islands to people
The Department of Conservation and the scientific community are not the only people to benefit from island restoration. On some islands the public can either participate in restoration or visit the island and observe the way that habitats and species’ abundances change over time. By being involved in these projects, New Zealanders will not only see plants and animals rarely found elsewhere, they will also understand more about how pests have modified environments on the three mainland islands. The lessons learned from islands emphasise why it is necessary to ensure that additional pest species are not introduced into New Zealand.
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