Island restoration

Islands, endemism and extinction

Isolated islands have been known to have greater levels of endemism since the 1970s when the theory of island biogeography, formulated by Robert MacArthur and E.O. Wilson, was developed. This higher occurrence of endemism is because isolation limits immigration of new species to the island, allowing new species to evolve separately from others on the mainland. For example, 71% of New Zealand's bird species (prior to human arrival) were endemic. As well as displaying greater levels of endemism, island species have characteristics that make them particularly vulnerable to human disturbance.

Many island species evolved on small islands, or even in restricted habitats on small islands. Small populations are vulnerable to even modest hunting, and restricted habitats are vulnerable to loss or modification of said habitat. More importantly, island species are often ecologically naive, that is they have not evolved alongside a predator, or have lost appropriate behavioural responses to predators. This often resulted in flightlessness, or unusual levels of tameness. This made many species susceptible to hunting (it is thought, for example, that moas were hunted to extinction in a few short generations) and to predation by introduced species. Some, such as the dodo, are thought to have become extinct because of the pressure of both humans and introduced animals. One estimate of birds in the Pacific islands puts the extinctions at 2000 species. Between 40 and 50% of the bird species of New Zealand have become extinct since 200AD.

The field of island restoration is usually credited with having been started in New Zealand in the 1960s, but other smaller projects, such as the restoration of Nonsuch Island in Bermuda (which began in 1962) have been going on for almost as long. Nevertheless, the program undertaken by the Department of Conservation (DOC) is one of the largest in the world. It began on Cuvier Island, where ecologists removed stock, goats, feral cats and finally, in 1993, Pacific rats. The success of the project resulted in similar projects around New Zealand. The advantages to the DOC were considerable; in addition to protecting species endemic to smaller islands, like the magenta petrel, islands near the mainland, once restored, could act as habitat for species of birds that were unable to survive on the mainland. Species like the takahē, where the remaining wild population was at considerable risk from feral cats and dogs, could be moved to these islands to safeguard the species.

Eradication of introduced alien species

One important aspect of island restoration is the removal of invasive alien species. Since these species are most often the reason that native fauna and flora is threatened, their removal is essential to the restoration project. From 1673 until 2009, 786 successful invasive vertebrate eradications have been recorded and in the last few decades the frequency of eradications and the size of islands from which invasive vertebrates have been eradicated has increased. A definitive list of past island restoration efforts exists as the Database of Island Invasive Species Eradications. In addition a list of the current invasive species present on the world's islands exists as the Threatened Island Database

Islands are particularly suitable for restoration as once cleared of an introduced species they can be kept cleared of these species by virtue of being an island. Species removal is intensive and expensive, and methods used must be carefully chosen as to not create too much impact on non-target species. Feral cats, goats and three species of rats are among the most damaging species introduced to islands (Moors & Atkinson 1984). The differences in size, lifestyle and behaviour preclude the use of the same techniques for all of them, but with many species a range of techniques needs to be used in order to ensure success. Larger animals, such as goats and pigs, can be effectively hunted; in the case of Round Island, in Mauritius, all the goats were eliminated by a single marksman. On larger islands ecologists use a Judas goat, where a radio collared goat is released into the wild. This goat is then followed and groups it joins are removed. To remove cats a combination of techniques is needed: hunting, trapping and poisoning. Cats are more difficult to hunt than goats and pigs, requiring the use of experienced hunters and night hunting.

Trapping is ineffective for rats, given their sheer numbers, and the only method that works is poisoning, which can be delivered into the field by broadcasting (by hand or from the air) or by the maintenance of bait stations. This method has been employed around the world, in the Falkland Islands, in the tropical Pacific, and off New Zealand, where over 40 islands have been cleared. This method is not without problems, especially if the rats share the island with other, native species of rodent that might take the bait as well, as has happened on Anacapa Island in the Channel Islands and Rat Island (Hawadax) in the Aleutian archipelago. In the Pacific poison intended for rats was taken by land crabs instead; the crabs were not affected by the poison but frustrated attempts to clear the rats.

The removal of invasive weeds is, in most cases, more difficult than removing animal species. One such eradication was that of sandbur, Cencrus echinatus, an introduced grass on Laysan. The grass, introduced to Laysan around 1961, had taken over 30% of the island by 1991, displaced the native bunchgrass, and reduced the breeding habitat of two endemic threatened species, the Laysan duck and Laysan finch, as well as those of several seabirds. The removal took ten years, with controlled spraying for the first year, then individual removal of plants, then, when few plants were being found, sifting of the sands around plants to remove seeds. The cost of the eradication program was $150,000 per year.

Invasive vertebrate eradication has many benefits besides conservation of species and has been found to align with 13 UN Sustainable Development Goals and 42 associated targets encompassing marine and terrestrial biodiversity conservation, promotion of local and global partnerships, economic development, climate change mitigation, human health and sanitation and sustainable production and consumption.

Strategies

There are three strategies to reduce the numbers of the introduced animals in order to lighten the problems caused by them: exclusion, control, and eradication. Exclusion is removal of introduced species in limited areas and focused on a local impact. Control has a meaning of mitigation by reducing the numbers of introduced species down to ecologically and economically less harmful level. Since it is not complete wiping out strategy, this has to be taken constantly and repeatedly. This strategy is said to be feasible but less cost effective due to its incompleteness. Eradication is the complete removal of all the individuals of the population, down to the last potentially reproducing individual, or the reduction of their population density below sustainable levels (J. H. Myerset al., 2000). For islands, it is usually the best strategy as it gives permanent effects, which potentially means most cost effective. However, it is still logistically and economically high cost. Also, this strategy is sometimes hard to accomplish depending on the environment of island and the alien species. Island Conservation exclusively focuses on the eradication of invasive vertebrate species from islands. Up until 2021, the organization had deployed teams to protect 1,195 populations of 487 species and subspecies on 64 islands.

Methods

Fencing is used for excluding the alien mammals. This method intends to limit the area for the mammals before control or eradication. Shooting is often used for large animals to control them. This method has meanings of gaining food supply and recreation. It requires accessibility in the field and experience for the hunters. Also, to maintain the effect, it requires good amount of manpower for long time, so it can be costly. Trapping is designed for medium-sized mammals which are hard to shoot at. This method is usually only for reduction not eradication. The advantage of this method is that it can be selective since it is possible to exclude or reduce the possibility of trapping native animals. However, trapping requires limited area to cover and limited population to capture.

Poisoning is very effective for small species. However, there are some disadvantages of this method. It is possible that untargeted animals take the poison. It is also necessary to consider the secondary poisoning that other animals are affected by poisoned species. This method can be costly if the area to cover is large.

Pathogen Introduction (parasitism) is one of the biological methods to eradicate alien species. It is very effective for limited species, but the viruses and bacteria need to be specified clearly for use.

Predator introduction is another biological method to eradicate the introduced species. It is less costly and environmentally clean, but it can cause greater problems, because it is possible that the introduced predator targets the native animals instead of alien species and its existence and its parasites can be new problems.

Competitor introduction is also a biological method to eradicate introduced carnivores. It can be very effective with good amount of information. In the end, the competitor has to be removed as well. Virus vectored immune-contraception is one of the newest method that is to infect introduced animals with genetically engineered viruses. This method is considered environmentally clean, low cost, selective, and ethical. However, it is not fully operational and the effect comes slowly.

Restoration of former habitat

In many cases the removal of introduced species is sufficient to allow a return to a pre-disturbance state, but generally active management, often in the form of replanting native flora and reintroduction of extirpated fauna is needed to achieve restoration goals. Planting of native species helps to replenish species that were either grazed or out competed. Species of animal can be translocated either from existing populations, or from captive bred populations. These reintroductions need to be carefully managed, particularly in the case of endangered species, where the potential benefits need to be weighed against the possibility of failure. Not all translocations succeed, and it may be necessary to help the reintroduced animals along with supplementary feeding or other kinds of management.

One other important aspect of restoration is prevention, that is, keeping invasive species from returning to a cleared island. This can be achieved by restricting access to the island in question (reducing possible instances of invasion) to more stringent quarantine methods. For example, in order to prevent invasive weeds from returning to Laysan, people working on the island must bring entirely new clothes to the island, which must be frozen prior to arrival.