Invasive alien species: the biggest threat to Pacific biodiversity

• What is an invasive species?
• Effects of invasive species
• How did they get there?
• What can we do about them?
• SPREP's work on invasives

The Pacific is biologically unique, because the isolated islands provide ideal conditions for the evolution of new species. As a consequence, Pacific islands have high numbers of “endemic” species - species that are restricted to only one or a few islands and found nowhere else in the world. Birds and plants illustrate this outstanding biological uniqueness – the Pacific has more than 400 endemic bird species, while about 30% of the native plant species are endemic.

Many of the unique plants and animals of this region are amongst the most endangered in the world, mainly because the tiny sizes of most of the islands also means the total populations of many species are naturally very small, which makes them especially vulnerable to any disturbance. For this reason, most of the recently extinct species were from islands. The Pacific currently has about 25% of the world’s threatened bird species and has already lost many species.

One of the key threats to species and ecosystems worldwide is land clearing or habitat loss. But on islands, invasive species pose an even greater risk.

What is an invasive species?

Some species arrived naturally on islands, such as by flying or floating there, or being carried as seeds by birds. These are “native” species, and they arrived only occasionally, over millions of years, after islands emerged from the sea. Rates of arrival of native species are very slow — often in the order of one species every 10,000 years. Since the arrival of humans on Pacific islands, other species have been carried there by people, either deliberately (as food, or timber, or ornamentals etc) or accidentally (such as pest insects). These are “introduced” species, sometimes also called “alien” or “exotic” or “non-native” species. The rate of introduction by people is much faster than the natural rate of arrival — typically more than one species per year, or 10,000 times the natural rate. Many introduced species are useful, and most of them do not cause serious environmental problems. However, some of them get out of control and can cause enormous ecological, economic or health problems. These are called “invasive” species, also known as “pest” species.

Invasive species are usually highly adaptable. They can live in a wide range of environments. They breed fast, spread easily, and quickly become widespread. When they arrive in a new country, they have usually left the diseases and predators that would have kept their numbers under control back in their home country.

One infamous example is the Brown Tree Snake, which was introduced to Guam accidentally in the late 1940s. Its introduction has resulted in the extinction of nine of the eleven original native bird species in Guam, along with five species of lizard. There are an estimated 80 million Brown Tree Snakes on Guam today, and by climbing on wires they cause power cuts every 4 –5 days, damaging electrical infrastructure and household appliances. The snakes also impact on health, as their bites pose a risk especially to children. The Brown Tree Snake's impacts result in research and control costs totalling over $US 5 million a year in Guam alone. This does not include the costs to Guam’s major trading partners who have to ensure that snakes hitchhiking in goods or on aircraft or ships from Guam are detected before they can establish new populations.

Invasive species can come from any group of living things, including plants, rats, mongooses, ants, snails, mosquitoes and disease agents. There are also invasive birds such as mynas, and invasive aquatic species, both freshwater and marine. Invasive plants include trees, shrubs, grasses, and vines, which are usually the top species on agricultural weeds lists.

Effects of invasive species

Invasive species have a range of effects on the environment and on humans. They threaten many species with extinction. They interfere with the species that make up natural ecosystems and change the way they function. They have negative impacts on the resources people rely on to live – food, clean water, and shelter. They carry diseases and can directly harm humans. They can impact on species we rely on for our livelihoods such as crops and farm animals. Invasive vines smother forest canopies, reducing the production of flowers and fruits which fruit bats and native birds depend on, and causing heavy losses in commercial forestry plantations. Some of them even damage buildings, bridges and other structures, or can reduce the tourist potential of an area by damaging the environment and other attractions. They obviously can have a great impact on Pacific islanders' traditional activities and modern livelihoods.

Of all animals that have become extinct since 1800, 90% were island birds, and of these, 90% are considered to have fallen victim to invasive species. Many endemic bird species are in trouble in the Pacific, some directly threatened by predators such as rats, cats and mongooses, others are threatened indirectly, by loss of habitat and food such as by introduced vines smothering their forests. Rats eat eggs and young birds, especially of ground breeding species.

An example of the economic impacts of invasives was seen in Samoa in the 1990s, when taro leaf blight, a fungal disease, arrived and devastated production of taro, a key part of the Samoan diet and economy. This is estimated to have cost Samoa more than the impact of three cyclones, $US 40 million, in lost domestic consumption, lost exports and the cost of measures to control the disease.

Regardless of where we live, invasive species can impact on us all. Every Pacific country has invasive species that cause problems and is at risk of getting new ones.

How do they arrive and spread?

Some invasives were introduced deliberately by people as food, or for medicinal or other uses. Some are introduced as pets or ornamental plants. Pacific islanders are great gardeners, and many plant invaders were originally introduced as ornamentals. A few species, such as mongooses, have been deliberately introduced in attempts to control other invasive pests. This is biological control, which, if done carefully, can be an effective method to control certain invasive species. However, if not carefully checked first, species introduced for biological control can become problems themselves.

Among the many accidental introductions, plant pests, ants and diseases have been introduced to many islands as contaminants of fruits, vegetables, soil, plants, timber and commercial feed, while others get around by hiding in ships’ cargo or on aircraft.

The rate of accidental introductions is increasing, as there is more movement of people and goods around the region. Quarantine often exists at the international borders of countries but not between islands within the country, so controls often do not prevent spread between islands within an archipelago. Increased movement increases the risk of invasion of new islands. Invasives can be carried in cars, on military equipment or used machinery, and in personal effects such as hiking boots and camping equipment. Yachts pose a risk in the Pacific. Over 2000 yachts visit the Vava'u islands of Tonga each year. Invasive species can be found in the ballast water of ships or clinging to the their hulls. The risk is particularly high for relatively natural uninhabited islands.

What can we do about them?

There are five steps that can be taken depending on the invasive situation.

1. Prevention is definitely better than cure — it is cheaper and usually easier to keep something out than to control it once it has established. It is also more effective at preventing impacts — excluding a pest results in no impacts of it. So exclusion by quarantine is the first line of defence.

2. Eradication. However, once a species has reached an island it must be managed. The next best option to preventing a species arriving is to eradicate it entirely from an island. This can often be done if the species is detected quickly enough after it arrives, or before it has spread very widely. Eradication is therefore the next step, where feasible. The two main advantages of eradication are that it reduces the pest impact to zero; also the costs of managing the pest reduce to zero once it has been completely eradicated.

If the pest has already spread too widely for eradication to be an option, then its population can be kept under control in various ways.

3. Containment or exclusion means preventing the pest from spreading out of or into a defined area. This can be used to keep important (but invasive) crop species from escaping from farmland, or to keep invasives from spreading into nature reserves or other natural areas.

4. Site-specific control means keeping the pest’s population below a certain level in defined areas, such as reserves or other natural areas, usually by using manual, mechanical or chemical methods of control.

5. Biological control means introducing a natural enemy of the pest, such as a predator or disease of it, to control its population. Poorly planned biocontrol, such as introducing mongooses and other organisms that attack a wide range of prey or hosts, has caused enormous problems on some islands, but properly researched biocontrol using carefully selected agents that attack only the target species and nothing else, can sometimes bring serious pests under control without causing additional problems. An advantage of biocontrol is that once established, the control agent often maintains itself, and no further cost is incurred in further control.

SPREP's work on invasives

The Pacific is a leader in using a regional approach to address invasive species. The Guidelines for Invasive Species Management in the Pacific, developed by SPREP at the request of and with the collaboration of its member countries, and endorsed by them in 2008, provides a framework for national and regional efforts to manage invasive species. The Pacific has also developed two networks for improving invasive species management in the region: the Pacific Invasives Partnership (PIP) is a coordinating body for international agencies that provide services to Pacific countries and territories, and the Pacific Invasives Learning Network (PILN) is a network for invasive species workers in the countries and territories themselves. SPREP coordinates both networks. SPREP also develops and manages multi-country invasive species projects, and assists its member countries and territories to develop their invasives programmes and projects, such as by providing expertise and access to funds.

Updated (21 April 2010)

Alan Tye, Invasive Species Adviser, SPREP