Invasive Alien Species

Islands and invasive alien species

Islands, such as Australia, New Zealand, Madagascar, the Hawaiian Archipelago, and the Galapagos, have a high proportion of endemic and specialized flora and fauna. The geographic isolation of islands limits immigration of new species, allowing established species to evolve with few strong competitors and predators.

Invasive alien species introduced by human activity have some of the most dramatic effects on isolated ecosystems such as islands and are a leading cause of species extinctions. Islands are more prone to invasion by alien species because of the lack of natural competitors and predators that control populations in their native ecosystems. In addition, islands often have ecological niches that have not been filled because of the distance from colonizing populations, also increasing the probability of successful invasions.

Invasive alien species pose a particular risk to small island developing States (SIDS) by threatening the ecosystems, livelihoods, economies and public health of inhabitants. Increased trade, tourism and transportation are significant vectors, and the most common pathways are ship ballast water, hull fouling, cargo containers and packaging materials, unprocessed commodities such as timber/agricultural goods, imported food species such as fish, horticultural/plant imports, waste material, military activities, and biological agents to combat pests.

The equilibrium theory of island biogeography

The equilibrium theory of island biogeography1 explains major factors that affect species diversity on islands and other isolated habitats (e.g., mountains surrounded by deserts, lakes surrounded by dry land, forest fragments surrounded by human-altered landscapes) under natural conditions. The theory of island biogeography states that species richness is maintained by equilibrium between opposing rates of colonization and speciation, and of extinction. This equilibrium between extinction and immigration rate is determined by two major factors: distance from the mainland and island size. Larger islands can support more species and have lower extinction rates than small ones because they cover larger areas, with a greater diversity of habitats and resources. Less isolated islands tend to support more species than remote ones, because they have higher rates of immigration. Many field experiments support this theory, however there are also exceptions.

Organisms that are capable of colonizing islands naturally are typically adapted to be dispersers, are small and numerous in their original habitat and have a propagule (seed, larva, spore, etc.) that can be dispersed over water, through wind or vectored by an animal from the mainland. Humans have introduced alien species that would not have been able to immigrate to islands under natural conditions; bearing little resemblance to island biota, their effects on “native” species are highly unpredictable. The most damaging invasive species to islands are rats, feral cats and mammalian herbivores.

Invasive plants and animals introduced by humans often do not follow island biogeography predictions. Invasive alien species frequently have effects that far exceed what would be predicted by the equilibrium theory of island biogeography. For example, alien species are entering Hawaii about 2 million times faster than the natural rate2. Immigration by a single alien species may cause numerous extinctions and drastically alter the physical environment. For examples see the Crazy Ant, the Brown Tree Snake, the Strawberry Guava, Miconia, Small Indian Mongoose (etc…) in IUCN’s 100 of the World’s Worst Invasive Species. Also see the presentation on invasive species “Islands biodiversity program: New Zealand case study” by the Department of Conservation.

1MacArthur R. W. & Wilson E. O. (1967). The Theory of Island Biogeography. Princeton University Press, Princeton.
2Hawaiian Ecosystems at Risk project (HEAR)

Island birds and invasive species

Invasive species of animals, plants and disease-causing microorganisms are a particular threat to island birds and are the most common contributory factor to global avifauna extinction. In the last 500 years, invasive alien species have contributed to the extinction of nearly half of global bird extinctions, mostly by introduced rats, cats and diseases3.

Small island species are most at risk from invasive alien species: 67% of globally threatened birds (GTB) inhabiting oceanic-islands are affected, compared to 30% of GTB affected by invasive alien species on continents. Over half of the endemic birds of the Hawaiian islands are now extinct, due to habitat loss, introduced predators and diseases. The distribution and abundance of native forest birds is limited by diseases such as avian malaria and small pox, from the introduction of several mosquito species3.

3BirdLife International

What can be done?


Prevention of introductions is the most cost-effective method against invasive alien species. Governments conduct customs checks, inspect shipments and set quarantine regulations to try to limit the entry of invasive species. The most common method for prevention is to target individual species; however, identifying major pathways that lead to harmful invasions is a more comprehensive approach. It is also important that the general public be educated to make informed decisions about how to limit introductions and their spread.


Eradication is often feasible for islands because they are confined to a limited geographic area, especially in the early stages of an invasion. With planning, adequate techniques and continuous efforts, it is possible to eradicate many types of invasive species. The most successful eradications have been for mammals, which are often the most damaging invasive alien species. However, plants and insects can be just as damaging and more challenging to eradicate.

Eradication is generally more environmentally sound and ethical than long-term control, which may involve the sustained use of toxins, trapping or shooting. Control can involve increased environmental risks and more animal deaths than a short eradication campaign.

Conditions for success include:
• Proper planning
• Commitment to complete eradication
• Removing the target species faster than they reproduce
• Preventing re-invasion

The ability to demonstrate benefits of eradication and gaining support from local people is often necessary for a successful eradication program.

For more information see Turning the tide: the eradication of invasive species (Proceedings of the international conference on eradication of island invasives; Occasional Paper of the IUCN Species Survival Commission No. 27. Veitch, C. R. and Clout, M.N., eds. 2002).


Eradication is often sufficient to allow the return of the original native wildlife. However, active management, such as replanting of native flora and reintroduction of fauna is frequently necessary to fully restore a damaged area. One an area is restored, prevention is also required to keep an invasive species from returning to the island.

New Zealand’s Department of Conservation (DOC) has one of the largest restoration programs in the world (see link to presentation above).

See What needs to be done on the Invasive Alien Species Portal.

See Partners for a list of organizations and initiatives implementing the Island Biodiversity programme, under Goal 6 (Invasive Alien Species).

For more information on invasive alien species see the Invasive Alien Species Portal; Experiences, Case Studies, and Assessments in the Australia and Pacific region; and Expertise.