Biodiversity on Islands (III): Extinction / Biodiversidad en Islas (III): Extinción

by  Nuria Serrano Vinagre

This is the last post of the series about Biodiversity on Islands. After talking about Island characteristics and Island Biogeography, let’s turn to Extinction on islands.

Cíes, by Anxo Resúa on Flickr (some rights reserved)

The rate of extinction on islands is very high; for example the 93% of birds’ extinctions since XVII century were on islands. The high percent of extinctions is explained because of the fragility of the ecosystem. That’s present on:

·         Small populations

·         Unstable demography

·         Genetic problems

·         Extreme environmental conditions

Although there were cases of extinctions by natural causes, during the last 500 years the main responsible is the human, direct or indirectly.

The only recorded case that is explained by natural causes is the example of St Kitts (West Indies): in 1899, 2 hurricanes caused the extinction of a subspecies of Puerto Rican Bullfinch (Loxigilla portoricensis).

In contrast, to show you one example of the human effect on islands biodiversity, let’s see the case of Panama Canal: since its construction in 1914, many new islands lost biodiversity, for example Barro Colorado Island (Panama) has lost 20% of birds.

There are 5 main causes of extinction on islands. 

1. Habitat loss, by direct destruction (for example, deforestation) or by introduction of species. Some of the most destructive introduced species were Rabbits (Oryctolagus cuniculus), Goats (Capra pyrenaica) and Sheep (Ovis aries), which are carried by sailor in their boats as power supply; destroyed whole islands during Colonization. Their high metabolic rate and the absence of competitors and predators (on the islands where they arrived) are the reasons for their success. 

2. Competition among different species

3. Entrance of pathogens

These last two are some of the most important causes of extinction. Moreover, in most cases they are result of introduced species. This is the example of Blue Lorikeet (Vini peruviana), a small lorikeet from French Polynesia and the Cook Islands, who is endangered primarily by Cats (Felis catus), Rats (Rattus sp.) and avian malaria carried by mosquitoes.

4. Hunting is another cause, especially with very rare and exotic species.

One of the saddest example of hunting is the case of Dodo (Raphus cucullatus), from Mauritius Islands, a flightless bird extinct about 1662. When humans settled there at the beginning of XVII century, Dodos were fearless of humans: they haven’t meet predators before. Colonizers picked up them and killed them because of their meat. At the same time, humans destroyed the Dodo's habitat forest and introduced competitors, especially the Pigs (Sus scrofa) and Macaques (Macaca sp.). These two impacts had on the Dodo population are actually considered as more severe than hunting; anyway, humans are the responsible for this extinction. 

5. Predation

Maybe, together the habitat loss; the most devastating cause of extinction on islands is the predation. Most cases, as well as it occurs with pathogens and competitors, predators are introduced species which do away with native species on islands without predators before. Thereby, the animal who is responsible of most extinctions on islands, specially on Pacific; is the Rat (Rattus sp.), who attacks eggs of birds and chicks.

In Spain, a serious problem is the case of the American Mink (Neovison vison) on Cíes Islands, on the Atlantic Islands of Galicia National Park. The American Mink is escaping from farms, and reaching islands by swimming. There, they attack many birds, specially the European Shag (Phalacrocorax aristotelis).

That ends the third part of the series about Biodiversity on Islands.

To sum up, these are the main ideas:

•  The special characteristics of island ecosystems are consequence of isolation and its origin (Continental vs. Oceanic islands)
• The distribution of species on islands depends on the size and the distance to the mainland.
• Humans cause (direct or indirectly) the high extinction patterns on these delicate ecosystems.

Understanding the function of Islands Ecosystems and the human impacts on them must be the first step to develop an efficient politic of conservation.

Bibliography and links:

• Case, T.J. 1978. A general explanation for insular body size trends in terrestrial vertebrates. Ecology, 59:  1–18.
• Crosby, A. W. 1986. Ecological Imperialism: the Biological Expansion of Europe 900–1900. Cambridge University Press.
• Lombardero, X. 2009. El visón americano ya está por toda Galicia y amenaza las aves acuáticas. La Voz de Galicia: 05/12/2009.
• Margalef, R. 1993. Teoría de los sistemas ecológicos. Universitat de Barcelona.
• Pianka, E. R. 1978. Evolutionary ecology. Harper & Row.
• Quammen, David. 1997. The Song of the Dodo: Island Biogeography in an Age of Extinctions. Simon & Schuster, New York.
• Sax, D. F. & Gaines S. D. 2008. Species invasions and extinction: The future of native biodiversity on islands. PNAS, 105: 11490-11497.
• Wihttacker, R. J.,  Fernández-Palacios, J. M. 2007. Island Biogeography: ecology, evolution and conservation. Oxford University Press.
• Vigo, R. S. 2012. La Xunta espera erradicar 50 visones en Islas Cíes y Sálvora. Atlántico: 06/04/2012.