Island biogeography

The study of island biogeography is a field within biogeography that attempts to establish and explain the factors that affect the species diversity of a particular community. In this context the island can be any area of habitat surrounded by areas unsuitable for the species on the island; not just true islands surrounded by ocean, mountains surrounded by deserts, lakers surrounded by dry land, forest fragments surrounded by human-altered landscapes. The field was started in the 1960s by the ecologists Robert MacArthur and E.O. Wilson, who created the theory of island biogeography. This theory attempted to predict the number of species that would exist on a newly created island.

Theory of Island Biogeography

The theory of island biogeography holds that the number of species found on an island ( the equilibrium number) is determined by two factors, the effect of distance from the mainland and the effect of island size. These would affect the rate of extinction on the islands and the level of immigration.

Islands closer to the mainland are more likely to receive immigrants from the mainland than those further away from the mainland. The equilibrium number of an island close to Africa is going to be larger than that of one found in the mid-Atlantic. This is the distane effect. The size effect reflects a long known realtionship between island size and species diversity. On smaller islands that chance of extinction is greater than on larger ones. Thus larger islands can hold more species than smaller ones. The play between these two factors can be used to establish how many species an island can hold at equilibrium.

The theory of island biogeography was tested by Wilson and his student Daniel Simberloff in the mangroves off Florida. Small islands of mangroves were surveyed then fumigated with methyl bromide to clear their insect and arthropod communities. The islands were then monitored to study the immigration of species to the islands (the experimental equivelant of the creation of new islands). Within a year the islands had been recolonised, and had reached equilibrium, with islands closer to the mainland having more species, as predicted.

Island Biogeography and Conservation

Within a few years of the publishing of the theory its application to the field of conservation biology had been realised and was being vigorously debated in ecological circles. The realisation that reserves and national parks formed islands inside human-altered landsacpes, and that these reserves could lose species as they 'relaxed towards equilibrium' (that is they would lose species as they achieved their new equilibrium number) caused a great deal of concern. This is particularly true when conserving larger species which tend to have larger ranges. A study by William Newmark, published in the journal Nature and reported in the New York Times, showed a strong correlation between the size of a protected and the number of species of mammals in National Parks in the U.S.. This led to the debate known as single large or several small (SLOSS), described by writer David Quammen as 'ecology's own genteel version of trench warfare'. In the years after the publication of Wilson and Simberloff's papers ecologists had found more examples of the species area-realtionship, and conservation planning was taking the view that the one large reserve could hold more species that several smaller reserves, and that larger reserves should be the norm in reserve design .This view was in particular championed by Jared Diamond. This led to concern by other ecologists, including Dan Simberloff, who considered this to be an unproven over-simplification that would damage conservation efforts. Habitat diversity was as or more important than size in determining the number of species protected.

References

The Song of the Dodo Island Biogeography in an Age of Extinctions by David Quammen