Insular biogeography

The study of island biogeography is a field within biogeography that attempts to establish and explain the factors that affect the species richness 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, but also mountains surrounded by deserts, lakes surrounded by dry land, forest fragments surrounded by human-altered landscapes. The field was started in the 1960s by the ecologists Robert MacArthur(he preffered not to be called bob) and E.O. Wilson, who coined the term theory of island biogeography, as this theory attempted to predict the number of species that would exist on a newly created island.

Theory

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 farther 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 distance effect. The size effect reflects a long known relationship between island size and species diversity. On smaller islands the 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 equivalent 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.

Research conducted at the rainforest research station on Barro Colorado Island has yielded a large number of publications concerning the ecological changes following the formation of islands, such as the local extinction of large predators and the subsequent changes in prey populations.

Factors that Influence Island Communities

Degree of isolation (distance to nearest neighbor, and mainland)
Length of isolation (time)
Size of island (larger area usually facilitates greater diversity)
Climate (tropical versus arctic, humid versus arid, etc.)
Location relative to ocean currents (influences nutrient, fish, bird, and seed flow patterns)
Initial plant and animal composition if previously attached to a larger land mass (e.g., marsupials, primates, etc.)
The species composition of earliest arrivals (if always isolated)
Serendipity (the impacts of chance arrivals)
Human activity

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 landscapes (habitat fragmentation), 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, known as ecosystem decay) 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 National Park -in the U.S.- and the number of species of mammals.

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 relationship, and conservation planning was taking the view that the one large reserve could hold more species than 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.

Island biogeography theory also led to the development of habitat corridors as a conservation tool to increase connectivity between habitat islands. Habitat corridors can increase the movement of species between parks and reserves and therefore increase the number of species that can be supported.

In species diversity, Island Biogeography most describes allopatric speciation. Allopatric speciation is where new gene pools arise out of natural selection in isolated gene pools. Island Biogeography is also useful in considering sympatric speciation, the idea of different species arising from one ancestral species in the same area. Interbreeding between the two differently adapted species would prevent speciation, but in some species, sympatric speciation appears to have occurred.

References

MacArthur, R. H. and Wilson, E. O. 1967. The Theory of Island Biogeography. Princeton, N.J.: Princeton University Press.
Newmark, W. D., A land-bridge island perspective on mammalian extinctions in western North American parks, Nature, 325, 430 - 432 (29 January 1987)
David Quammen. 1997. The Song of the Dodo: Island Biogeography in an Age of Extinctions. Scribner. ISBN 0-684-82712-3
Allan A. Schoenherr, C. Robert Feldmeth, Michael J. Emerson. 2003. Natural History of the Islands of California. University of California Press.