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. So it does not only apply to true islands surrounded by ocean, but also mountains surrounded by deserts, lakes surrounded by dry land, or forest fragments surrounded by human-altered landscapes. The field was started in the 1960s by the ecologists Robert MacArthur 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 rate of immigration and the rate of extinction. These rates are mainly determined by the size of the island and its distance from the mainland.

Islands closer to a mainland are more likely to receive immigrants from the mainland than those farther away. The result is that there are more opportunities for species to establish a new population on the island. This is the distance effect. The size effect reflects a relationship between island size and the ability for a population (or a species) to survive. On smaller islands, species go extinct faster than on larger ones, presumably because they have smaller populations because of the small size of the island. The net effect of these factors is that large islands close to a mainland are colonized more often, and those colonists are more likely to survive than on small islands far away from a mainland. The play between these two factors can be used to establish how many species an island will hold at equilibrium.

The theory of island biogeography was tested empirically by Wilson and his student Daniel Simberloff in the mangroves off Florida. Small islands of mangroves were first completely surveyed to no what species they held. The islands were then fumigated with methyl bromide to clear their insect and arthropod communities. In effect, they became new islands. The islands were then monitored to study the immigration of species to the islands. Within a year, the islands had been recolonised and 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 theory's publishing, its application to the field of conservation biology had been realised and was being vigorously debated in ecological circles. The realization that reserves and national parks sometimes formed a sort of island inside human-altered landscapes (habitat fragmentation), and that these reserves could lose species as they 'relaxed towards equilibrium' (known as ecosystem decay) caused a great deal of concern. These "islands" have a particular tendency to lose larger species, which tend to also 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.

These concerns 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 found more examples of the species-area relationship, and conservation planning was taking the view that larger reserves could hold more species than many 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 could damage conservation efforts. A counter view among some ecologists was that habitat diversity was just as, or perhaps more, important than size in determining the number of species protected.

In terms of generating new species diversity, Island Biogeography mostly assumes allopatric speciation. Allopatric speciation is where new gene pools (i.e., a founding population on an island), diverge from their source gene pool over time through genetic drift or natural selection.

References

• MacArthur, R. H. and Wilson, E. O. 1967. The Theory of Island Biogeography. Princeton, N.J.: Princeton University Press.
• 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.