Island Biogeography 182 version
Biodiversity is a measure of the amount of variety in organisms, generally the variety in an
ecological community. Biodiversity takes into account both the total number of different species
(species richness) and the relative numbers of different species (species abundance). Biodiversity
tends to increase with habitat size, environmental variation (for example, topographic relief or
climatic microclimates), and latitude (for example, there are more species in the tropics than at
cooler climates).
Biogeography is the science that attempts to document and understand spatial patterns of
biodiversity. In studying biogeography researchers seek to understand distribution patterns of
organisms and the processes of evolution, dispersal and extinction that result in those patterns.
The field of island biogeography is the brainchild of Robert McArthur and Edward O. Wilson.
The basis of island biogeography supposes there is a large source area of species and surrounding
the source area is a series of islands of different sizes and distances from the source area. Species
disperse from the source area to the islands. Thus, the biodiversity found on an island is a
function of (1) how close the island is to the mainland, and (2) how large the island is. As you
might imagine, larger islands tend to have more species than smaller islands because there is
greater habitat diversity and, therefore, more resources available. In the theory's simplest form
species from the source area disperse to an island at a particular rate (the immigration curve),
depending on the island’s size and distance from the source area.
Biodiversity on island habitats is thought by many researchers to reach an equilibrium that is
maintained by the new arrival of some species (immigration) while other populations are
disappearing from the islands (extinction). Some of the most interesting research on biodiversity
has examined species richness on island habitats (e.g., Simberloff and Wilson 1970).
Though extinction of local populations may occur, dispersing individuals from other habitat
sources can reintroduce the same species. This is referred to as the rescue effect. The rescue
effect can be important in maintaining higher levels of biodiversity in smaller habitats than
expected when nearby source populations of species can disperse to these habitats. The rescue
effect is thought to be closely linked with proximity and dispersal abilities of source populations.
This has implications for conservation biology, since the closer two or more populations are to
each other the less likely that any of them will suffer extinction, as population A may be rescued
via migration from individuals in population B.
Though studies such as MacArthur and Wilson’s original model were for islands, ecologists have
found the same trends on mainland habitqats. Mainland habitats oftern have large environment
variations within a single ecosystem, creating mosaics fo ecological island-like habitat patches.
Also, further alterations of habitats by humans, now estimated to have greatly altered at least 50%
of all terrestrial ecosystems, has fragmented many formerly large and continuous habitats. These
anthropomorphic (human induced) alterations increase island-like separations of mainland
populations, which pose dispersal problems and reductions and extinctions for many species.
Dispersal abilities vary widely from species to species. A formibable barrier to dispersal for one
species (ex. Snails crossing roads), may be negligible to other species that can cross mountains or
oceans without much difficulty (ex. Many birds).
