PATTERNS OF SPECIES RICHNESS ON ISLANDS
A major focus of island biogeography has been to unravel the factors that determine species richness on islands. ftere is a well-known relationship between island area and species richness; in general, larger islands have greater species richness than do smaller islands. But island area alone does not give a complete picture of insular species richness. Compared to less remote islands, very isolated islands have lower diversity than one would predict based only on their areas. fte 19th century botanist Joseph Hooker noted the latter pattern with regard to plant diversity on islands, and since then this pattern has been verified for various animal taxa as well.
In 1967, ecologists Robert MacArthur and E.O. Wilson published their influential book, The Theory of Island Biogeography, in which they proposed that island species richness is determined by rates of species immigration and extinction. ftey reasoned that, as species richness (S ) on an island increases, immigration of new species (I ) decreases because it becomes more likely that any newly-arriving individuals belong to a species that is already present on the island. fte extinction rate (E ), on the other hand, will increase with S because more species means smaller population sizes for each species, increasing the risk of extinction. Immigration and extinction result in continual turnover in the species composition of an island, but the balance of these two factors results in a dynamic equilibrium level of species richness, S* (Fig. 6.4).
Figure 6.4: According to MacArthur and Wilson’s equilibrium theory of island biogeography, species richness (S*) on an island is a result of immigration rate (I ) equaling extinction rate (E ). (Attribution: Shawn Meagher, Western Illinois University)
MacArthur and Wilson then included the effects of island isolation and size on immigration and extinction rates. ftey reasoned that immigration is affected primarily by distance of an island from a mainland source of immigrants, and extinction is determined primarily by island area. Islands that are close to a source of colonizers are more likely to receive immigrants because these islands represent a “closer target;” larger islands are less likely to have extinctions because they have more resources and environmental stability than do smaller islands. ftis means that greater species richness will be found on larger islands near the mainland and less richness on smaller islands far from the mainland (Fig. 6.5). ftus, MacArthur and Wilson’s theory tied together island area and degree of isolation into a predictive model of island species richness.
Figure 6.5: As shown by the figure on the left, larger islands will have lower extinction rates (E ), and therefore greater equilibrium species richness (S*), than will smaller islands. In the figure on the right, we see that islands near a mainland source of colonizers will have greater immigration rates (I ) and greater equilibrium species richness than will more isolated islands. (Attribution: Shawn Meagher, Western Illinois University)
fte theory of island biogeography has had a huge impact on biogeography and ecology, as witnessed by the new research it has generated and the many thousands of citations the book has received. fte theory has been criticized for treating individual species as identical units, without taking into account differences in life histories, resource requirements, physiological constraints, and so forth. But like any scientific theory, the theory of island biogeography attempts to reveal general principles at the expense of such complex details that might mask patterns. Tests of the theory have been inconclusive, with some studies supporting the theory and others not. In general, the relationships between island size and isolation and species richness have been supported for a variety of taxa. Some studies of bird richness on islands have suggested that substantial species turnover and dynamic equilibrium does occur over time, but these studies may have been affected by anthropogenic changes on the islands. Some have expressed doubt that an island ever reaches equilibrium species richness because of the frequency of disturbances.
Of major importance is the fact that applications of the theory of island biogeography are not restricted to oceanic islands alone, but have great ramifications for insular terrestrial habitats as well.