(iii) Equilibrium theory of island biogeography

The three processes outlined above (species-area, species-isolation, species extinctions and turnover) were integrated into a single theory of how they might all interact to determine species diversity on islands by two American ecologists in the mid-1960’s: E.O. Wilson and Robert MacArthur - (although the essence of the theory was first contained in a PhD thesis of Eugene Munroe in 1948) - represented by the figure below.

One curve shows the theoretical decline in immigration rate of species onto an island, the other shows the increase in the extinction rate, both as a function of increases in total species diversity on the island. Where the two curves intersect represents the "equilibrium" point where losses from extinction are exactly balanced by the gain of new species by immigration. The corresponding value on the x-axis represent the equilibrium number of species found on an island, the corresponding point on the y-axis represents the equilibrium "turnover" rate in species composition. These curves yielded a theorectical construct under which one could, with knowledge of extinction and immigration, predict the species diversity on islands.

Of course, not all islands are the same - they vary in size and isolation from continental sources as we have seen. MacArthur and Wilson's theory integrated this variation into a "family" of curves that represented theoretical predictions of species diversity and turnover rate as functions of immigration and extinction which themselves are influenced by island size and isolation. A family of curves is depicted below. The numbers "1", "2", and "3" represent intersection points between the curves for "near/small", "far/small", and "near/large" islands. Note that these variable conditions make distinct predictions about the equilibrium numbers of species and turnover rate on each kind of island, i.e. for S, the rank is 2 > 1 > 3. For turnover rate, the relative ranks are: 1 > 3 > 2. Make sure that you can rationalize these relative rankings based on the differences in size and isolation.

(iv) Impacts/significance of the equilibrium theory of island biogeography

What was the impact of MacArthur and Wilson’s theory of island biogeography?

It offered a possible explanation for the tremendous variation in species diversity across habitats observed in nature; a fundamental problem in ecology.

The theory lead to testable predictions about how factors such as island size and isolation influenced species diversity. The theory is another example of "quantitative biogeography"; where descriptive buiogeography is augmented by more rigorous, hypothesis testing of alternativbe explanations for biogeography pattern (generally lacking before hand).

For instance, at equilibrium, the theory predicted the following rank order of species diversity:

Where S = species diversity, LN = large, near islands, LF = large, far, SN = small, near, and SF = small, far

Also, species turnover rates were predicted to rank as:

The outlining of testable hypotheses led to a huge increase in publications trying to test the theory (over 2,000 between 1967-1987).

The theory reinvigorated the potential role of current day ecological factors (immigration, extinction, dispersal, habitat diversity) in explaining species diversity using islands as model systems.

The theory had (has) relevance to conservation biology, especially with respect to the nature and design of wildlife reserves. For example, the figure below depicts different designs for reserves based on the number of patches that should be erected for a given size of available area and the degree of interconnectedness among them that might promote the greatest chances for persistence of species. In each case (a-d), the design on the left is the preferred one. Scenarios (A) and (C) in particular are based on ideas from island biogeography (larger areas can maintain more species, isolated areas likely experience greater extinction rates, etc). Scenario (A) is often described as the "SLOSS" debate (is a "single, large" refuge better "or several small"?).

Species-area relationships have also been used to try and estimate species losses from habitat loss through human activity (see Brook et al. 2003).