The Foundations of the Equilibrium Theory

What Does the Theory Propose?

The equilibrium theory of island biogeography posits that the number of species found on an island is not static but is instead maintained at a dynamic equilibrium. This equilibrium is achieved when the rate at which new species colonize the island equals the rate at which resident species go extinct. The theory is elegantly simple yet powerful, allowing ecologists to predict how changes in island size and isolation affect biodiversity.

Colonization Rate

• Definition: Colonization (or immigration) is the arrival of new species from a source pool, such as a mainland or a larger island.

• Pattern: When an island is empty, the colonization rate is at its maximum because every new arrival is a new species. As the number of species on the island increases, the colonization rate drops because fewer new species remain to be added.

Extinction Rate

• Definition: Extinction is the loss of species already present on the island.

• Pattern: When there are few species on the island, the extinction rate is low. As more species accumulate, competition for limited resources intensifies, leading to a higher extinction rate.

The Dynamic Equilibrium

The intersection point where the colonization rate equals the extinction rate is the equilibrium number of species. At this point, the number of species on the island remains relatively stable, although the actual species present may change over time due to ongoing immigration and extinction events.

What the Theory Does NOT Include

It’s important to note that MacArthur and Wilson’s original model does not focus on factors such as birth and death rates, resource consumption, predation rates, or speciation and hybridization rates. While these processes are important in ecology, the equilibrium model specifically emphasizes the balance between colonization and extinction as the primary drivers of island species richness23410.

Why Island Size and Distance Matter

• Larger islands tend to have lower extinction rates because they offer more habitats and resources, supporting larger populations and reducing the risk of random extinctions.

• Islands closer to the mainland have higher colonization rates because they are easier for new species to reach.

• Smaller islands and more isolated islands have higher extinction rates and lower colonization rates, respectively, leading to lower equilibrium species richness.

Practical Applications

The equilibrium theory of island biogeography has been applied to:

• Conservation planning: Understanding how habitat fragmentation affects biodiversity.

• Reserve design: Prioritizing larger and better-connected reserves to maximize species richness.

• Predicting species turnover: Anticipating changes in community composition over time.

Conclusion: The Correct Answer

According to MacArthur and Wilson’s equilibrium model of island biogeography, the number of species on an island represents a balance between colonization rate and extinction rate. This core concept has shaped decades of ecological research and remains fundamental to our understanding of biodiversity patterns on islands and in fragmented landscapes.

Correct answer:
(3) colonization rate and extinction rate.