The Core Principles of Island Biogeography

MacArthur and Wilson’s equilibrium theory posits that the number of species on an island is determined by a dynamic balance between two processes:

• Immigration: The arrival of new species from a source region (usually the mainland).

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

The interplay between these processes, influenced by island size and distance, determines the equilibrium number of species an island can support.

Why Island Size and Distance Matter

1. Island Size

• Larger islands have more habitats, resources, and space, which can support larger populations and a greater variety of species.

• Larger islands also have lower extinction rates because populations are less likely to be wiped out by random events or competition.

• Smaller islands support fewer species due to limited resources and higher extinction rates.

2. Distance from the Mainland

• Islands closer to the mainland are more easily reached by dispersing species, resulting in higher immigration rates.

• Isolated islands (far from the mainland) are harder to reach, so they receive fewer new species, resulting in lower immigration rates and, ultimately, fewer species overall.

The Classic Prediction

According to MacArthur and Wilson’s equilibrium theory, larger islands and islands closer to the continent (mainland) are expected to have more species than smaller and isolated islands134689. This is because the combination of higher immigration and lower extinction creates a higher equilibrium species richness.

This prediction is consistently supported by empirical studies across a wide range of island types and taxa. While there are exceptions—such as the “small-island effect,” where very small islands may not follow the expected pattern—the general rule holds true for most islands and fragmented habitats.

Practical Implications

• Conservation Planning: When designing nature reserves or managing habitat fragments, larger and better-connected areas are prioritized to maximize biodiversity.

• Understanding Extinctions: Small, isolated islands are more vulnerable to species loss, making them priorities for conservation intervention.

• Predicting Invasions: Islands closer to the mainland are more susceptible to colonization by both native and invasive species.

Answer to the Question

Given the options:

1. Larger islands and islands closer to continent are expected to have more species than smaller and isolated islands.

2. Smaller islands and islands far from the continent are expected to have more species than larger and isolated islands.

3. Smaller islands and islands closer to the continent are expected to have more species than far away smaller and isolated islands.

4. More species are expected on all islands irrespective of their size and distance from the continent.

The correct answer is:

(1) Larger islands and islands closer to continent are expected to have more species than smaller and isolated islands.

This matches the predictions and foundational concepts of MacArthur and Wilson’s equilibrium theory of island biogeography134689.

Conclusion

MacArthur and Wilson’s equilibrium theory remains a guiding principle in ecology, conservation, and biogeography. By recognizing the importance of island size and proximity to the mainland, we can better understand, preserve, and manage biodiversity in both natural and human-altered landscapes.