The Core Principles of Island Biogeography

1. Immigration Rate

• Higher for islands close to the mainland: Islands near the mainland are easier for new species to reach, so the immigration curve is higher.

• Lower for islands far from the mainland: More isolated islands are harder to colonize, so the immigration curve is lower.

2. Extinction Rate

• Lower for large islands: Large islands offer more resources, habitats, and space, reducing competition and supporting larger populations. This means the extinction curve is lower.

• Higher for small islands: Small islands have limited resources and space, leading to more competition and higher extinction rates. The extinction curve is higher.

Interpreting the Graph

When you look at a graph with a high immigration rate (solid line) and a high extinction rate (dotted line), you’re seeing the signature of an island that is:

• Close to the mainland (high immigration)

• Very small (high extinction)

This is because proximity to the mainland makes it easy for species to arrive, but the small size of the island makes it difficult for populations to persist due to limited resources and increased competition.

Why This Combination Matters

This scenario is crucial in understanding why some islands, despite being easy for species to reach, still have relatively low species richness. The high extinction rate on small islands counteracts the high immigration rate, leading to a dynamic equilibrium where species are constantly arriving and disappearing.

Theoretical and Practical Implications

• Conservation: Small, isolated reserves may lose species quickly even if they are close to source populations, highlighting the importance of both size and connectivity in conservation planning.

• Habitat Fragments: The same principles apply to habitat fragments in terrestrial landscapes, such as forest patches surrounded by farmland or urban areas.

Supporting Evidence from the Literature

MacArthur and Wilson’s theory has been supported by decades of ecological research. Studies consistently show that:

• Large, near islands have the highest species richness.

• Small, far islands have the lowest species richness.

• Small, near islands have moderate richness, with high turnover due to frequent immigration and extinction events14511.

Conclusion

According to MacArthur and Wilson’s equilibrium theory of island biogeography, an island that is close to the mainland and very small will experience both high immigration rates and high extinction rates. This dynamic influences the number of species present and the rate at which species turnover occurs. Understanding these principles is essential for effective biodiversity management and conservation in both island and fragmented terrestrial ecosystems.

Correct answer:
(1) It is close to the mainland and is very small.