Overview

Island Biogeography is a field of ecology that examines the factors influencing species richness and diversity on islands and isolated habitats. The theory, formalized by Robert MacArthur and E.O. Wilson (1967), explains how island size, distance from the mainland, immigration, and extinction rates shape the biodiversity of islands.

Key Concepts

1. Species-Area Relationship

  • Larger islands support more species due to greater habitat diversity and resources.
  • The relationship is often expressed as:
    S = cA^z
    Where:
    • S = number of species
    • A = area
    • c and z = constants

2. Distance Effect

  • Islands closer to the mainland have higher immigration rates.
  • Remote islands have fewer species due to lower colonization rates.

3. Equilibrium Theory

  • The number of species on an island is a balance between immigration and extinction.
  • Immigration decreases as more species are present (fewer new arrivals).
  • Extinction increases as more species compete for limited resources.

Diagram: The Island Biogeography Model

Island Biogeography Model

Factors Affecting Island Biodiversity

Factor Description Example
Island Size Larger islands = more species Madagascar vs. Seychelles
Isolation Distant islands = fewer species Galápagos vs. Caribbean islands
Habitat Diversity Varied habitats = more niches Hawaii’s volcanic landscapes
Human Impact Introduced species, habitat loss Rats on Pacific islands
Evolutionary History Adaptive radiation, speciation Darwin’s finches

Data Table: Species Richness on Selected Islands

Island Area (km²) Distance from Mainland (km) Native Bird Species Endemic Species
Madagascar 587,041 400 285 115
Galápagos 8,010 1,000 56 25
Seychelles 455 1,600 21 12
Hawaii 28,311 3,700 65 33
Channel Is. 350 30 15 5

Case Studies

1. Galápagos Islands

  • Famous for Darwin’s finches, which evolved to exploit different food sources.
  • Isolation led to high rates of endemism.

2. Krakatau (Indonesia)

  • Volcanic eruption in 1883 wiped out all life.
  • Recolonization studied over decades; species richness increased rapidly, then stabilized.

3. Florida Keys Experiment

  • Simulated island defaunation by fumigating mangrove islands.
  • Monitored recolonization; confirmed equilibrium theory predictions.

4. Hawaiian Honeycreepers

  • Adaptive radiation produced diverse species from a single ancestor.
  • Many species now endangered due to habitat loss and introduced predators.

Surprising Facts

  1. Species Turnover is Constant: Even if the total number of species remains stable, the actual species present can change frequently due to ongoing immigration and extinction.
  2. Small Islands Can Have High Endemism: Despite fewer species overall, small islands often harbor unique species found nowhere else.
  3. Human-Made Islands Mimic Natural Patterns: Artificial islands (e.g., in urban wetlands) show similar colonization and extinction dynamics as natural islands.

Recent Research

  • 2022 Study: “Rapid Biodiversity Loss on Islands Accelerated by Invasive Species and Climate Change” (Nature Ecology & Evolution, 2022).
    • Found that islands are losing endemic species at a rate 3x higher than continental areas.
    • Highlights the vulnerability of island ecosystems to global change.

The Most Surprising Aspect

Dynamic Equilibrium:
The most surprising aspect is that island biodiversity is not static. The composition of species is in constant flux, even when the overall number remains stable. This turnover means islands are continuously shaped by both ecological and evolutionary processes, making them hotspots for speciation and extinction.

Additional Insights

  • Rescue Effect: Islands closer to the mainland can “rescue” declining populations through frequent immigration, reducing extinction rates.
  • Nestedness: Smaller islands often have a subset of species found on larger islands, but not always the same ones.
  • Genetic Drift: Isolation leads to rapid genetic changes and speciation.

Diagram: Immigration and Extinction Rates

Immigration and Extinction Rates

References

  • MacArthur, R.H. & Wilson, E.O. (1967). The Theory of Island Biogeography.
  • Nature Ecology & Evolution (2022). “Rapid Biodiversity Loss on Islands Accelerated by Invasive Species and Climate Change.” Link

Quick Recap

  • Island biogeography explains species diversity using area, isolation, immigration, and extinction.
  • Islands are natural laboratories for studying evolution and extinction.
  • Human impacts and climate change are accelerating biodiversity loss.
  • The dynamic equilibrium of species turnover is the most surprising and important feature.

Study Tip

Focus on understanding the interplay between area, isolation, and species turnover. Use case studies to illustrate theoretical concepts. Consider recent research for current trends in island biodiversity.