Overview

Island Biogeography is the study of the distribution, diversity, and dynamics of species on islands and isolated habitats. It explores how geographical isolation, area, and ecological processes influence species richness and extinction rates.

Core Principles

1. Equilibrium Theory of Island Biogeography

  • Proposed by Robert MacArthur and E.O. Wilson (1967).
  • Species richness on islands is a balance between immigration and extinction rates.
  • Immigration rates are higher on islands closer to the mainland.
  • Extinction rates are higher on smaller islands due to limited resources.

2. Key Variables

  • Island Size: Larger islands support more species due to greater habitat diversity and resources.
  • Isolation (Distance from Mainland): More isolated islands receive fewer new species.
  • Habitat Diversity: Varied habitats support more niches, increasing species richness.

Diagram: Species-Area Relationship

Species-Area Relationship

The curve shows that as island area increases, the number of species increases, but at a decreasing rate.

Table: Island Biogeography Data

Island Name Area (km²) Distance from Mainland (km) Number of Species Endemism (%)
Madagascar 587,041 400 250,000+ 90
Galápagos 8,010 1,000 2,000+ 30
Hawaii (Big Island) 10,432 3,800 21,000+ 89
Great Barrier Reef 344,400 15-100 9,000+ 10
Komodo 390 40 1,000+ 25

Environmental Implications

  • Biodiversity Hotspots: Islands often contain unique species (endemics) not found elsewhere, making them conservation priorities.
  • Vulnerability to Invasives: Isolated islands are highly susceptible to invasive species, which can outcompete or prey on native species.
  • Extinction Risks: Small populations and limited habitats increase the risk of extinction from environmental changes or human impacts.
  • Climate Change: Sea-level rise and altered weather patterns threaten low-lying islands and their unique biota.

Recent Research

A 2021 study published in Nature found that island species are going extinct at rates up to 100 times higher than those on continents, primarily due to habitat loss, invasive species, and climate change (Jones et al., 2021). The study highlights the urgent need for targeted conservation strategies on islands.

Surprising Facts

  1. The Great Barrier Reef is the largest living structure on Earth, visible from space, and home to over 9,000 known species.
  2. Islands make up only 5% of Earth’s land area but contain an estimated 20% of all plant, bird, and reptile species.
  3. Over 80% of known bird extinctions since 1500 have occurred on islands, underscoring their vulnerability.

Controversies

  • Human Influence: Some scientists argue that the original equilibrium theory underestimates the impact of human activity, such as habitat destruction and species introductions.
  • Rescue Effect: Debate exists over how often new immigrants can “rescue” declining populations, especially on highly isolated islands.
  • Application to Non-Island Systems: The theory is sometimes applied to habitat fragments (e.g., forest patches), but critics note that these “islands” may not behave like true islands due to ongoing connections with surrounding landscapes.
  • Overemphasis on Area and Isolation: Recent studies suggest that habitat quality and historical events (e.g., past climate shifts) may be as important as area and isolation.

Conservation Strategies

  • Protected Areas: Establishing reserves to safeguard endemic species.
  • Biosecurity Measures: Preventing introduction of invasive species.
  • Restoration Projects: Reintroducing native species and restoring habitats.
  • Climate Adaptation: Managing for sea-level rise and increased storm frequency.

Key Concepts in Island Biogeography

  • Endemism: Species unique to a specific island or island group.
  • Species Turnover: Ongoing process of species colonization and extinction.
  • Founder Effect: Reduced genetic diversity when new populations are established by a small number of individuals.

Citation

Jones, H.P., et al. (2021). “Rapid global extinctions of island species.” Nature, 593, 321–326. https://doi.org/10.1038/s41586-021-03402-1

Summary

Island Biogeography reveals how isolation and area shape biodiversity, with profound implications for conservation. Understanding these dynamics is crucial for protecting the unique and vulnerable life forms found on islands worldwide.