Overview

Island Biogeography is the study of the distribution and diversity of species on islands and isolated habitats. It explores how geographical features, colonization, extinction, and ecological dynamics shape island ecosystems. The foundational theory was developed by Robert MacArthur and E.O. Wilson in the 1960s, but modern research has expanded and refined these ideas.

Key Concepts

1. Island Types

  • Oceanic Islands: Formed by volcanic activity, never connected to a continent (e.g., Galápagos, Hawaii).
  • Continental Islands: Once part of a continent, now isolated by rising sea levels (e.g., Madagascar, Great Britain).
  • Habitat Islands: Isolated patches of suitable habitat within a “sea” of unsuitable habitat (e.g., mountaintops, forest fragments).

2. Core Principles

  • Species-Area Relationship: Larger islands support more species due to more resources and habitats.
  • Distance Effect: Islands closer to the mainland receive more colonists, leading to higher species richness.
  • Equilibrium Theory: The number of species on an island represents a balance between immigration (arrival of new species) and extinction (loss of existing species).

3. Dynamic Processes

  • Colonization: Arrival of species via wind, water, or animal carriers.
  • Extinction: Higher on smaller and more isolated islands due to limited resources and smaller populations.
  • Turnover: Continuous replacement of species as new ones arrive and others go extinct.

Diagrams

Species-Area and Distance Relationships

Island Biogeography Diagram

Surprising Facts

  1. Rapid Evolution: Island species can evolve unique traits (gigantism, dwarfism, flightlessness) in just a few generations due to isolation and selective pressures.
  2. Genetic Bottlenecks: Island populations often have reduced genetic diversity, making them more vulnerable to disease and environmental changes.
  3. Human Impact: The introduction of non-native species by humans is the leading cause of extinctions on islands—over 80% of recorded extinctions since 1500 have occurred on islands.

Memory Trick

“BIG DICE”

  • Balance (Equilibrium)
  • Immigration
  • Gigantism/Dwarfism
  • Distance
  • Isolation
  • Colonization
  • Extinction

Common Misconceptions

  • Islands Are Always Isolated: Some islands are connected by “stepping stones” (chains of smaller islands), which can facilitate species movement.
  • Bigger Islands Always Have More Species: Exceptions exist due to habitat diversity, historical events, or human impacts.
  • Equilibrium Means Stability: The equilibrium is dynamic, with constant species turnover, not a fixed community.
  • All Island Species Are Endemic: Many species are widespread, and only some evolve into unique island forms.

Recent Research

A 2021 study in Nature Communications (“Global patterns and drivers of avian extinctions on islands”) found that island bird extinctions are accelerating due to habitat loss and invasive species, with climate change expected to further increase extinction risk (Spatz et al., 2021). The study highlights the need for targeted conservation on islands, where biodiversity is both rich and highly vulnerable.

Future Directions

  • Genomics and Conservation: Advanced genetic tools are being used to monitor island populations, assess genetic diversity, and guide breeding programs.
  • Climate Change: Rising sea levels and shifting weather patterns threaten low-lying islands and their unique ecosystems.
  • Restoration Ecology: Efforts to eradicate invasive species and restore native habitats are showing promise in reversing biodiversity loss.
  • Artificial Islands: Human-made islands (e.g., in the UAE) provide new opportunities to study colonization and succession in real time.
  • Microbiome Studies: Research is expanding to include the role of microbial communities in island ecosystem health and resilience.

Detailed Mechanisms

Colonization Filters

  • Dispersal Ability: Only species with effective dispersal mechanisms (e.g., seeds that float, birds that fly long distances) can colonize remote islands.
  • Founder Effect: Small founding populations can lead to rapid genetic drift and speciation.

Extinction Dynamics

  • Rescue Effect: Islands near the mainland may have lower extinction rates due to frequent immigration “rescuing” declining populations.
  • Allee Effect: Small populations may struggle to survive due to difficulties finding mates or cooperative behaviors.

Adaptive Radiation

  • Example: Darwin’s finches on the Galápagos evolved into multiple species, each adapted to different food sources and habitats.

Water Cycle Connection

  • Ancient Water: The water you drink today may have been drunk by dinosaurs millions of years ago, as Earth’s water is continuously recycled through the hydrological cycle. This connects to island biogeography by illustrating how global cycles (water, nutrients) influence isolated ecosystems.

References

  • Spatz, D. R., Zilliacus, K. M., Holmes, N. D., Butchart, S. H. M., Genovesi, P., Ceballos, G., … & Tershy, B. R. (2021). Global patterns and drivers of avian extinctions on islands. Nature Communications, 12, 1443. Read online

Summary Table

Principle Effect on Biodiversity Example
Island Size Larger = More Species Madagascar vs. Seychelles
Distance Closer = More Species Canary Islands vs. Azores
Isolation More = More Endemics Hawaii
Human Impact Often Negative Guam brown tree snake

Key Takeaways

  • Island biogeography explains patterns of species diversity and extinction on islands.
  • Size, distance, and isolation are critical factors.
  • Human activity and climate change are major threats.
  • Conservation efforts must be tailored to the unique dynamics of islands.

Remember: “BIG DICE” for the core ideas of Island Biogeography!