19_Island Biogeography

Island Biogeography Overview

Island biogeography examines the distribution and diversity of species on islands. It is a key area of interest for ecologists seeking to understand how isolation and size of an island can affect the species composition found there. This is crucial in conservation efforts and can inform strategies for preserving biodiversity.

Species Richness on Islands

Size of Islands

  • Small Islands: Generally support fewer species than larger islands.

  • Large Islands: Tend to support greater species richness due to larger area and resources.

Relative Species Richness

  • Species richness on Pacific islands decreases as the distance from the mainland increases. Therefore, islands closer to mainland sources have a higher diversity of species.

Ecological Significance of Islands

  • Islands serve as isolated ecosystems, allowing conditions for unique evolutionary processes and biodiversity to develop.

  • Due to their boundaries, they can be seen as natural laboratories for ecological research, helping scientists understand broader ecological principles.

The Equilibrium Theory of Island Biogeography

  • Developed by Robert MacArthur and Edward O. Wilson in the 1960s, it articulates how species diversity on islands is affected by colonization and extinction rates.

Key Concepts

  1. Species-Area Effect: Larger islands support more species. Extinction curves increase as island size decreases.

  2. Isolation Effect: Islands that are farther from the mainland are less likely to be colonized due to distance, resulting in fewer species than islands that are nearer.

  3. Species Turnover: Refers to the rate of change in species composition; islands exhibit constant turnover despite maintaining a relatively stable number of species over time.

Dynamics of Colonization and Extinction

  • Species richness changes over time based on two key processes:

    • Colonization: New species arriving on the island, which decreases as the number of existing species increases. The colonization rate formula is:

      • ( C - (C/P)S )

    • Extinction: Species disappearing from the island, which increases as more species are present. The extinction rate formula is:

      • ( (E/P)S )

The Rate of Change in Species Richness

  • The equation ( dS/dt = C - (C/P)S - (E/P)S ) describes the dynamics between colonization and extinction, where at equilibrium, the change in species richness is zero.

  • At equilibrium, species richness can be calculated using: ( S_{equil} = \frac{CP}{C+E} )

Different Island Types

  • Land Bridge Islands: These islands were once connected to the mainland and typically host higher species richness compared to islands that have never been connected due to their larger initial species pool and easier access.

  • Volcanic Islands: These islands grow toward a species equilibrium through the process of colonization from the mainland, leading to unique ecological dynamics.

Experimental Tests of the Theory

  • Dan Simberloff's Research: Conducted studies on small mangrove islands to test predictions, specifically observing recolonization patterns following the removal of species. Results indicated that closer islands to the mainland recovered species richness faster due to easier access for new colonists.

Application to Conservation Biology

  • Understanding the principles of island biogeography is vital for conservation strategies, especially in a world where habitat fragmentation and isolation due to human activities are prevalent. Insight into these dynamics can help in setting priorities for conservation efforts and establishing protected areas.