Island Biogeography

Island biogeography explores ecological phenomena related to island size and isolation, which significantly influence biodiversity and species distribution. Understanding how these factors shape ecosystems on islands is vital for conservation efforts and biodiversity protection in isolated environments.

Historical Context of Island Recovery
Landscape Changes Over Time:

  • 1956: Mature forest remaining = 81%

  • 2001: Mature forest remaining = 14%
    Evidence indicates a severe decline in wildlife populations due to habitat loss directly caused by human activities such as logging, urban development, and agriculture. This has serious implications for biodiversity on islands where species often have limited ranges and specialized habitats.

Key Recovery Milestones:

  • 1978: The first species was listed as endangered in Canada, highlighting the need for protective measures.

  • 1988: Formation of the Recovery Team, composed of specialists aimed at revitalizing endangered species and restoring their habitats.

  • 1996: The first evidence of significant wildlife decline was reported, prompting increased conservation efforts.

  • 2006: Release of 31 marmots from captive breeding programs into their native habitat marked a pivotal recovery effort for the species, showcasing human intervention's role in reversing biodiversity loss.

Definition of an Island
Types of Islands:

  • Oceanic Islands: Typically formed through volcanic activity and characterized by their complete isolation from continental land masses, leading to unique ecosystems with endemic species.

  • Sky Islands: Mountain regions isolated by lower land, creating microclimates that promote distinct ecological communities. These islands often serve as critical refuges for species adapted to specific environmental conditions.

  • Habitat Islands: Areas of habitat surrounded by inhospitable environments (e.g., urban areas or agricultural lands) that can hinder the movement of species and jeopardize their survival.

General Definition: An island is any habitat area surrounded by an inhospitable matrix that affects the species occurring within that habitat. This matrix creates barriers that limit genetic exchange and colonization opportunities for species.

Historical Hypotheses of Flora Dispersal to Islands

Two prominent hypotheses for how plant species colonize oceanic islands include:

  1. Seeds are carried across oceans via ocean currents, winds, or birds, enabling species to reach isolated lands.

  2. Islands were formerly connected to continents, providing pathways for flora migration during periods of lower sea levels when land bridges existed. This historical connection shaped the evolution and diversity of island flora.

Theories by Founders of Island Biogeography

Robert H. MacArthur and E. O. Wilson developed theoretical frameworks on island ecology that provided critical insights into how geographical isolation and habitat size influence species richness and ecological dynamics. Their work laid the foundation for the field of biogeography, offering predictive models for species distribution.

The Krakatau Islands Case Study
Pre and Post-1883 Eruption Events:

  • Before the Eruption: The Krakatau Islands exhibited rich biodiversity, including a wide variety of plants and animal species.

  • After the Eruption: The 1883 volcanic eruption resulted in total destruction of species habitats, leading to a stark reduction in biodiversity.

Initial Recovery (1884 - 2011):

Following the eruption, slow immigration and establishment of various plant and animal species began, indicating resilience in ecological systems.

  • 1886: Observational studies began with 9 flowering plants established; this number gradually increased over the years, indicating a trend toward dynamic equilibrium in species recovery.

Species-Area Relationship (SAR)

  • Larger areas generally contain more species, highlighting the importance of habitat size in conservation strategies.

Mathematical Representation:

S=cAzS = cA^z
Where:

  • SS = number of species (species richness)

  • cc = constant representing the ecosystem/taxa

  • AA = area

  • zz = slope of log-transformed power model, illustrating how species richness changes relative to area size.

Patterns of Species Extinction on Islands

Small islands exhibit higher extinction rates due to:

  • Fewer Niches and Resources: Limited resources lead to increased competition among species.

  • Smaller Populations: Smaller populations are more vulnerable to extinction due to stochastic events and genetic bottlenecks, which can significantly impact long-term viability.

Species-Isolation Relationship (SIR)

Less isolated islands experience higher rates of immigration from the mainland. As isolation increases, species richness tends to decrease, underscoring the importance of connectivity for maintaining biodiversity.

The Equilibrium Model of Island Biogeography
Key Elements:

  • The rate of immigration and extinction determine species richness on islands.

  • As species richness increases,

    • Immigration rate typically decreases due to limited resources and suitable habitats.

    • Extinction rates increase due to heightened competition among species.

At Equilibrium:

A steady state is depicted by: S=I=ES = I = E
Where:

  • SS = equilibrium number of species. This model helps predict biodiversity patterns on islands depending on their area and isolation.

Effects of Environmental Changes

Sudden disruptions such as hurricanes or climate shifts can temporarily push species counts above or below stable equilibrium points. These events can initiate turnover in community structure while contributing to overall stability in species richness over time, emphasizing the resilience of island ecosystems in the face of environmental changes.