Factors Affecting Colonization and Extinction

Introduction

Key Concepts

The Theory of Island Biogeography

Developed by ecologists Robert MacArthur and E.O. Wilson in the 1960s, the Theory of Island Biogeography explains how species diversity on islands is a balance between immigration (colonization) and extinction rates. The theory posits that larger islands closer to the mainland tend to have higher species richness due to easier access for colonizers and more diverse habitats that support larger populations, reducing extinction rates.

Colonization Factors

• Distance from Source: Islands closer to the mainland or source populations have higher colonization rates due to shorter dispersal distances for organisms.
• Isolation: The degree of isolation affects the likelihood of species reaching the island. Greater isolation typically results in lower colonization rates.
• Dispersal Mechanisms: Species with effective dispersal methods, such as birds or wind-dispersed plants, are more likely to colonize new islands.
• Availability of Habitat: Diverse and suitable habitats on islands can attract a variety of species, increasing colonization rates.

Extinction Factors

• Island Size: Smaller islands have limited resources and smaller populations, making species more susceptible to extinction due to stochastic events and reduced genetic diversity.
• Environmental Fluctuations: Islands are often subject to harsh and variable climates, which can increase extinction rates.
• Human Impact: Introduction of invasive species, habitat destruction, and overexploitation by humans can significantly elevate extinction rates.
• Population Size and Genetics: Smaller populations are more vulnerable to inbreeding and genetic drift, leading to decreased adaptability and higher extinction probabilities.

Species-Area Relationship

The species-area relationship is a fundamental principle in island biogeography that describes the positive correlation between the size of an island and the number of species it can support. Mathematically, it is often expressed as: $$ S = cA^z $$ where:

• S: Number of species
• A: Area of the island
• c, z: Constants that vary depending on the region and taxa

Equilibrium Theory of Island Biogeography

The equilibrium theory suggests that the number of species on an island represents a balance between the rate of new species colonizing the island and the rate of existing species going extinct. This balance is influenced by factors such as island size and distance from the mainland: $$ \text{Balanced Species Diversity} = \text{Immigration Rate} - \text{Extinction Rate} $$ As islands increase in size or proximity to the mainland, the immigration rate increases and extinction rate decreases, leading to higher species diversity.

Implications for Conservation Biology

Understanding the factors affecting colonization and extinction is pivotal for formulating conservation strategies. Protecting larger habitats, reducing isolation barriers, and mitigating human impacts can enhance species colonization and reduce extinction risks. Additionally, creating wildlife corridors can facilitate dispersal and maintain genetic diversity, promoting ecosystem resilience.

Case Studies

Galápagos Islands

The Galápagos Islands exemplify the principles of island biogeography. Their isolation has led to unique species evolution, while their varying sizes and habitats support diverse ecosystems. However, human activities and introduced species pose significant threats to their biodiversity, highlighting the delicate balance between colonization and extinction.

Madagascar

Madagascar's long-term isolation has resulted in high endemism but also makes its species highly vulnerable to extinction. Habitat destruction and lack of natural dispersal pathways exacerbate the risks, emphasizing the need for targeted conservation efforts.

Mathematical Models in Island Biogeography

Mathematical models, such as the Makovicky model, extend the equilibrium theory by incorporating variables like habitat diversity and environmental stochasticity. These models help predict species diversity changes over time and assess the impact of different conservation strategies.

Human Influence on Colonization and Extinction

Human activities significantly alter colonization and extinction dynamics. Habitat fragmentation, pollution, climate change, and the introduction of invasive species disrupt natural processes, often accelerating extinction rates while hindering natural colonization. Conservation policies aim to mitigate these impacts by protecting habitats, controlling invasive species, and supporting sustainable ecosystems.

Comparison Table

Summary and Key Takeaways