Regional Biogeography Lecture Summary

Outline

• Species Area Relationship
    - Species Area Curves
    - Species Isolation Relationship

• Equilibrium Theory of Island Biogeography
    - The Graphical Model
    - Model Predictions
    - Experimental Tests
    - Additions to the Model

Species Area Relationship

• Definition: Species richness increases with the area sampled.

• Documentation: This relationship has been documented across various spatial scales, ranging from small ponds to entire continents.

• History: The first species-area curve was plotted by HC Watson for British plants.

• Reasons for Positive Species-Area Relationship:
    1. Habitat Diversity: Larger areas contain more habitat types, enabling more resources/niches and the support of specialized and rare species.   2. Resource Availability: Larger areas supply more resources to sustain more individuals within a species, reducing extinction vulnerability.   3. Home Range Requirements: Some species require large areas for their home ranges.

• Documentation Venue: This relationship is particularly noted in island contexts and analogous habitats surrounded by less hospitable environments (e.g., lakes, mountain tops).

Species Area Curves

• Description: Plots species richness (S) against the area (A) of a sample.

• Mathematical Relationship: The relationship can be modeled using linear regression as follows:
    $S = zA + c$
    Where:
    - $z$ = slope of the line (indicating how steeply species richness increases with area)
    - $c$ = y-intercept (constant)

• Log Transformation: Both S and A require log transformations to linearize the relationship.

• Interpretation: Higher values of $z$ correspond to a steeper slope, meaning greater differences in species richness as area increases.

Species-Isolation Relationship

• Definition: Species richness is negatively correlated with the distance from the main source of species (such as the mainland for islands).

• Observation: More isolated islands or habitat patches typically exhibit lower species richness than those closer to the source.

• Confounding Factors: Area and isolation often confound each other, making it difficult to isolate their effects.

• Reasons for Richness Decline:
    1. Dispersal Probability: The likelihood of a colonist reaching an island diminishes with increased distance/isolation.
    2. Local Extinction Risk: Isolated patches face a higher risk of local extinctions without replacement from source populations.

Case Study: Pacific Islands

• Application: Examined bird species richness related to island area across islands in the Pacific at varying distances from New Guinea.

Key Figures in Biogeography

• E. O. Wilson:
    - Educational Background: PhD from Yale, focus on competition and coexistence among warblers.   - Career: Professor at Princeton and Harvard, notable for contributions to systematics and biogeography, later concentrated on conservation.

• Robert MacArthur: Associated with foundational concepts in biogeography.

Equilibrium Theory of Island Biogeography

• Framework: Integrates immigration and extinction dynamics concerning island species populations.

• Variables:
    - Mainland Species Pool (P)   - Island Species (S)   - Species Immigration Rate (I)   - Species Extinction Rate (E)

• Immigration and Extinction Dynamics:
    - Immigration is maximized on empty islands without existing species, decreasing as islands accumulate species.   - With an increase in species (S), the variety of potential incoming immigrants decreases, challenging successful colonization.   - Extinction risk escalates as more species occupy the island, due to competition and reduced population size for individual species.

Equilibrium Predictions

• Equilibrium Point (S*): Refers to the balance between immigration and extinction rates.
    - $I = E$: Stability in species richness occurs when these rates are equal.

• Turnover Dynamics: Not just the number of species but the change in species composition over time due to ongoing colonization and extinction processes.

Impacts of Island Size and Distance on Species Dynamics

• Predictions when changing island parameters:
    - Further Island Distance: If an island is moved further from the mainland, immigration will decline.   - Larger Island Area: Extinction rates should decrease on larger islands due to more available resources and habitat, leading to a smaller pool of potential extinctions.

• Predicted Outcomes:
    - Small, isolated islands are expected to harbor fewer species compared to larger, proximate ones, with intermediate outcomes for other combinations of size and isolation.

Practical Applications and Exercises

• Group Work: Complete a worksheet together and submit via Canvas by 11:10 AM.

• Equilibrium Species Richness Calculation: Participants need to consider species richness in context with island size and isolation, including turnover rates and impacts of natural disasters such as hurricanes on species population dynamics.

Experimental Tests of Theory

• Dan Simberloff's Study:
    - Research on the effect of isolation distance regaining arthropod species richness on mangrove islands in the Florida Keys.
    - Removals were performed on selected islands to analyze recolonization rates and species turnover post-disturbance.

Limitations of the Theory

• Critiques of the equilibrium theory include:
    - The need for more nuanced measures of isolation and area beyond simple distance.
    - Greater consideration of species interactions and dynamics in immigration and extinction.

• Importance of considering biogeographical realities to enhance the robustness of theoretical predictions.

Summary

• This seminar presents an overview of the Equilibrium Theory of Island Biogeography and emphasizes significant concepts regarding species area and isolation relationships, along with their effects on biodiversity. Discussions on practical applications and experimental validations of the theory illustrate its relevance in understanding ecological frameworks.