Biogeography: Species Diversity and Global Patterns

Biogeography Overview

Outline

  • Biogeography: study of variation in species composition and diversity among geographic locations

  • Main Topics Covered:
      1. Dispersal and immigration
      2. Environmental conditions
         - Species supply filter
      3. Species interactions
         - Local community
         - ECOLOGY 6e, Figure 19.4

Scales of Diversity

  • Types of Diversity:
      - Alpha Diversity (α):
        - Species richness of local communities (e.g., average richness per hectare, per habitat patch, per site)
        - This represents "within-habitat diversity" or community richness.
      - Beta Diversity (β):
        - Change (or turnover) in species composition over small distances (e.g., between adjacent habitats)
        - Known as "between-habitat diversity"
        - Highest where habitats vary over fine spatial scales (i.e., heterogeneous habitats, high topographic complexity).
      - Gamma Diversity (γ):
        - Total species richness over a large geographic area such as biome, continent, or ocean basin.
        - Represents the combined influence of alpha and beta diversity.
        - Describes “Regional richness” or “Regional species pool.”

Key Concepts in Biogeography

Importance of Biodiversity Patterns

  • Species richness and composition vary with latitude:
      - Lower latitudes have higher species diversity than higher latitudes.

  • Species richness and composition can also differ between continents, even when latitudes are similar.

  • The same community type or biome may exhibit variations in species richness and composition depending on its geographical location.

Factors Influencing Diversity

  • Global Patterns: Species diversity across biogeographic regions.

  • Latitudinal Gradients: Species diversity increases towards the tropics and decreases towards the poles.

Beta Diversity and Its Drivers

  • Beta Diversity: Refers to spatial turnover in diversity.

  • Elevated beta diversity patterns usually correlate with increased topographic variation and other environmental gradients.

  • Example: A hypothesis regarding a driver of large-scale patterns of beta diversity could include environmental conditions affecting species dispersal and interactions.

Hypothetical Example of Diversity Calculation

  • The relationship between alpha, beta, and gamma diversity can be expressed mathematically as:
      - extγ=extβimesextαext{γ} = ext{β} imes ext{α}

  • Where:
      - α: local richness
      - β: species turnover
      - γ: regional richness

Connections to Conservation Efforts

  • To conserve species, consider:
      - Highest individual local species richness (S) for protecting a particular island.
      - Gamma diversity (γ) for protecting larger areas such as island groups.
      - Endemic species protection aligned with beta diversity measurement.

Global Biogeographic Regions

  • Earth’s landmasses can be categorized into six biogeographic regions:
      1. Nearctic
      2. Neotropical
      3. Palearctic
      4. Ethiopian
      5. Oriental
      6. Australasian

  • Each region has distinct biotas differing in species diversity and composition which are often influenced by tectonic plate boundaries.

Plate Tectonics and Biogeography

  • Illustrated patterns show historical events:
      - Pangaea: A single landmass that began breaking apart around 251 million years ago.
      - By the mid-Cretaceous period, it had differentiated into two significant landmasses: Laurasia and Gondwana.

  • The geographical distribution of species today reflects this historical fragmentation.

Latitudinal Diversity Gradient Hypotheses

  • Major hypotheses explaining the latitudinal gradient in species diversity include:
      1. Species Diversification Rate Hypothesis:
         - Rate of speciation minus extinction higher in tropics.
      2. Species Diversification Time Hypothesis:
         - Tropical regions have had more time for species accumulation due to climatic stability.
      3. Productivity or Carrying Capacity Hypothesis:
         - Increased resources due to higher productivity promote greater biodiversity.

Implications of Hypotheses

  • Increased land area, stable temperatures, and climatic conditions in tropical regions lead to lower extinction rates and higher speciation rates.

  • Productivity and resource availability contribute to sustaining larger population sizes and greater diversity among species.

Conclusion & Questions

  • Questions raised regarding the implications of understanding biogeography for conservation and species management, as well as understanding how global patterns of diversity manifest in specific ecological contexts.