Biogeography and Species Diversity Summary

Biogeography Overview
  • Biogeography is the study of species composition and diversity across different geographic locations.
  • It examines patterns in species diversity which vary according to global, regional, and local scales.
Case Study: Amazon Rainforest
  • Species Diversity: One hectare of Amazon rainforest has more plant species than all of Europe; the Amazon River hosts more fish species than the entire Atlantic Ocean.
  • Deforestation Impact:
    • Road building began in the 1960s, converting 15% of the rainforest into pastureland, towns, roads, and mines.
    • Significant loss of species due to habitat fragmentation.
Dynamics of Forest Fragments Project (BDFFP)
  • Initiated by Thomas Lovejoy in 1979 to determine the minimum area of rainforest required to sustain species diversity.
  • Forest plot sizes in the study varied from 1 to 1,000 hectares.
  • Findings revealed:
    • Even minimal distances between forest fragments lead to species isolation.
    • Fragmentation causes "edge effects" such as increased light exposure, higher temperatures, and more susceptibility to disease and fire.
Spatial Scales in Biogeography
  • Global Scale: Entire world; isolation of species occurs due to large distances between regions.
    • Rates of speciation, extinction, and dispersal vary by geographic locations.
  • Regional Scale: Areas with uniform climates; species tied to their regions by dispersal.
    • Regional species pool (gamma diversity) is the total species present in a region.
  • Local Scale: Represents communities where species interactions and physiology are key.
    • Local diversity (alpha diversity) and beta diversity (turnover of species between communities) are fundamental concepts.
Species Diversity Concepts
  • Alpha Diversity: Diversity within a community.
  • Beta Diversity: Diversity between communities.
  • Gamma Diversity: Total diversity or regional diversity pool.
  • Local species richness can be influenced by the regional species pool, affecting community composition.
  • A slope of 1 in plotting local vs. regional species richness indicates all species in a region can be found in all communities (unlikely in nature).
  • Observations suggest:
    • Community richness often determined by regional species pool.
    • Local processes limit species diversity at saturation points.
Biogeographic Regions and Patterns
  • Wallace identified six biogeographic regions corresponding to tectonic plates, showing patterns of species diversity influenced by geographic area and isolation.
  • Species diversity shows a negative relationship with latitude, typically decreasing towards the poles.
  • Hypotheses for High Tropical Diversity:
    1. Higher diversification rates in tropical regions.
    2. Longer evolutionary time spans in stable tropical climates.
    3. Elevated productivity due to abundant resources.
Species-Area Relationships and Island Biogeography
  • The species-area relationship suggests increased richness with the area sampled—larger islands supporting more species.
  • Isolation as an island (or any isolated habitat) affects species diversity; closer to the mainland means higher immigration rates.
  • Equilibrium Theory of Island Biogeography (MacArthur and Wilson):
    • The number of species on an island results from the balance of immigration and extinction rates.
    • Predictions based on island size affecting extinction rates and distance affecting immigration rates.
  • Case Studies:
    • Krakatau and Simberloff and Wilson's mangrove experiments showed recovery patterns of species post-disturbance, supporting the theory.
Summary
  • Global diversity patterns generally conform to: 1) diversity decreases with latitude and 2) diversity varies across continents.
  • Local diversity is closely tied to the regional species pool.
  • "Island" environments experience diversity influenced by size and distance from sources of species."