Biogeography

Central questions

• What accounts for the patterns of distribution of mammals?
• How did they get to where we now find them?
• Did they evolve where we now find them, or did they evolve elsewhere and move?

Key observations:

• Similar habitats on separate continents have very similar mammalian taxa, often the same species
  • arctic fauna
  • convergence between australian marsupials and eutherian mammals (e.g., marsupial moles and North American eutherian moles and African golden mole)

How do we make sense of this? It has important implications for our understanding of mammalian systematics and evolution...

1. Where are mammals now? Seven basic geographical regions (Fig. 5.2):
   1. Palearctic
   2. Nearctic (Regions 1 + 2 often combined = Holartic)
   3. Ethiopian
   4. Oriental
   5. Australian
   6. Neotropical
   7. Oceanic (oceans + isolated islands)
   
   
   
2. What kind of fauna is associated with each region?
   Each shares much of its diversity with other regions.
   In general
   • diversity in tropics > diversity in temperate regions
   • diversity of well-connected places > diversity of isolated places
   
   
   
3. Abiotic processes that have affect the distribution of mammals
   1. continental drift (Fig. 4.11)
   2. climate change
   
   
   
4. Biotic processes that affect the distribution of mammals
   1. dispersal - an individual event
      1. Why disperse?
         • avoidance of inbreeding
         • avoidance of competition for breeding opportunities
         • avoidance of competition for resources
      
      
      
   2. migration - a population level event
      1. Why migrate?
         • large scale climatic changes
           • whale migration
           • Serengeti herbivore migrations
           • Southward migration of many species during ice ages
      
      
      
   3. faunal interchange - moving into a brand new geographic region
      Often happens simultaneously with multiple species
      1. Why does it happen?
         • climatic changes
         • continental drift
         • other events that result in the removal of a dispersal barrier (e.g., decreases in sea level during ice ages)
      
      
      
   4. mechanisms of dispersal
      1. corridor routes
      2. filter routes
      3. sweepstakes routes (e.g., rafting)
         • Madagascar
      
      
      
   5. factors influencing ability to disperse, migrate or colonize a new region
      1. habitat type (terrestrial vs. aquatic)
      2. mode of locomotion (flying vs. terrestrial)
      3. body size
      4. generalized vs. specialized dietary habits
   
   
   
5. So...similarities that we see today among separated land masses are the result of
   • fairly recent movement of mammals between land masses
   • isolation of widespread mammals on separate land masses (vicariance)
   • convergent evolution of once divergent lineages
   
   
   
6. How it all fits together:
   1. Mammals evolved at a time (>200MYA) when the present-day continents were a single land mass (Pangaea)(Fig. 4.11)
   2. About 180 MYA the northern land mass (Laurasia) started drifting northward away from southern land mass (Gondwanaland), and individual continents started drifting apart from each other
   3. About 65 MYA South America and Antarctica/Australia drifted off from Gondwanaland
   4. Even more recently, South America moved west, India moved north
   5. ...so during the course of mammalian evolution species became geographically isolated; some species invaded new land masses when continents came into contact
   
   
   
7. Examples
   1. Bering land bridge between Palearctic and Nearctic land masses
      • Existed for most of the last 65 MY
      • Examples of species that crossed
      
      
      
   2. Neotropical faunal interchange (Fig. 5.13)
      • N. and S. America have been separated for most of the last 65 MY by water
      • 3 MYA a land bridge through Panama developed
      • North to South interchange:
        • Carnivores (foxes/wolves, cats, bears, mustelids)
        • Atriodactyls (camels, peccaries)
        • Perissodactyls equids, tapirs)
        • Rodents
        • Shrews
        • Rabbits
      • South to North interchange:
        • armadillos, sloths, anteaters
        • Hystricomorph rodents (porcupine, nutria)
        • Opossum
        • Primates
      
      
      
   3. Marsupials
      • Thought to have evolved in N. America
      • dispersed throughout N. and S. America and through Antarctica (100 MYA)
      • to Australia (~65 MYA)
      • Australia became geographically isolated and marsupials radiated widely
      • Antarctica became really cold, and marsupials went extinct
      • S. American marsupials persisted, while N. American marsupials went extinct
      • Appearance of the Panama land bridge allowed the ancestor of the Virginia opossum to move into N. America from S. America