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
  • E.g., arctic fauna
  • E.g., similarities 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:
   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 disrupts dispersal and gene flow
   2. climate change - places limits on where mammals can live
      • due to continental drift
      • due to ice ages
   
   
   
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?
         • seasonal changes in temperature and food availability
           • whale migration
           • Serengeti herbivore migrations
         • Large scale climatic changes such as the ice ages - resulted in Southward migration of many species
      
      
      
   3. Faunal interchange - moving into a brand new geographic region
      1. Often happens simultaneously with multiple species
      2. Often goes in both directions (i.e., some species go one way, while other species go the other way)
      3. Why does it happen?
         • climatic changes (e.g., human dispersal)
         • continental drift
         • other events that result in the removal of a dispersal barrier (e.g., decreases in sea level during ice ages or volcanic activity that created a land bridge in Central America about 13 MYA, see Fig 2.2)
      
      
      

      WORKSHEET: How might the factors listed below influence ability to disperse, migrate or colonize a new region? habitat type (terrestrial vs. aquatic, tropical vs. temperate, arid vs. wet) mode of locomotion (flying vs. terrestrial) body size dietary habits (generalized vs. specialized)

      
      
      
      
   4. Categories of dispersal/migration/faunal interchange
      1. corridor routes
      2. filter routes
         
         
         

         WORKSHEET: What kinds of biotic and abiotic factors might create a filter? What about man-made things?

         
         
         
         
      3. sweepstakes routes (e.g., rafting)
         
         
         

         WORKSHEET: What kinds of factors might create a sweepstakes route? How do man-made things both create sweepstakes routes and provide ways of successfully dispersing across sweepstakes routes?

      
      
      
   
   
   
5. So...similarities that we see today among separated land masses are the result of
   • fairly recent movement of mammals between land masses (faunal interchange)
   • isolation of widely distributed mammals on separate land masses (vicariance)
   • convergent evolution of once divergent lineages
   
   
   

   WORKSHEET: Case studies of mammals with differing lifestyles, modes of locomotion, body size, and dietary habits

   
   
   
6. How it all fits together - Explore how climate and the position of Earth's tectonic plates have changed as mammals were evolving
   1. Mammals evolved at a time (>200MYA) when the present-day continents were a single land mass (Pangaea)
   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 some widely distributed species became geographically isolated (vicariance); some species invaded new land masses when continents came into contact (faunal interchange)
   
   
   
7. Examples
   1. Mammals of Madagascar
      • Colonization of Madagascar by mammals from the African mainland (an example of a sweepstakes route)
      
      
      
   2. Bering land bridge between Palearctic and Nearctic land masses
      • Existed for most of the last 65 MY
      • Examples of species that crossed
      
      
      
   3. Marsupials (see how the position of the land masses relates to marsupial evolution and movements)
      • Thought to have evolved in N. America or Asia (some confusion because the oldest fossils were found in Asia, but the most numerous fossils were found in N. America)
      • dispersed throughout N. and S. America and through Antarctica (100 MYA)
      • to Australia (50 MYA)
      • Australia became geographically isolated (an example of vicariance) and subsequently 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 (an example of faunal interchange)
      
      
      
   4. Neotropical faunal interchange
      • N. and S. America have been separated for most of the last 65 MY by water
      • somewhere between 13 and 3 MYA a land bridge through Panama developed
      • North to South interchange:
        • Carnivores (foxes/wolves, cats, bears, mustelids)
        • Artiodactyls (camels, peccaries)
        • Perissodactyls (equids, tapirs)
        • Rodents
        • Shrews
        • Rabbits
      • South to North interchange:
        • armadillos, sloths, anteaters
        • Hystricomorph rodents (porcupine, nutria)
        • Opossum
        • Primates
      
      
      
      

      WORKSHEET: Neotropical faunal interchange