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IV. Community Ecology (Who's who and who's not)

B.  How important are species interactions
2. Patterns in species numbers

Are there predictable patterns among communities, and, if so, can interactions between species explain these patterns?

Examples of patterns in the number of species within communities

  • Increasing species number in tropical regions:
  • Ant species:

      Brazil 222
      Trinidad 134
      Cuba 101
      Utah  63
      Iowa 73
      Alaska 7
      Arctic Alaska 3

    Three general types of explanations:

  • Theory of island biogeography:

  • The theory was formalized by MacArthur and Wilson to explain two trends in the number of species found on an islands:

    1. Number of species increases with island size (e.g. ants, reptiles, birds, plants)


    S = number of species on island
    A = Area (size) of island
    b = coefficient varying w/ taxa and region
    z = slope varying w/ degree of isolation (and dispersal abilities)


    2. Number of species decreases with distance of island from the mainland


    What causes these trends exist?

    Robert MacArthur and E.O. Wilson explain this as dynamic equilibrium between extinction and immigration:

    Turnover rate is rate at which one species is lost and another regained when at equilibrium. Why does immigration decrease with increasing species number?  Why does extinction increase with increasing species number?

    How do island size and distant from mainland affect these rates?


    Other possible causes for trends:

    What is "diversity"  as measured by ecologists?
    Typically "diversity" in ecology refers not only to species number, but also how evenly the number of individuals per species comprise the total ('eveness'). 

    For example, which is most "diverse" according to the above definition?

  • Community A with 5 species, each with 100 individuals 
  • Community B with 5 species, 400 of one species and the other spp. w/ 25 each
  • Community C with 4 species, each with 125 individuals

  • Several different indices have been developed so that both species richness and eveness are incorporated into a single community number so that diversity among communities can be compared (for example, the Shannon Diversity Index)

    Diversity can also be partitioned into alpha, beta, and gamma diversity.