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Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Mechanisms of Coexistence

This question has long been asked by ecologists, and is important now if we wish to discover how humans and other species can coexist.

Four primary mechanisms underlying coexistence exist:

  • Niche relations.  Species coexist because they have different environmental requirements (soil, water, climate, etc.), so that no species will competitively exclude another.  These mechanisms operate only over distances in  which individuals directly interact with another (spread of roots in a plant, the size of a home range in an animal).
  • Habitat heterogeneity.  Species coexist  because of different environmental conditions found within a region, so that a species will have at least one place where it can not be competitively excluded.  The more combinations of environmental  conditions, the greater the number of species which can coexist.  This factor operates over distances in which environmental change is most pronounced (10-meter to 1000 kilometer)
  • Mass effect.  Species coexist because they are constantly dispersing in from other habitats they disperse to.  If a bog and dry prairie are neighbors, there will be a few dry prairie plants in the bog.  These strays will soon be eliminated from the habitat, but by the time that happens, more individuals have colonized.  this effect will be most pronounced in regions where many different habitats are in close proximity, and will be most pronounced at 1/2-5 kilometer distances.
  • Trophic equivalency.  Species coexist because habitats are isolated from each other, limiting the number of potential competitors, and the rate of competitive exclusion.  If isolation between habitats is frequent, many species with similar niches may thus be able to coexist.  This effect is most often thought to be important between continents.  However, it may also be important at much smaller distances (100 kilometers or less.)
  • These explanations can be put into two major classes:

  • Environment and niche relations:  coexistence is possible because of differences between requirements of species and differences in the environment (Niche relations and Habitat heterogeneity)
  • Spatial and temporal constraint:  coexistence is possible because species have not been able to locate all potential habitats due to dispersal limitation and/or historical factors  (Mass effect and Trophic equivalency)
  • Of these, the first class of explanations have been most thoroughly investigated by ecologists.  Only within the last few decades have the latter explanations been formally considered.

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    Created 2 September 2011, Last Update 02 September 2011

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