OCR AS Biology

Revision Notes

4.2.4 Measuring Species Richness & Species Evenness

Measuring Species Richness & Species Evenness

  • One measure of biodiversity is the species diversity in a given area
  • There are two key components of species diversity:
    • Species richness
    • Species evenness
  • Species richness is a measure of the number of different species within a given area
    • An area with a greater number of species will have a greater species richness
    • For example, a tropical rain forest has a very high number of different species so it would be described as being a species-rich area
  • However, species richness can be a misleading indicator of diversity as it does not take into account the number of individuals of each species
  • Species evenness is a measure of the relative abundance of the different species within a given area
    • An area in which all the species have similar abundances will have a greater species evenness
    • In the example below, Area 1 and Area 2 both contain 4 tree species
    • However, Area 2 is actually dominated by one species and in fact, one of the species is very rare (only one individual)
    • Although the two areas have exactly the same species richness, Area 1 has a higher species evenness (and therefore a higher overall species diversity) than Area 2
    • This example illustrates the limitations of using just species richness on its own
  • Conservationists often favour the use of an index of diversity (known as Simpson’s Index) as it takes into account both species richness and evenness

Exam Tip

To measure species richness you simply count the number of different species present.

To measure species evenness you count the number of different species present and the number of individuals of each species (basically any sampling method that takes into account the abundance of each species).

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