Measuring Species Richness & Species Evenness (OCR AS Biology): Revision Note

What is 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

Area 1 and Area 2 have the same species richness but different species evenness. As it has a higher species evenness, the overall species diversity of Area 1 is higher than that of Area 2, as species diversity takes both richness and evenness into account.