4.2.4 Measuring Species Richness and Species Evenness

Question 1

What is species richness and species evenness?

Answer 1

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