Species richness & Species evenness

Species richness

The number of different species in a given area

Species evenness

The proportion of the community that is made up of an individual species

Difference between species richness and evenness?

Species richness measures the number of different species while species evenness tells the proportion of the community made up of an individual species.

2 communities could have the same number of species, but the proportion of each community made up by each species could vary, resulting in different biodiversities.

Species biodiversity

A meadow and a wheat field both contain 25 species (Species richness). In the meadow, all the species may be equally abundant (so species evenness is high), but in the wheat field, 95% of plants are a single species of wheat. The meadow therefore has greater biodiversity.

How to measure species richness?

Count the number of different species in a given area.

Why is using species richness not good enough to determine the biodiversity of the habitat?

The number of species in a habitat is qualitative data, however it does not take into account the number of individuals in each species. To include this, you need to estimate the evenness, which is quantitative.

How to estimate the biodiversity of a habitat?

Simpson's Index of Biodiversity

Result of Simpson's Index of Biodiversity

Will be between 0 and 1

What does a high Simpsons index mean (close to 1)?

- High biodiversity, high species richness, high species evenness

- Many species present

- Ecosystem is stable

- Likely to withstand a change

What does a low Simpsons index mean (close to 0)?

- Dominated by 1 species

- Change in 1 species likely to affect the whole habitat

- Ecosystem is unstable

- Unlikely to withstand a change

When filling out a Simpson's Index table, what do you have to do?

Write the numbers to the same decimal places as the numbers in the table