Species richness is the number of different species in a community, while species evenness describes how equally individuals are distributed among those species. Together, they determine species diversity. This is core SL and HL content in IB ESS subtopic 3.1, Biodiversity and evolution.
| Measure | What it describes | Simple example |
|---|---|---|
| Species richness | The number of species present | A woodland containing 10 tree species has greater richness than one containing 6. |
| Species evenness | The similarity of species' relative abundances | Evenness is high if each tree species has similar numbers; it is low if one species dominates. |
| Species diversity | The combined effect of richness and evenness | A community is generally more diverse when it has both many species and a balanced distribution. |
Consider two sampled communities, each containing 100 organisms from four species. Both have the same richness because each contains four species.
- Community A contains 25 individuals of each species. It has high evenness.
- Community B contains 85 individuals of one species and 5 individuals of each remaining species. It has low evenness.
Community A therefore has greater species diversity even though the two communities have identical richness. Greater diversity can contribute to ecosystem resilience because ecological functions are less dependent on one dominant species.
A common misconception is that biodiversity means only the number of species. This is incorrect: counting species measures richness, but ignores their relative abundances. In fieldwork, abundance data collected using random quadrats can be used to compare both components.
IB ESS also uses Simpson's reciprocal index to combine richness and evenness into a quantitative measure. A higher index value indicates greater diversity, but the result is most meaningful when comparing sites or monitoring the same ecosystem over time.