Species diversity is a measure of the diversity within an ecological community that incorporates both species richness (the number of species in a community) and the evenness of species' abundances. Species diversity is one component of the concept of biodiversity.
Influence of Species Richness on Species Diversity
Species diversity is influenced by species richness. All else being equal, communities with more species are considered to be more diverse. For example, a community containing 10 species would be more diverse than a community with 5 species.
Influence of Evenness of Species Diversity
Species diversity is also influenced by the relative abundance of individuals in the species found in a community. Evenness measures the variation in the abundance of individuals per species within a community. Communities with less variation in the relative abundance of species are considered to be more “even” than a community with more variation in relative abundance. Consider the following two communities.
All five species in Community A have the same abundance, whereas there is great variation in abundance across the five species in Community B. For this reason, we would consider Community A to be more even.
All else being equal, communities with greater evenness are considered to have greater species diversity. Even though species richness of the two communities is equal (species richness = 5 in each community), Community B is less diverse than Community A because most of the individuals in Community B are members of the same species.
Measurement of Species Diversity
Determining which community has greater species diversity is easy when either species richness or evenness is held constant while the other parameter varies, but often communities will vary in both richness and evenness. Scientists have developed a variety of mathematical equations (or indices) that incorporate both species richness and evenness into a single measure of species diversity (e.g., the Shannon-Wiener Index and Simpson’s Index). Different diversity indices assign different weightings to species richness and evenness, so the so most useful index to choose depends on the circumstances.
- Magurran, A.E. 1988. Ecological Diversity and its Measurement. Princeton University Press, Princeton, NJ. ISBN: 0691084912