Species evenness describes the commonness or rarity of a species; it requires knowing the abundance of each species relative to those of the other species within the

community A community is a social unit (a group of people) with a shared socially-significant characteristic, such as place, set of norms, culture, religion, values, customs, or identity. Communities may share a sense of place situated in a given g ...

. Abundance values can be difficult to obtain. Area-based counts, distance methods, and

mark and recapture Mark and recapture is a method commonly used in ecology to estimate an animal population's size where it is impractical to count every individual. A portion of the population is captured, marked, and released. Later, another portion will be captu ...

studies are the three general categories of methods for estimating abundance. Species evenness is combined with

species richness Species richness is the number of different species represented in an community (ecology), ecological community, landscape or region. Species richness is simply a count of species, and it does not take into account the Abundance (ecology), abunda ...

, (the number of species in the community), in order to determine

species diversity Species diversity is the number of different species that are represented in a given community (a dataset). The effective number of species refers to the number of equally abundant species needed to obtain the same mean proportional species abundan ...

, which is an important measure of community structure. Community structure in turn provides the quantitative basis needed to create hypotheses and experiments that help to increase understanding of how communities work. To demonstrate the contributions of species richness and species evenness to species diversity, consider the following hypothetical example, in which there are two meadow communities, each containing four species of butterflies. Hence, both communities have the same butterfly species richness, however their species evenness differs. In community A, one of the species constitutes 80% of the individuals in the community, while the remaining three species comprise only 20% of the individuals in the community. In community B, the number of individuals are evenly divided among the four species, (25% each). Therefore, community A has lower species evenness than community B. When comparing the two communities, even though each has the same species richness of four species, community B has the higher species diversity because it has higher species evenness. The Shannon index is the most commonly used way to quantitatively determine species diversity, H, as modeled by the following equation:

H = - \sum_^S  \ln

The Shannon index factors in both species evenness and species richness, as represented by the variables p_i and s, respectively. The lowest possible value of H is zero, and the higher a community’s H value, the greater its species diversity. If calculated for each of the two hypothetical butterfly communities described previously, the lower Shannon index value (H) would belong to community A, thereby confirming mathematically that this community has lower species diversity than community B. Furthermore, given that both communities have the same species richness, it is the lower species evenness in community A that drives the difference in species diversity.

References

{{DEFAULTSORT:Species Evenness Measurement of biodiversity

See also

References