Measuring Diversity: Richness, Evenness and Turnover Between Sites
Counting species is not enough, because two communities with the same number of species can differ in how evenly individuals are spread. Indices combine richness and evenness, and a separate measure compares which species differ between sites.
What a learner can do afterwards
- Calculates a diversity index from abundance data and explains what evenness contributed.
- Compares two communities with equal richness and different evenness.
- Distinguishes diversity within a site from turnover between sites.
1 · Read
Species diversity includes richness and evenness. Richness is the number of species present. Evenness describes how individuals are distributed among species. Compare two forests, each with 10 tree species. In one, every species has equal numbers of trees. In the other, 90 percent of the trees belong to one species. Richness is equal, but the first forest has greater evenness and species diversity.
Picture two meadows, each with 5 species and 100 plants. The first holds 20 plants of each species. The second holds 92 of one species and 2 of each other species. Richness ties at 5, but the first meadow has greater species diversity because its abundances are more even. The chart shows the first meadow.
Compare sites using similar sampling methods and effort. Count individuals per species, then compare richness, evenness and the species lists. Diversity within a site is alpha diversity. Beta diversity describes differences in species composition among sites, including turnover when species replace others. If two occupied sites share no species, they have complete turnover. A species found at only one sampled site may still occur elsewhere in the world.
Richness, evenness and local uniqueness do not by themselves rank conservation priorities. Also examine threats, habitat condition, species' geographic ranges and likely protection benefits. If other relevant conditions are equal and the aim is to prevent extinction, a site containing a threatened species found nowhere else has a stronger claim than one containing only widespread species. Losing its only habitat could cause extinction.
Evenness compares how evenly individuals are distributed across species. Equal counts give equal shares and high evenness. To find a species' share, divide its count by the total count. Diversity can help maintain ecosystem functions when species have overlapping roles but respond differently to disturbance. Another species may maintain a function when one declines, but recovery is not guaranteed.
Equal counts give equal shares: each species has 3 of the 9 individuals.
The Gini-Simpson index combines richness and evenness: D = 1 - sum(p²), where p is each species' share of all individuals. Square each share by multiplying it by itself, add the squares, then subtract from 1. For counts of 6 and 4, the shares are 0.6 and 0.4. D = 1 - (0.36 + 0.16) = 0.48. Counts of 5 and 5 give 0.50. With equal richness, greater evenness raises this index. Other indices, such as Shannon's, also use richness and evenness.
The counts total 10, so the species' shares are 6/10 and 4/10.
Richness counts species, evenness describes how individuals are distributed, and turnover compares sites.
Lesson corrections, worked examples and revised questions written for this dataset.
Reading summaries adapted from Khan Academy articles, with original explanatory framing. The hypothetical introduction example and index flashcard were written for this dataset. Exploring Ecosystems: Tropical Rainforest Diversity is by California Academy of Sciences, linked via YouTube through the academy corpus record. License terms for retained third-party material are unverified.
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