Species Evenness Calculators
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Pielou's Evenness Index (J')
J' = H' / H'_max = H' / ln(S)
H' = Shannon-Wiener diversity index = −Σpᵢ ln(pᵢ). S = species richness. H'_max = ln(S) (maximum possible H' when all species are equally abundant). J' ranges from 0 (one species dominates completely) to 1 (all species equally abundant).
Example: 3 species with proportions 0.7, 0.2, 0.1: H' = −(0.7 ln 0.7 + 0.2 ln 0.2 + 0.1 ln 0.1) = −(−0.2496 − 0.3219 − 0.2303) = 0.802. H'_max = ln(3) = 1.099. J' = 0.802/1.099 = 0.730.
Other Evenness Metrics
- Simpson's evenness: E = (1/D) / S = 1/(D × S); D = Σpᵢ²; ranges 0–1
- Hill numbers-based evenness: E = ⁰D / ᵍD = S / ᵍD; ratio of q=0 to q=g Hill numbers
- Camargo's evenness (E_C): based on pairwise absolute abundance differences; robust alternative to Pielou's J
Evenness vs. Richness
Richness = number of species. Evenness = distribution of individuals. Both contribute to diversity: H' = evenness contribution + richness contribution. Communities with low evenness (one dominant species) have lower biodiversity than communities with equal richness but high evenness. Disturbed or stressed communities often show reduced evenness — one or few stress-tolerant species become dominant.
Glossary
Frequently Asked Questions
Species evenness measures how equally individuals are distributed among species. Pielou's J = H'/ln(S): Shannon diversity (H') divided by the maximum possible Shannon diversity for that species number (ln(S)). J ranges 0–1: J = 1 means all species have identical abundance; J → 0 means one species dominates. Example: community with S = 4 species, abundances 50, 25, 15, 10 (N = 100): pᵢ = 0.50, 0.25, 0.15, 0.10; H' = −Σpᵢ ln(pᵢ) = 1.279 nats; ln(4) = 1.386; J = 1.279/1.386 = 0.923 (high evenness).
Species richness (S) = the count of distinct species in a community. Species evenness = how equally individuals are distributed among those species. Two communities can have identical richness but very different evenness and diversity. Example: Community A: 4 species with 250, 250, 250, 250 individuals — J = 1.0 (maximum evenness). Community B: 4 species with 970, 10, 10, 10 individuals — J ≈ 0.28 (low evenness). Both have S = 4, but community A has much higher Shannon diversity. Community B is dominated by one species; the rare species contribute little to diversity indices.
Disturbance and stress typically reduce evenness by allowing one or a few tolerant species to become dominant while sensitive species decline. Intermediate disturbance hypothesis: maximum diversity (including evenness) occurs at intermediate disturbance levels — too little disturbance → competitive dominance by few species (reduced evenness); too much disturbance → only disturbance-tolerant specialists survive (again low evenness); intermediate disturbance → diverse coexistence with high evenness. Water pollution, eutrophication, and agricultural intensification often reduce evenness in plant, invertebrate, and microbial communities by creating competitive advantages for pollution-tolerant or nutrient-responsive species.
High species evenness supports ecosystem function through several mechanisms: (1) Greater functional redundancy — multiple species performing similar functional roles means the system is more resilient to loss of any one species. (2) Niche complementarity — when species are equally abundant and each uses resources slightly differently, total resource use is maximized and productivity is higher. (3) Stability — more even communities show less extreme population fluctuations because no single species dominates, reducing the impact of any one species' population cycle on the whole community. Studies in grasslands and forests show that more even communities maintain higher productivity and are more resistant to drought, invasion, and disease outbreaks.