Shannon Diversity Index Calculator
Computes the Shannon diversity index (H′) from the individual counts of up to three species in an ecological community. Use it in field ecology to compare habitat quality or monitor biodiversity over time.
About this calculator
The Shannon diversity index (H′) quantifies both the richness and evenness of species in a community. It is calculated as: H′ = −Σ (pᵢ × ln(pᵢ)), where pᵢ is the proportion of individuals belonging to species i (i.e., count of species i divided by the total count). The natural logarithm (ln) is standard; some disciplines use log₂ or log₁₀, which changes the scale but not the interpretation. H′ = 0 means the community contains only one species; higher values indicate greater diversity. For three species, the formula expands to: H′ = −(p₁ ln p₁ + p₂ ln p₂ + p₃ ln p₃). The theoretical maximum for three equally abundant species is ln(3) ≈ 1.099. Evenness (whether species are represented equally) strongly influences H′ alongside species count.
How to use
A meadow survey records: Species 1 = 40 individuals, Species 2 = 35 individuals, Species 3 = 25 individuals. Total = 100. Step 1 — p₁ = 40/100 = 0.40; p₂ = 35/100 = 0.35; p₃ = 25/100 = 0.25. Step 2 — H′ = −(0.40 × ln(0.40) + 0.35 × ln(0.35) + 0.25 × ln(0.25)) = −(0.40 × (−0.916) + 0.35 × (−1.050) + 0.25 × (−1.386)) = −(−0.3664 − 0.3675 − 0.3466) = 1.0805. Enter 40, 35, and 25 in the respective fields to get H′ ≈ 1.0805.
Frequently asked questions
What is a good Shannon diversity index value for an ecological community?
H′ values typically range from 0 (a single-species community) to around 4–5 in highly diverse tropical ecosystems, though values above 3 are considered high in most temperate habitats. Most natural communities fall between 1.5 and 3.5. However, 'good' is context-dependent: a recovering disturbed site might show improvement even from H′ = 0.8 to 1.2, while a pristine old-growth forest might reach H′ = 3.8. Always compare H′ values within the same ecosystem type and sampling method for meaningful conclusions.
How does species evenness affect the Shannon diversity index compared to species richness?
The Shannon index captures both richness (how many species are present) and evenness (how equally individuals are distributed among species). A community with 10 species where one species accounts for 99% of individuals will have a very low H′, possibly lower than a 3-species community with equal abundance. This sensitivity to evenness is both a strength and a limitation. If you want to separate the two components, calculate species richness (S) directly and compute Pielou's evenness index J′ = H′ / ln(S) alongside H′.
Why does the Shannon diversity formula use natural logarithm rather than log base 2 or log base 10?
The choice of logarithm base changes the units of H′ but not the relative ranking of communities. Natural logarithm (ln) gives H′ in 'nats', log₂ gives 'bits', and log₁₀ gives 'decibels'. Ecology conventionally uses ln because it connects naturally to information theory as formalised by Shannon. Some community ecology software defaults to log₂, so always check which base was used before comparing H′ values across studies. This calculator uses the natural logarithm, consistent with the most common ecological literature.