Margalef Richness Index Calculators

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The Margalef richness index (D) is a simple diversity measure that corrects species richness for sample size: D = (S − 1) / ln(N), where S is the number of species observed and N is the total number of individuals sampled. It adjusts for the fact that larger samples always detect more species. Higher D = greater diversity. The Margalef index is less commonly used than Shannon or Simpson indices in modern ecology, but remains a quick, simple measure for comparing sites when sample sizes differ. It is particularly common in aquatic ecology and marine biology.

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Margalef Richness Index Formula

D = (S − 1) / ln(N)

S = number of species; N = total individuals; ln = natural logarithm. Example: survey finds 25 species in 300 individuals: D = (25−1) / ln(300) = 24 / 5.703 = 4.21.

Interpretation

D > 5: high diversity. D 2–5: moderate diversity. D < 2: low diversity. These are very approximate — context matters. D = 0 when S = 1 (only one species). D increases with increasing S and decreasing N.

Comparison with Other Indices

  • Shannon H': accounts for both species richness and evenness; most widely used
  • Simpson D: probability that two random individuals are same species (1-D = diversity)
  • Species richness (S): simple count; strongly affected by sampling effort
  • Margalef D: corrects S for sample size; intermediate in complexity

Limitations

Margalef assumes the number of species increases logarithmically with the number of individuals (log-normal species abundance distribution). This assumption may not hold for all communities. For robust diversity comparisons, rarefaction or non-parametric estimators (Chao1) are preferred over Margalef.

Glossary

Margalef Richness Index (D)
D = (S−1)/ln(N); corrects observed species richness for sample size; S = species count; N = total individuals; larger D = greater diversity; common in aquatic ecology.
Species Richness (S)
The count of species in a sample; increases with sampling effort; requires rarefaction or estimators (Chao1) for fair comparison between sites with different sample sizes.
Rarefaction
Statistical standardization of species richness to equal sampling effort; more rigorous than Margalef correction; implemented in R packages (iNEXT, vegan); preferred for modern diversity comparisons.

Frequently Asked Questions

D = (S − 1) / ln(N). S = number of species observed; N = total individual count. Example: benthic macroinvertebrate survey: S = 18 species; N = 450 individuals: D = (18−1)/ln(450) = 17/6.109 = 2.78. Compare two sites: Site A: 18 species, 450 individuals → D = 2.78. Site B: 20 species, 1,200 individuals → D = (20−1)/ln(1200) = 19/7.090 = 2.68. Site A has slightly higher Margalef D despite having fewer species because sample size is smaller, penalizing site B for its larger sampling.

Species richness (S) increases with sample size — larger samples inevitably detect more species. A raw species count is biased: a site sampled with 1,000 individuals will appear more diverse than the same site sampled with 100. Margalef's formula (S−1)/ln(N) partially corrects for this by dividing by ln(N) — a natural logarithm of sample size, because species accumulation typically follows a log relationship with sample effort. This correction allows fairer comparisons between sites sampled at different intensities, though rarefaction provides a more rigorous statistical correction.

Margalef D: based only on species count (S) and total individuals (N); doesn't use relative abundances; simple to calculate; corrects species richness for sample size. Shannon H' = −Σpᵢ ln(pᵢ): uses relative abundances (pᵢ) of each species; accounts for both richness and evenness; more information-theoretically sound; the standard diversity index in most ecology studies. Shannon is more sensitive to rare species than Margalef. When sample size is equal across sites, Shannon provides a more complete picture of diversity. Margalef is useful when: only species lists and total counts are available; quick field comparisons are needed; working with aquatic ecology literature that historically used Margalef.

Limitations: (1) Assumes log-normal species accumulation: (S−1)/ln(N) only corrects for sample size if species accumulate logarithmically — this is not always true. (2) Ignores evenness: two communities with the same S and N but very different abundance distributions get the same D. (3) Sensitive to taxonomic resolution: depends on how finely species are identified (genus vs. species vs. morphospecies). (4) Not widely used in modern ecology: Shannon H' and Simpson's D, plus rarefaction and non-parametric estimators (Chao1, ACE), provide more robust and widely interpretable diversity estimates. Modern preference: report species richness (S), Shannon H', and Pielou's J' (evenness) with rarefied samples rather than Margalef D alone.