Margalef Index Calculators

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The Margalef diversity index (D_Mg) is a simple biodiversity measure that quantifies species richness relative to the total number of individuals sampled, accounting for the fact that more individuals sampled will typically yield more species. Proposed by Ramón Margalef in 1958, it is calculated as: D_Mg = (S − 1) / ln(N), where S is the number of species and N is the total number of individuals. Higher values indicate greater diversity. Margalef index is widely used in ecology, marine biology, and environmental impact assessments as a quick, standardized richness metric.

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Margalef Index Formula

D_Mg = (S − 1) / ln(N)

S = number of species; N = total number of individuals; ln = natural logarithm. Example: sample contains 15 species and 200 individuals: D_Mg = (15 − 1) / ln(200) = 14 / 5.298 = 2.64.

Interpreting Margalef Values

There are no universal thresholds, but relative comparisons are meaningful. In benthic macroinvertebrate studies, values below 2 are sometimes interpreted as poor quality; 2–5 as moderate; above 5 as good biodiversity. These thresholds are context-dependent — tropical systems naturally have higher values than arctic or deep-sea systems.

Margalef vs. Other Richness Indices

  • Menhinick index: D_Mn = S / √N — simpler formula but more sensitive to sample size
  • Shannon-Wiener (H'): Accounts for both richness and evenness, not just species count
  • Simpson's index: Emphasizes dominance — opposite focus from Margalef

Limitations

Margalef index increases with sample size (N) even when true diversity is the same, because larger samples typically reveal more species (species-area effect). Comparing communities sampled with very different effort requires caution. Rarefaction is preferred when sample sizes differ substantially because it standardizes richness estimates to equal sampling effort.

Glossary

Margalef Index (D_Mg)
A biodiversity richness measure: D_Mg = (S − 1) / ln(N); S = species number, N = total individuals; corrects species count for sample size using the natural logarithm.
Species Richness
The total number of species in a community or sample; the simplest biodiversity measure; increases with sample size, requiring correction (Margalef, rarefaction) for fair comparison.
Rarefaction
A method for standardizing species richness estimates across samples of different sizes by subsampling to the smallest common sample size; preferred over richness indices for unequal sampling effort.

Frequently Asked Questions

Margalef index D_Mg = (S − 1) / ln(N), where S is species number and N is total individuals. It corrects raw species count for sample size because larger samples yield more species. Example: 12 species in 150 individuals: D_Mg = (12−1)/ln(150) = 11/5.011 = 2.19. Higher values indicate greater species richness relative to sampling effort. Compare values from sites sampled with similar effort for meaningful interpretation.

A higher Margalef index indicates greater species diversity relative to the number of individuals sampled. High values are typical in species-rich, mature, or undisturbed ecosystems — tropical forests, coral reefs, and unpolluted rivers. Low values suggest disturbed, stressed, or species-poor communities — heavily polluted rivers, monoculture habitats, or early successional communities dominated by a few abundant species. Values should always be interpreted relative to the ecosystem type and sampling context.

Margalef index (D_Mg = (S−1)/ln(N)) measures only species richness corrected for sample size — it does not consider how evenly individuals are distributed among species. Shannon-Wiener (H' = −Σpᵢ × ln(pᵢ)) integrates both species richness and evenness — a community with 10 species all equally abundant has higher H' than one with 10 species but one dominant species. Margalef is simpler and faster; Shannon-Wiener is more informative because it captures community structure beyond just species count.

Margalef index partially corrects for sample size using ln(N), but this correction is imperfect — different communities with identical true diversity but different sample sizes will still give different D_Mg values. Rarefaction standardizes by randomly subsampling all communities down to the smallest sample size, then comparing species counts. This produces directly comparable richness estimates. Alternatively, rarefaction curves show how species richness accumulates with sampling effort, allowing comparison of communities at any standardized sample size.