Fish Biodiversity Index Calculator
Calculate Shannon-Wiener diversity (H'), Simpson's index, Pielou's evenness, and Margalef's richness from species abundance data.
About this calculator
Biodiversity isn't a single number — this calculator reports four classic ecological indices side by side because each captures a different aspect of a community's structure from the same up-to-eight species counts. Shannon-Wiener H' sums, across all species, each species' proportional abundance (pi) times its natural log, negated — it rewards both having more species present and having them evenly represented, so a sample with two species split evenly scores lower than one with five species split evenly, but a sample dominated by one species scores lower still even with the same species count. Simpson's D takes the opposite framing: it's the sum of each species' proportional abundance squared, which approximates the probability that two randomly picked individuals belong to the same species — so higher D means lower diversity, which is why the calculator also reports 1-D as an inverted, more intuitive "diversity" figure where higher is always better.
Pielou's evenness (J') normalizes Shannon's H' by the maximum possible diversity for that many species (ln of species richness), isolating how evenly abundance is distributed independent of how many species you found. Margalef's index, (S-1)/ln(N), estimates richness while correcting for sample size, since larger samples tend to turn up more species by chance alone. If no species counts are entered, every index returns zero rather than an undefined value — a genuinely empty sample and "no diversity" aren't the same thing, so don't read a zero as a survey finding nothing alive.
Inputs
Results
Shannon-Wiener H'
1.72
How to Use This Calculator
- Enter the number of species observed in the survey transect or station.
- Input the abundance (individual count) for each species.
- Review the Shannon diversity index (H'), Simpson's index (D), and species evenness (J').
- Compare indices across sites or time periods to assess biodiversity change.
- A Shannon H' above 3 indicates high diversity; below 1.5 indicates low diversity or dominance.
How the result changes with Species 1 count
| Species 1 count | Shannon-Wiener H' |
|---|---|
| 23 | 1.81 |
| 34 | 1.77 |
| 68 | 1.6 |
| 113 | 1.39 |
What each input means
- Species 1 count
- Number of individuals of the first species in the sample.
- Species 2 count
- Number of individuals of the second species.
- Species 3 count
- Number of individuals of the third species.
- Species 4 count
- Number of individuals of the fourth species.
- Species 5 count
- Number of individuals of the fifth species. Enter 0 if fewer species.
- Species 6 count
- Number of individuals of the sixth species. Enter 0 if fewer species.
- Species 7 count
- Number of individuals of the seventh species. Enter 0 if fewer species.
- Species 8 count
- Number of individuals of the eighth species. Enter 0 if fewer species.
What each result means
- Shannon-Wiener H'
- Shannon-Wiener diversity index using natural log. Higher values = more diverse. Typical range 0-4.
- Simpson's diversity (1-D)
- Simpson's diversity index. Ranges 0-1; closer to 1 means higher diversity.
- Simpson's D (dominance)
- Simpson's dominance index. Probability that two random individuals belong to the same species.
- Pielou's evenness (J')
- Ratio of observed to maximum possible diversity (0-1). 1.0 = perfectly even distribution.
- Species richness (S)
- Total number of species with at least one individual.
- Total individuals (N)
- Sum of all individual counts across species.
- Margalef's index
- Margalef's species richness index: (S-1)/ln(N). Accounts for sample size.
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator report both Simpson's D and 1-D?
Simpson's D is the sum of each species' squared proportional abundance, which approximates the chance that two randomly picked individuals are the same species — so a higher D actually means lower diversity, which is counterintuitive. The calculator also reports 1-D as an inverted version where higher numbers consistently mean higher diversity, matching how Shannon's H' and Pielou's J' already behave.
Why can two samples have the same Shannon H' but very different species counts?
Shannon's H' weighs both richness (how many species) and evenness (how balanced their abundances are), so a sample with fewer species that are perfectly evenly distributed can score similarly to a sample with more species that's dominated by one or two of them. That's exactly why the calculator also reports Pielou's evenness (J') separately, to isolate the evenness component from raw species count.
Why does Margalef's index matter if I already have species richness (S)?
Raw species richness doesn't account for sample size — a larger sample tends to turn up more species just by chance, even from an equally diverse community. Margalef's index, (S-1)/ln(N), corrects for this by dividing adjusted richness by the natural log of total individuals counted, making it more comparable across surveys with different total catch sizes.
What does it mean if the calculator returns all zeros?
That happens only when every species count entered is zero or blank — the engine treats a genuinely empty sample as undefined rather than as "zero diversity found in nature," and returns zero for every index as a placeholder. It's a data-entry state, not an ecological finding, so don't interpret it as a survey documenting an empty habitat.
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