🌿 Calculators

Whittaker Beta Diversity

Calculates Whittaker's beta diversity (βw), the total species richness of the sample set divided by the mean number of species per sample, minus one. It measures species turnover between samples: zero means identical composition, and high values indicate communities that differ sharply. Enter the total richness and the per-sample mean.

Total species richness (gamma) Mean species per sample (alpha)

Result

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How much the species list shifts from plot to plot

Every floristic or faunal survey report lists the mean richness per plot and the pooled species list for the whole area. The number that usually goes missing sits between the two: how much composition shifts from one plot to the next. Two areas can average exactly 12 species per plot while one of them holds a mosaic of distinct communities and the other repeats the same assemblage everywhere. Without a beta figure, the report describes both the same way, and the management call rests on an incomplete reading.

The calculation runs βw = S / ᾱ − 1, where S stands for total richness across the sample set (gamma diversity, the consolidated species list) and ᾱ for the mean species count per sample. Output carries no unit and ranges from 0, when every plot shares the same composition, up to N − 1 with N plots, when no species repeats between them. The values already loaded in the form, 48 species overall and 12 per plot on average, return 3.0000 — equivalent to four plots with nothing in common. Whittaker published the index in 1960 as S/ᾱ; the variant with the −1, adopted here, has become standard and reads zero when turnover vanishes.

Sampling design drives the index, and that limitation bites hardest: S grows with the number of plots while ᾱ grows with plot size, so a βw from two studies with different effort will not compare directly. The maths treats the matrix as presence and absence, which means a species recorded once weighs as much as the canopy dominant. The most frequent slip is typing the richness of one particular plot into the second field instead of the average across all of them. Should ᾱ come in at zero or below, the page flags it and skips the result.

Frequently asked questions

What does the 3.0000 shown with the default values mean?

It means the pooled list for the area holds four times as many species as a typical plot: 48 divided by 12 gives 4, minus 1 gives 3. Reading the index as a count of communities helps — 3 matches what four completely distinct plots would yield, with not a single shared species. Anything under 1 points to plots that resemble each other closely, while 3 or 4 upward describes a mosaic, patches of vegetation that barely overlap.

Another textbook prints the formula without the −1. Which one holds?

Both circulate. Whittaker defined βw = S/ᾱ back in 1960, which reads 1 when plots match perfectly; the form with −1, used here, shifts that origin to zero and measures turnover on its own. Converting takes one step: add 1 to the figure this page returns and you have the original version. What breaks down is comparing a value computed one way against a value computed the other way, a mix-up that shows up often in literature reviews. State in your methods which convention you picked.

Can abundance data go in instead of species counts?

Not on this page. Whittaker built βw on presence-absence lists, and the tool sticks to that: both fields expect species counts and nothing else. When abundance matters, a forest where one species covers 80% of the individuals say, reach for a different index such as Bray-Curtis dissimilarity or Jost decomposition with Hill numbers. Running both and comparing pays off: a wide gap between them usually exposes rare species inflating βw.

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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.