Shannon index

What is the Shannon index?

The Shannon index (also called the Shannon-Wiener or Shannon-Weaver index) is one of the most widely used measures of alpha diversity in ecology and microbiome research. It comes from information theory: Claude Shannon defined entropy in “A Mathematical Theory of Communication”, published in the Bell System Technical Journal in 1948. Ecologists later borrowed the formula to quantify how uncertain you would be about the species of an individual picked at random.

The more types of microorganism there are, and the more evenly they are spread, the greater that uncertainty and the higher the index.

How it is calculated

The formula is:

H′ = −Σ pᵢ · ln(pᵢ)

where pᵢ is the relative abundance of each taxon, its share of the total. In practice:

• If only one taxon is present, H′ = 0.
• For a fixed number of taxa, the index peaks when they are all equally abundant.
• Rare taxa carry more weight than they do in the Simpson index.
• The value changes with the logarithm base (natural, base 2 or base 10), so a report should state which one it uses.

Pielou’s evenness is sometimes derived from it (H′ divided by the logarithm of richness) to isolate how balanced the community is.

How to interpret it

• There is no universal “good” value. The index depends on technique (16S or metagenomics), taxonomic level, sequencing depth and how sequences are grouped (OTUs or ASVs). Compare it only with the same laboratory’s reference interval.
• Low is not a diagnosis. Observational studies associate low gut diversity with dysbiosis and recent antibiotic exposure, but a low value does not point to a specific cause.
• High is not the same as healthy. At some body sites, such as the healthy vagina, low diversity is expected.

Example

A functional medicine doctor orders two stool tests six months apart from the same laboratory for a patient with long-standing bloating. The Shannon index moves from below the reference interval to within it, alongside dietary changes and some improvement in symptoms. The doctor reads this as a consistent trend, without crediting the change in the index for the improvement, and bearing in mind that the microbiota also fluctuates on its own.

Shannon index at Xenogene

According to its product page, the Gut Microbiota test assesses bacterial diversity as part of a metagenomic stool analysis using Nanopore sequencing. Indices such as Shannon summarise that diversity in a single figure that is handy for follow-up, but reading it is a job for the requesting clinician, together with the clinical history and the other findings in the report.