How to calculate Shannon Wiener Diversity Index

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How to calculate Shannon Wiener Diversity Index



Shannon–Wiener Diversity Index Explanation

  • Diversity is often thought of as the number of species (richness), but this is incomplete.
  • Two communities can have the same number of species but very different diversity depending on how evenly species are distributed.
  • A community dominated by one species is less diverse than one where species are evenly distributed.



Richness

  • Number of species
  • Example: 4 species vs 4 species → same richness




Evenness

  • How evenly species are distributed
  • Example:
    • 25%, 25%, 25%, 25% → high evenness
    • 70%, 10%, 10%, 10% → low evenness

Diversity (Shannon)

  • Combines richness + evenness
  • Higher value = more diverse ecosystem

Example from text

Community 1 (balanced)

  • All species = 25%
  • Shannon ≈ 1.39

Community 2 (dominated)

  • One species = 70%
  • Others small
  • Shannon ≈ 1.06

Even though both have 4 species,
Community 1 is more diverse






To estimate the percentage of cover of each species, this might seem tricky at first—how do you come up with numbers for each percentage? A useful approach is to mentally divide the quadrat into four quarters, or 25% sections. If a species fills one of these squares, you can estimate it as 25% cover; if it fills two squares, that would be 50%. If it fills half a square, you could estimate about 12.5%. Percent cover is somewhat subjective, so when comparing sites, it’s best for the same person to estimate values consistently, or for a group to reach a consensus to minimize bias.






For example, imagine observing a plot with five different species. One species might occupy about half the area (50%), while another grassy species covers around 35% of the space. A third species might account for roughly 10%, with the remaining two species covering about 3% and 2%, respectively. These percentages are useful because they already represent proportions, and when summed, they equal 100% (or 1.0), reflecting full coverage of the plot. These proportions can then be plugged into the Shannon–Wiener diversity equation to calculate a diversity index, which in this example yields a value of approximately 1.049.





If the total plant cover does not equal 100%, adjustments are necessary. For instance, suppose four species together only cover 37% of the plot: one species at 25%, another at 10%, and two smaller ones at 1% each. Since these do not sum to 100%, they must be normalized by dividing each species’ coverage by the total coverage (37%). This converts them into true proportions—for example, 25% divided by 37% becomes approximately 0.67. After converting all species in this way, their proportions should sum to 1.0. These adjusted values can then be used in the Shannon equation. In this case, the resulting diversity index might be around 0.83.

It is important to note that having bare ground does not inherently reduce diversity. A plot can have significant uncovered space and still maintain high diversity if the species present are evenly distributed. In this example, diversity is low not because of bare ground, but because the species are unevenly distributed, with some contributing very little to overall abundance.

https://www.youtube.com/watch?v=lbnXPI05qNI

 

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