Summarizing values to Describe Plant
communities
Approaches to Estimating Diversity and Dominance
Several indexes and quantitative measures of biodiversity have been
developed. The simplest approach is to express diversity as the number of
species on a site or community — called species richness.
Diversity or dominance can be calculated with density, cover, or biomass.
However, density is the most commonly used
variable to assess plant diversity, therefore the examples in this module will be
based on density.
ShannonWiener Index (H’)
 Most commonly used index of diversity in ecological studies
 Values range from 0 to 5, usually ranging from 1.5 to 3.5
 Calculated:

Where:
n_{i} = number of individuals or amount (e.g., biomass or
density) of each species (the i^{th}^{ }species)
N = total number of individuals (or amount) for the site, and
ln
=
the natural log of the number. 
 Advantages:
» Relatively easy to calculate
» Fairly sensitive to actual site differences
 Disadvantage:
» There are several instances where H’ is similar between sites
even though sites are different.
For Example: Average Plant Density (plants/m^{2}) for an
upland and lowland site
Simpson’s Index (λ)
 λ is a measure of dominance.
Therefore, (1λ) estimates species diversity.
 Gives the probability that any two individuals drawn at random from an
infinitely large community belong to different species.

Where:
n_{i} = number of individuals or amount of each
species (i.e., the number of individuals of the i^{th} species)
N = total number of individuals for the site 
 Advantages and Disadvantages:
» Less sensitive to species richness and heavily weighted towards
the most abundant species
» Generally, less sensitive than ShannonWeiner H' to real changes
in diversity
