Principles of Vegetation Measurement & Assessment
and Ecological Monitoring & Analysis


Veg Sampling
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Estimating Biodiversity

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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.

Shannon-Wiener 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:
= number of individuals or amount (e.g., biomass or density) of each species (the ith species)
= 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/m2) for an upland and lowland site

  UPLAND LOWLAND          
Greasy Grass 3 7   H' for Upland Site = 1.099
Fuzzy Forb 3 2   H' for Lowland Site = 0.530
Spiney Shrub 3 0          
     CLICK HERE for detailed calculations

Simpson’s Index (λ)

  • λ is a measure of dominance. Therefore, (1-λ) estimates species diversity.
  • Gives the probability that any two individuals drawn at random from an infinitely large community belong to different species.
= number of individuals or amount of each species (i.e., the number of individuals of the ith 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 Shannon-Weiner H' to real changes in diversity
Greasy Grass 3 7   λ for Upland Site = 0.167
Fuzzy Forb 3 2   λ for Lowland Site = 0.611
Spiny Shrub 3 0    
     CLICK HERE for detailed calculations

Summary Questions

  1. What plant attributes can be used to estimate and calculate biodiversity?

  2. How is dominance related to diversity?

  3. What information would you need about an ecosystem to calculate diversity?

Advanced Questions:

  1. In the example below, which site (Clay Loam or Sandy Loam) has the greatest species richness?

  2. Which site (Clay Loam or Sandy Loam) has the greatest evenness of species?

  3. Which site has the greatest diversity as estimated by the Shannon Weiner Index (H')?

  4. Which site expresses the greatest level of dominance  by a few species (or lowest evenness) as estimated with a Simpson Index (λ)?

Example Diversity based on shrub dominance of a Clay Loam and a Sandy Loam Site in South Texas.
(Shrubs expressed as plant density as plant per hectare)

Shrub (common name)

Clay Loam Sandy Loam

    Scientific plant name

blackbrush 156 65    (Acacia rigidula)
guajillo 176 55    (Acacia berlandieri)
catclaw acacia 43 45    (Acacia greggii)
granjeno 56 32    (Celtis pallida)
whitebrush 25 67    (Aloysia lyciodes)
kidneywood 15 25    (Eysenhardtia texana)
elbowbush 1 70    (Forestieria cuntjolia)
wolfberry 0 28    (Lycium berlandieri)
shrubby bluesage 0 40    (Salvia bullotaeflora)

CLICK HERE for answers and detailed calculations

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