Biodiversity Calculations

Species Richness, S = total number of different species found

In the example S = 10 for study plot one

Question for Students – What does species richness indicate? Does the species richness number give a complete picture of the difference between the two study plots? (Species richness is a measure of biodiversity, the higher the number, the greater the biodiversity. So if biodiversity is good, a high number is good. Species richness does not describe the distribution of plants in area studied, just the raw number of types found).

Dominance, D = (total number of organisms) ÷ (the number of the most abundant)

In the example D = 822 plant organisms ÷ 756 short grass plants = 1.09

Questions for Students – What does the dominance number show that is not shown by the species richness number? (Dominance is a way to consider whether or not the different plant types are evenly distributed. In the example, study plot one is dominated by short grass, but in other area, the plants might be more evenly distributed.)

What are the possible values for the dominance number? (The dominance number has to be > 1. If all the plants are one kind, D = 1. If even distributions are desirable, then bigger is better. Student can discuss whether or not that holds true all the time.)

What does a dominance number of 1.0 indicate? (All plants are the same type)

Sorensen Number, S = 2c , where a = the number of species in plot 1 (species richness) (a + b) b = the number of species in plot 2 (species richness) c = the number of species in common in plot 1 & 2

In the example, a = 10. If the second study plot has a species richness of 8, then b = 8. If the two plot have two plants in common then c = 2.

Then S = (2 x 2) ÷ (10 + 8) = 4 ÷ 18 = 0.22

Questions for Students – What does the Sorensen number show that is not shown by the species richness number or dominance number? (The Sorensen number compares to different areas. You can compare two areas by looking at the species richness or the dominance numbers for each, but the Sorensen number gives information about how much the two plots have in common.)

What values for the Sorensen number are possible? (The Sorensen number must fall between 0 and 1. It is zero when there are no species in common, c = 0, and 1 when all species are in common c = (a + b)/2. If the two study plots are within a relatively close distance, as they would be if studied at school, a Sorensen number close to zero represent high habitat diversity).