5.4 Measuring species diversity

As we have seen before, the ecosystems depend on the contribution and distribution of the individual organisms that live within them. An ecosystem with a high level of biodiversity is more resistant to environmental change and to human impacts.
But how can we measure the biodiversity? A variety of techniques and scales have been created in order to measure biodiversity, one of them are thebiodiversity indices. The main idea of a biodiversity index is to obtain a quantitative estimate of biological variability that can be used to compare biological entities in space or in time. There are different kinds of diversity indices and scientists can use more than one. Here we discuss three of them:

– Species Richness (S)
– Species Evenness (E)
– Simpson’s index (D)

The Species Richness index is the most common type of biodiversity index. It is the number of different species present in an area. This index is a count of species, and it doesn’t take into account the abundance of the species. The more species are present in a sample, the ‘richer’ the area. Just add them up! In the Fig.1_Ses5.4 we have a community with 5 species (Species A, B, C, D, E) with different abundance.

Fig1_Ses5.4 Species richness. Eva Turicchia

The Species Evenness is another method to measure biodiversity. Evenness expresses how uniformly the individuals in a community are distributed among the different species. It gives the information about the relative abundance of individuals belonging to the different species. For example, a study site where the 99.9% of the individuals belong to the same species it is not equally distributed (Fig.2 & 3_Ses5.4).

Fig2_Ses5.4 Species equally distributed. Eva Turicchia
Fig3_Ses5.4 Species not equally distributed. Eva Turicchia

The Simpson’s index combines both species richness and species evenness in one number. A community dominated by one or two species is considered to be less diverse than one in which several different species have similar abundance. As species richness and evenness increase, so diversity increases. The Simpson’s index ranges from 0 to 1, where 0 means no diversity and 1 means infinite diversity. It uses the value D to indicate the probability that two randomly selected individuals in the community belong to the same species. The greater the value of D, the lower the diversity. The less is the value of D, the higher the diversity.

Simpson’s index (D) 1-D = [ Σ n(n-1) / N(N-1)] n = total number of organisms of a particular species

N = total number of organisms of all species

Σ = add up!

Low species diversity values could suggest that there are relatively few successful species in the habitat, the environment is quite stressful and only few organisms are really well adapted to that environment As such food webs are relatively simple in this habitat, and its transformation would probably have quite serious effects. On the other hand, high species diversity could suggest that there is a large number of successful species and a more stable ecosystem than the low value habitat, there is a complex food web and environmental changes are less likely to be damaging to the ecosystem.

Example of application of the indices:

Species n (n-1) n(n-1)
Species A 5 4 20
Species B 12 11 132
Species C 7 6 42
Species D 4 3 12
Species E 10 9 90
Σ n(n-1) 296
N (N-1) N(N-1)
38 37 1406

1- D = Simpson’s index = 1 – (296 / 1406) = 0.8

S = Number of species = 5 (Species A, B, C, D, E)