Diversity index

Diversity indices are quantitative measures that are designed to evaluate an area regarding the number and distribution of species present on it. There are many different types of indices. Some indices are really simple and only take into account the number of species present in the area (richness), while others also take into account the evenness of the community (Shannon and Simpson indices).

The Shannon index (H’, see Figure 1) takes into account the number and evenness of the species in the whole community.

The Shannon index is calculated by using the proportion of individuals of one species with respect to the total number of individuals of all species together (pi) and multiplied by the natural logarithm of that same number. This is done for each of the species and then all the numbers are added. Values of the Shannon index in natural systems typically vary between 0.5 and 5. Typically, areas with Shannon index values below 2 are considered low diversity areas, while areas with a Shannon diversity index over 3 are considered high diversity areas.

Although diversity indices are very useful to evaluate the diversity of an area, they might hide some information. The importance of endemic species is especially prone to be diluted in such indices. Endemic species are species with a very restricted geographical distribution, so they can be found in a certain area and not anywhere else in the world. For the index, an endemic species is just one species more in the area, however, it is more than that, since removing it from the area would mean the extinction of the species. Consequently, the endemic condition needs to be taken into account when decisions need to be taken.