Biodiversity And How It Is Measured

Biodiversity is the number of species in a given area. The largest measure of biodiversity is the estimated total number of species on the planet (somewhere around 10 million, although estimates of as high as 40 million are not unreasonable). Other scales are often more useful such as how many species occur in a country or mountain range or nature preserve. Ecologists have several measures for comparing diversity in different places.

Species Richness is the number of species found in a particular community. Measures of species richness such as the Shannon Diversity Index are weighted to show that one community if made of many species that all are equally numerous versus another community that has just as many species, but a small number are extremely numerous and the rest are quite rare. Species Richness can also be viewed at different special scales. Alpha Diversity is the number of species in a particular designated area suchas a park or reserve. For example, in the diagram below, Area 1 in Region 1 has an Alpha Diversity of 5. Region 1 has an average Alpha Diversity of 6 ((5+6+7)/3). Gamma Diversity is the number of species in a larger geographic area such as a mountain range or continent. For example, Region 1 has a Gamma Diversity of 7 (ABCDEFG). Beta Diversity is a measure of how species composition changes along an environmental or geographic al gradient such as moving from headwaters to mouth of a river. For example, Region 1 has a Beta Diversity of 1.2 indicating that most of the species that occur at one end of the gradient still occur at the other, in other words, there is low species turnover.

Region 1:     Area 1          Area 2          Area 3               Alpha     Gamma     Beta

Species:          BCDEF        ABCDEF      ABCDEFG              6                7            1.2

Region 2:     Area 1          Area 2          Area 3

Species:           ABC              DEFG            DGHIJ               4               10           2.5

Region 3:     Area 1          Area 2          Area 3

Species:           ABC               DEF                GHI               3                 9            3.0

As can be seen by comparing Region 1, Region 2 and Region 3 to each other, different arrangements of species can be represented with different measures of diversity. For example, while Region 2 has the highest total number of species (Gamma Diversity), Region 1 has the highest number of species per specific location (Alpha Diversity), and Region 3 has the highest turnover of species across the whole region (Beta Diversity).

There are other measures of diversity that are also useful in ecology. Genetic Diversity indicates how much variation exists at the DNA level in a species or population and is often used as a measure of how evolutionarily adaptable that species or population is. Ecological Diversity is a measure of how many different types of communities occur in a given ecosystem.