Assessment of biodiversity at the population level
Assessing biodiversity at the population level produces a 'biodiversity index'. This can be used to monitor the biodiversity of a habitat over time and to compare biodiversity to different habitats.
An example of this is Simpson's index, which describes the biodiversity of motile (moving) organisms, such as invertebrates in a stream. The commonst way of calculating it gives a numerical value. The higher the value, the higher the biodiversity.
If you collect water samples from a stream and identify and count all the organisms you can see, Simpson's diversity index, S, can be calculated from the formula:
- N = the total number of organisms present
- n = the number in each species
In order to calculate S, the total of organisms (N) is counted for and N(N-1) can be calculated. For each species, n(n-1) is calculated and the values added to give ∑n(n-1),
You can work this out to compare the biodiversity of different habitats. More habitats means that there are more ecological niches. This means that more species can be accommodated and thereore the biodiversity is higher.
Assessment of biodiversity with polymorphic loci
An examination of genes and alleles can give an assessment of biodiversity at the genetic level. This approach focuses on the alleles present in the gene pool of the population, and not the individuals.
Number of alleles
A gene's position on a chromosome is its locus. A locus shows polymorphism if it has two or more alleles at frequencies greater than would occur by mutation alone. If a gene has more alleles, its locus is more polymorphic than if there were fewer.
In some plants:
- Gene T controls the height, there are two different alleles.
- Gene S controls whether or not pollen can germinate on the stigma of a flower of the same species. In one species of poppy, gene S has 31 different alleles.