Species diversity is another level at which biodiversity can be investigated (see the last article about genetic diversity for the first level). Today we will look at what species diversity means and how it can be measured.
A few definitions
Before we start, there are a few words which will pop up through this article (and topic) which you need to have a good understanding of:
- Habitat – an area where an organism lives.
- Population – all the organisms of a single species in a habitat.
- Community – consists of all the populations (of different species) in a habitat.
- Abundance – the number of individual organisms of a species in a habitat.
What is species diversity?
Species diversity takes into account both species richness and species evenness:
- Species richness is the number of different species in a community.
- Species evenness is the abundance of each species in a community.
This is because it is important to know whether a species has five individuals or a thousand individuals, as well as how many different species there are. An index of diversity can calculated using species richness and species evenness. The larger the diversity index, the higher the species diversity. The below equation is used. N(N-1) is calculated once for the whole community, whereas n(n-1) must be calculated for each species.
Note that some A-Level Biology specifications have a slightly different equation (Simpson’s Index of Diversity) so watch out for that.
Measuring species diversity
In order to do the above calculation, we first have to measure species richness and evenness in a habitat. Normally it would be impossible to count every single organism in a habitat, so sampling techniques are used to gain a representative sample of the habitat to make calculations.
Most often, sampling is done randomly to avoid bias. For example, you might use a random number generator to generate random coordinates at which to place quadrats in a field. The number of species and the number of individuals of each species can be counted in the quadrat, and the data used to estimate what the values would be for the whole habitat. Of course, you would want to take repeat samples and calculate a mean. The more samples the better.
Sometimes, non-random sampling is required. For example, if the habitat has an environmental gradient (e.g. down a hillside or down a beach towards the sea) then it would be better to use systematic sampling. This would involve placing a transect (a line) across the habitat and putting the quadrats at fixed intervals along the transect. It allows you to see how the species change across the environmental gradient.
Two other types of non-random sampling are stratified and opportunistic. Stratified sampling can be used when a habitat has different types of areas within it, such as areas of bare rock within a field. If the habitat was 10% bare rock, you would want 10% of your samples to come from the bare rock areas so that you are taking proportional samples. Opportunistic sampling is where the scientist chooses where to take the samples from. It’s easy, but can lead to biased results which is not good science.
Summary
- Species diversity is a measure of biodiversity which takes into account species richness and species evenness.
- An index of diversity can be calculated to compare between habitats.
- To measure species diversity, samples must be taken. This can be done randomly or non-randomly depending on the area being sampled.
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