Most ecologists recognize two aspects of biodiversity that must be considered when we try to quantify biodiversity.
One is Species Richness, which is the number of species in a community. The other is Relative Abundance or Equitability, which is the evenness with which the individuals are spread out among the species in a community.
The measurement is just a count of the number of species found when the observers sample the community. It is represented by "S".
The only real complication is that the number depends to some degree on the size of the sample the observers use.
When you sample you take the individuals one by one, selecting them randomly, and record the species of each individual. At first you get new species fairly regularly, but as time goes on more and more of the individuals are repeats of species you've already found. You may never stop finding new species, they just become more and more uncommon.
The graph below illustrates how the species richness "S" increases as the number of individuals examined increases: rapidly at first, then more slowly.
When this happens you have to decide how many individuals you're going to count before you say that you've reached "S"
Techniques are available for using the shape of the curve to estimate the number at which it would ultimately level off. Or you can decide to accept as "S" the number of species you have when the number of additional new species falls below a certain percentage (say 1%) of the additional individuals you count.
Species richness measurements provide useful information. They have the advantage of being very graphic and easy to understand and can be easily explained to general audiences.
They are often the only type of biodiversity information you can obtain unless you can go out and do your own field work. And in many circumstances the species richness number is enough to allow you to do comparisons, especially between geographic areas or habitats.
But by themselves species richness (S) numbers can be misleading. Think about two communities, each with the same number of species but where one has one very common species with only a few individuals of the other species while the other community has equal numbers of each species. Intuitively we recognize that the community where the individuals are spread out equally among the species is a more diverse community than the one where almost all of the individuals belong to one species.
To correct for this we need to consider relative abundance or equitability.
We know that in most communities some species are going to be common, others less common, and still others rare. But the extent to which this is true varies from community to community and from situation to situation.
There are mathematical formulas for measuring how evenly the individuals are distributed among the species and in some cases they are useful. However, by themselves they have the disadvantage that they only measure equitability; they do lose the important information about species richness.
What is more useful is some measurement that combines species richness and equitability.
One measurement ecologists use for this purpose is the Shannon-Weiner or Information Index. It a mathematical formula derived from information theory that was developed by engineers to analyze how efficiently data could be transmitted along telephone lines.
The index, H', is given by the formula:
Although this looks complicated it just means that for each species you determine the proportion (p) that it is of the whole, then multiply that number times the natural log (ln) of that number. This gives you p ln p. You do this for each species in the community then add up all of the resulting numbers. Since the number comes out negative we turn it into a positive number for convenience.
|click for an example which shows the differences in S and H' in four hypothetical communities|
The information index takes into account the eveness of the species distribution as well as the absolute number of species. For many types of technical studies it may be the better measure to use. However, it is not very intuitive and may not be as useful as the species richness index S when discussing diversity with a general audience.
In a study by the Boquet River Association both indices were used to study the effect of embedding sediments on stream macrobenthos communities.
Several "matched pairs" of stream segments were selected. In each pair the characteristics of the stream and river bottom were very similar, except that one had more sediments embedded in the rocks than the other. We sampled the macrobenthos using kick nets, then counted and identified the numbers of the different species.
The species richness S was about the same for the members of the pairs that had a lot of sediment as for those that had little sediment. But the information index H' was lower (less diversity) for the member of the pair that had a lot of sediment when compared to the member of the pair with little sediment.
We interpreted this to mean that while the stress to the macrobenthic community was not severe enough to extirpate any of the species it was great enough that only a few members of the community were able to do well while the rest were only surviving in reduced numbers. If we had used only the species richness index we would have missed this.