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Source: Based on Ricklefs (1993).

Source: Based on Ricklefs (1993).

The Shannon-Weaver index, H, is based on information theory. It gives more weight to rare species than does Simpson's index. It has the advantage of being normally distributed, making it possible to use statistical tests of significance to compare different values.

Table 14.9 shows examples for several different distributions of five species. Note that for H it is necessary to ignore zero values in the calculation (you can't take the log of zero).

The Shannon-Weaver index is often reported as its exponential form, since eH is equal to S when the populations are evenly distributed. Both D and eH vary from 1.0 when only one species is represented, to S if the species are evenly distributed. The Shannon-Weaver index can be used to compute an evenness parameter, Pielou's index, e. The evenness index varies from zero for maximally uneven, to 1.0 for perfectly uniform distribution:

Because e depends explicitly on the number of species, its value is affected by including species with zero population in the count. Potentially, we could add any number of species which are absent from the ecosystem. Therefore, there would have to be some independent criteria for including such species, such as because they were formerly present in that ecosystem or because they are typically found in similar ecosystems.

A graphical approach that gives more information is to plot the probability distribution of the species. This plot is called the dominance-diversity curve. It is made by sorting the species by p,, and plotting p, vs. rank order. Evenness will be related to the slope of the curve.

Food web diversity has also been defined as a diversity index computed with abundances classified by trophic level instead of species.

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