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3.1.2 Diversity indices

Diversity indices were popular in the 1970s. Diversity
indices are a more complex refinement of measures of
taxonomic richness that also take account of the pattern of
distribution of abundances across the taxa, principally the
evenness of abundances. They were originally developed
in the search for parameters that encapsulated the
properties of ecological communities. Pollution and other
environmental pressures are usually associated with less
even distributions: in stressed environments, many taxa
cannot cope so are absent, but the few tolerant taxa that can
occur do so in greater abundance because of the reduced
competition. The classic example is severe organic pollution
in which the few species that can tolerate very low oxygen
concentrations and silt are found in very great abundance,
supported by the nutrition provided by the organic matter
and the bacteria that also thrive on it. Asellus aquaticus
Because of their more complex derivation, diversity
indices are more difficult to interpret – in simplest terms
because they confound richness and evenness, but
more fundamentally because the properties of biological
communities that they attempt to describe are themselves
complex and only partially understood (Green 1979). (72) It
is much easier to interpret separate measures of richness
(such as number of taxa) and evenness – of which the most
widely used is that by Pielou (1969) (73) – in the same way
that it is easier to interpret WHPT ASPT and WHPT NTaxa
separately rather than combined as a score. Green (1979) (72)
(Figure 5.2) provides a critique of diversity indices, which
continue to be used. The Shannon-Weiner index is the most
widely used diversity index for water quality assessment
and it is a component of the Intercalibration Common Metric
index ICMi (see Section 3.1.7).

Other fundamental ecological mathematical and ecological
diversity analysis methods can be found in publications by
Pielou (1969), (73) Pielou (1975) (74) and Pielou (1977). (75)

Figure 5.2
Front cover of Roger Green’s book Sampling design and statistical
methods for environmental biologists. A highly recommended
(72)
introduction to survey design. (Illustration from Wiley)

Brychius elevatus adult

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