Page 218 - Freshwater-Biology-and-Ecology-Handbook

CHAPTER 5 3.1.1 – 3.1.2 3.1.1 Taxonomic richness

The most straightforward measure of taxonomic richness is
the number of species, genera or taxa. This is the simplest
measure of diversity to interpret, but care is still needed.
There is usually an assumption that greater taxonomic
richness indicates better environmental quality, but this is
not always so.

For example, in base-poor rivers that are naturally
oligotrophic, mild organic pollution can increase taxonomic
richness by increasing the availability of nutrients directly
and by reducing the bioavailability of toxic metals that may
be present naturally; but the increased richness is at the
expense of naturally occurring species that are intolerant of
such conditions. Higher than normal taxonomic richness can
indicate enrichment, but it can also indicate unusually high
habitat diversity which is characteristic of some of the best
sites, including nature reserves.

An issue with measures of taxonomic richness is knowing
what taxa are included. Not every species or genus of
invertebrate is readily identifiable in the aquatic stage.
WHPT NTaxa is a standardised measure of richness at
family level as the taxa included are already pre-defined.

Indices of taxonomic richness are sensitive to the
sampling method because larger samples will contain
more invertebrate taxa and some habitats support greater
richness than others. It is therefore most important that
any comparison of differences in taxonomic richness are
based on samples collected in the same way. Measures of
taxonomic richness generally have lower precision than
other indices because they are more sensitive to sampling
variation caused by sampler variation and the patchy
distribution of invertebrates.

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