Page 220 - Freshwater-Biology-and-Ecology-Handbook

CHAPTER 5 3.1.3

3.1.3 Sensitivity indices

The earliest and still the most widely used sensitivity to other pressures, including natural environmental
indices respond to organic pollution from sewage. These pressures, so a poor value of a particular index does not
indices therefore respond to organic loading, but also to necessarily mean that there is an environmental problem
siltation and the toxic effects of ammonia, although the or that it is caused by the pressure that the index is
main influence for animals is low oxygen concentration, designed to respond to.
and for plants it is more abundant nutrients and shading.
Because they respond to multiple pressures, indices Some indices are more accurate than others. The accuracy
of organic pollution are widely used as general quality of many depends on the type of water body – most river
indices. However, they are insensitive to acidification quality indices work best in shallow streams with gravel beds.
and toxic metal pollution because many of the taxa most The precision of indices can also depend on how they are
intolerant of low oxygen concentration and siltation are derived: those based on average values across many taxa
tolerant of metals and acidity. Indices have also been are more precise that those based on one or a few indicator
developed specifically for other environmental pressures taxa. This is a common property of averages and it is
including metal pollution, acidification, siltation and low therefore recommended that such indices are accompanied
flow. Sládeček (1973) (76) describes how indices can be by the number of taxa on which the index was based to
derived for particular pressures. The most common types provide an indication of precision. However, number of taxa
of sensitivity indices are calculated either as scores (the is not a good measure of precision of averaged indices that
sum of sensitivity values), which therefore vary not only use weighting factors. Index values derived from relatively
according to sensitivity but also to taxonomic richness, or small data sets or the expert opinion of a few experts will be
average scores: average score per taxon, such as WHPT less reliable than those based on the experience of many
ASPT and LIFE, or per individual organism, eg saprobic experts over a long period, or very large data sets covering
indices (see Section 3.1.6). the full spectrum of water bodies in which the indices are
used. It is therefore still necessary for the final interpretation
Some sensitivity indices are based not only on values to be made by an ecologist, particularly where they are used
relating to sensitivity but also on weightings. Weightings to make expensive environmental management decisions or
are factors, usually multipliers, used to adjust the sensitivity as evidence for legal prosecutions.
values for individual taxa. Weightings can relate to the
narrowness of response and therefore the ability of the Sensitivity indices are generally less prone to error variation
taxon to indicate a particular band of quality (for example, caused by sampler variation and the sampling method.
the indicator weightings used in saprobic indices), or to Indices like BMWP-ASPT and presence-only WHPT-ASPT
abundance. Washington (1984) (77) describes different are particularly robust and, being averages, they have
categories of biotic indices in more detail. Many authors do relatively low error variation. However, sensitivity indices
not differentiate scores, weightings and index values but in which the index values vary according to abundance are
treat them synonymously. sensitive to sampling method. Indices such as WHPT-ASPT,
LIFE and PSI should only be calculated from standard
Because biotic indices simplify complex data, much RIVPACS samples. The presence-only version of WHPT-
information that is useful for interpreting biological data ASPT described in Section 3.1.5 can be calculated from
is lost. Many indices are designed to relate to a particular other types of sample (including Surber and artificial
environmental pressure, but all indices will respond substrates) because it does not depend on abundance data.

220 | Freshwater Biology and Ecology Handbook
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