Page 217 - Freshwater-Biology-and-Ecology-Handbook

3

DATA ANALYSIS

3.1 Biotic indices

Biotic indices are numerical values that relate the presence Finally, invertebrates respond to the integrated effect of all
of taxa to environmental pressures. Their role is to simplify environmental pressures (both natural and anthropogenic)
complex biological data so that ecologists can explain their and it is impossible to apportion impact to individual
results to environmental managers who may have little pressures unless an environmental pressure is so severe
st
knowledge of ecology. Since the turn of the 21 century, that it is overwhelming. This will increasingly be the case as
indices have also been used to define ecological quality gross pressures are eliminated by environmental regulation.
objectives and compliance with them, notably for the The corollary, that the impact of one environmental pressure
European Water Framework Directive. can be mitigated by reducing other unrelated pressures, is
true, but not widely recognised.

Biotic indices Hellawell’s 1978 book (56) gives a comprehensive and detailed
overview of biotic indices and other metrics used for river
Biotic indices are numerical values that relate the management up to that date. A critical review of biotic indices
presence of taxa to environmental pressures. (including diversity, sensitivity and similarity indices), was
Their role is to simplify complex biological data so that written by Washington (1984). (71)
ecologists can explain their results to environmental
managers who may have little knowledge of ecology.
There are two types of biotic indices:

• parameters that describe the biological community
Although their format is intentionally very simple (usually as a whole
a single number or letter), biotic indices are actually very • those based on the sensitivity of taxa to
complex, and most do not behave as parameters on a environmental pressures.
continuous scale of equal intervals. Biotic indices should
not be used as a basis for statistical analysis.

Users should be wary of using biotic indices, particularly Most indices that describe the community as a whole are
without understanding the extent of the data from which they measures of diversity, of which taxonomic richness is the
were derived: comprehensiveness, reliability, geographical simplest index.
and stream type coverage; the statistical properties of their
format (average, score, percentage); the impact of sampling
and analytical error on the index and the magnitude of that
error; the way in which the environmental pressure that the
index is designed to respond affects invertebrates; other
environmental pressures that have the same effect, cause
the same environmental pressures or interact with each
other (for example, reducing the availability of oxygen,
increasing siltation, altering the availability of metal ions or
nutrients); and other environmental pressures that co-occur
at sites where the pressure of interest occurs. The fact
that an index sensitive to a particular pressure indicates an
impact does not necessarily mean that pressure is present
at a site.

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