Page 221 - Freshwater-Biology-and-Ecology-Handbook

Terminology

Score Index value Average score per taxon

An index based on the sum of index values, A value relating The value of an index
much like the score of a game: football score, to the sensitivity expressed as a score divided by
cricket score. Examples include of a taxon to an the number of taxa – examples
BMWP-score, Chandler-score. In the same environmental include ASPT (average
way that I don’t recommend using diversity pressure. BMWP-score per taxon),
indices because they confuse richness WHPT ASPT, and LIFE. If you
and evenness, I don’t recommend using Sometimes called call an index value a sensitivity
scores because they confuse richness and a sensitivity score. score, the term ASPT still
sensitivity – it’s far better to use number makes sense as the average
of taxa, and average score per taxon. sensitivity score per taxon.

Proportional indices Weighting Abundance related index values

The value of an A multiplier used to increase the influence Take abundance into account
index expressed of a taxon. Indicator weightings increase the by assigning different index
as a proportion influence of taxa with narrower distributions values to different abundances
or percentage of across the environmental gradient – for of each taxon. Values for each
total: examples example, the indicator values used in abundance level of each taxon
include PSI, saprobic indices. In abundance weightings, are derived independently,
SPEAR and TRPI. a greater weighting factor is usually applied as if they were different
to taxa with greater abundance. Abundance taxa – examples include WHPT.
weightings are sometimes confused with
abundance related index values.

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