Biological field and laboratory methods for measuring the quality of ...

BIOLOGICAL METHODS
Organic nutrients and sludge deposits are frequently
associated. The responses shown are by
no means simple or fixed and may vary depending
on a number of factors including:
• a combination of stresses acting together or
in opposition,
• indirect effects, such as for example the
destruction of highly productive vegetative
substrate by temperature alterations, sludge
deposits, turbidity, chemical weed control,
• the physical characteristics of the stressed
environment, particularly in relation to substrate
and current velocity.
Data on standing crop and numbers of taxa
may be presented in simple tabular form or
pictorially with bar and line graphs, pie
diagrams, and histograms. Whatever the method
of presentation, the number of replicates and
the sampling variability must be shown in the
tables or graphs. Sampling variability may be
shown as a range of values or as a calculated
standard deviation, as discussed in the
Biometrics Section of this manual.
Data on standing crop and number of taxa are
amenable to simple but powerful statistical
techniques of evaluation. Under grossly stressed
situations, such analyses may be unnecessary;
however, in some cases, the effects of environmental
perturbations may be so subtle in comparison
with sampling variation that statistical
comparisons are a helpful and necessary tool for
the evaluative process. For this purpose,
biologists engaged in studies of macroinvertebrates
should familiarize themselves with the
simple statistical tools discussed in the
Biometrics Section of this manual.
5.1.3 Diversity
Diversity indices are an additional tool for
measuring the quality of the environment and
the effect of induced stress on the structure of a
community of macroinvertebrates. Their use is
based on the generally observed phenomenon
that relatively undisturbed environments
support communities having large numbers of
species with no individual species present in
overwhelming abundance. If the species in such
a community are ranked on the basis of their
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numerical abundance, there will be relatively
few species with large numbers of individuals
and large numbers of species represented by
only a few individuals. Many forms of stress
tend to reduce diversity by making the environment
unsuitable for some species or by giving
other species a competitive advantage.
The investigator must be aware that there are
naturally occurring extreme environments in
which the diversity of macroinvertebrate
communities may be low, as for example the
profundal fauna of a deep lake or the black
fly-dominated communities of the high gradient,
bed rock section of a torrential stream. Furthermore,
because colonization is by chance,
diversity may be highly variable in a successional
community; for this reason, diversity indices
calculated from the fauna of artificial substrate
samplers must be evaluated with caution. These
confounding factors can be reduced by comparing
diversity in similar habitats and by exposing
artificial substrate samplers long enough for a
relatively stable, climax community to develop.
. S S S-l
IndIces, such as N' Log N' and Log N where
S = number of taxa and N = total number of
individuals, are merely additional means of summarizing
data on total numbers and total taxa in
a single numerical form for evaluation and
summarization. They add no new dimension to
the methods of data presentation and analyses
discussed above and, in addition, are highly
influenced by sample size. Sample size in this
context relates to the total number of organisms
collected (an uncontrollable variable in most
macroinvertebrate sampling), not to the area or
volume of habitat sampled. Do not use such
indices for summarizing and evaluating data on
aquatic macroinvertebrate communities.
There are two components of species
diversity:
• richness of species
• dis tri bu tion of individuals among the
species.
It is immediately obvious that the second
component adds a new dimension that was not
considered in the methods for evaluating data