State of the Bay Report 2010-Final - Anchor Environmental

8.3.2.3 Diversity Indices
A number of indices (single numbers) can be used as measures of community structure;
these include the total number of individuals (N), total number of species (S), the total biomass (B),
and the species equability or evenness, which is a measure of how evenly individuals are distributed
among different species. Diversity indices provide a measure of diversity, i.e. the way in which the
total number of individuals is divided up among different species. Understanding changes in benthic
diversity is important because increasing levels of environmental stress generally decrease diversity.
Two different aspects of community structure contribute to community diversity, namely
species richness and equability (evenness). Species richness refers to the total number of species
present while equability or evenness expresses how evenly the individuals are distributed among
different species. A sample with greater evenness is considered to be more diverse. It is important
to note when interpreting diversity values that predation, competition and disturbance all play a role
in shaping a community. For this reason it is important to consider physical parameters as well as
other biotic indices when drawing a conclusion from a diversity index.
The following measures of diversity were calculated for each sampling location using
PRIMER V 6:
The Shannon-Weiner diversity index (H’): H’ = - Σipi(log pi) (1)
Where pi is the proportion of the total count arising from the ith species. This is the
most commonly used diversity measure and it incorporates both species richness and
equability.
The Pielou’s evenness index (J’): J’ = H’observed / H’max
Where H’max is the maximum possible diversity which would be achieved if all
species were equally abundant (= log S). This is the most common expression of equability.
The Margalef’s index (d) of species richness: D = (S-1)/ log N (3)
Where S is the total number of species and N is the total number of individuals.
Species richness is often simply referred to as the total number of species (S), but this is very
dependent on sample size. The Margalef’s index thus incorporates the total number of individuals
(N) and is a measure of the total number of species present for a given number of individuals.
8.3.2.4 Integration of Indices with Environmental Variables
The aim of these analyses was to determine how the environmental variables (metal
concentrations, organic content of sediment, grain size) relate to the observed biological patterns in
macrobenthic community structure. This was done in two ways. Firstly, the concentrations of
individual environmental variables were superimposed onto biotic MDS plots as circles of varying
diameter (the larger the circle, the higher the concentration). These are known as ‘bubble plots’ and
they allow one to easily identify the sites at which certain contaminants are elevated, as well as to
determine if contamination patterns have any correlation to biotic structure. The second approach
was to run principal component analysis (PCA) on environmental data, and determine if there was
any contamination gradient among the sediment samples.
Principal component analysis has been widely used for the analysis of environmental data as
it can ‘unmask’ significant relationships between variables and relationships between samples
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon 158
(2)