State of the Bay Report 2010-Final - Anchor Environmental

these species usually provide the largest contribution to biomass. Smaller r-selected, opportunistic
species with a shorter life-span are also represented, and usually dominate numerically but make a
small (often insignificant) contribution to overall biomass (Warwick 1993). However, in an area
affected by pollution (or some other form of disturbance), the large and long-lived (k-selected)
species are less favoured and opportunistic (r-selected) species eventually become dominant in both
biomass and numbers. When cumulative contributions by species to overall abundance and biomass
are plotted together on the same graph, in the case of undisturbed communities the curve for
biomass generally lies above the curve for abundance for its entire length (Figure 8.3a). However,
under moderate or low levels of pollution, the large competitive species are eliminated and the
inequality between abundance and biomass dominants is reduced so that the curves coincide closely
and may cross one another (Figure 8.3b). As pollution becomes more severe, benthic communities
become increasingly dominated by one or a few small species and the abundance curve lies above
the biomass curve throughout its length (Figure 8.3c).
Cumulative %
100
50
0
UNDISTURBED
1 5 10
a b c
MODERATELY
DISTURBED
GROSSLY
DISTURBED
1 5 10 1 5 10
Species Rank (log scale)
Figure 8.3. Hypothetical ABC curves for species biomass and abundance showing undisturbed, moderately
disturbed and grossly disturbed conditions (after Warwick 1993).
When examining large numbers of ABC curves, the area between the abundance curve and
biomass curve is important, and is represented by the W statistic (Warwick 1993). Algebraically, the
W statistic ranges between –1 and +1 with W = +1 for even abundance across species but biomass
dominated by a single species (undisturbed) and W = –1 for patchy abundance and biomass being
compiled of many species (severely disturbed) (Clarke and Warwick 1994).
For 2010 data, the W statistic was calculated for each individual sampling station within
Small Bay, Big Bay and Langebaan Lagoon to indicate the level of disturbance at different locations.
The average W-statistic was also then calculated for Small Bay, Big Bay and Langebaan Lagoon for
1999, 2004, 2008, 2009 and 2010. In order to test if the observed changes in W-statistic were
statistically significant (p