Some methods account for the variation of organism's biomass and abundance as a response to environmental disturbances. They consist of a comparison between the curves resulting from ranking the species as a function of their representativeness in terms of both their abundance and biomass.
ABC method. This method, introduced by R. M. Warwick in 1986, is based on the assumption that, for a given community, the distribution of the number of
individuals and the biomass from each species do not show the same pattern of variation pattern. The graphics allow plotting the interval of species (in the abscissae axis), arranged in decreasing order according to a logarithmic scale, against the cumulative dominance curves (in the ordinate axis).
Three different situations can occur as a function of the degree of disturbance affecting the community:
1. In a nondisturbed system, a relatively low number of relatively large individuals from few species will contribute to most of the biomass, and at the same time, the distribution of the individuals among the different species is more equitative. Graphically, the biomass curve will be plotted above the abundance one, indicating higher numeric than biomass diversity.
2. In communities under moderate disturbance conditions, the biomass cumulative curve will not show such an important contribution of just a few species represented by a low number of individuals as in the previous case, but instead abundances increase. Graphically, the biomass and abundance curves come out intersected.
3. In the case of communities under intense disturbances, few species represent most of the individuals, all of a small size, which explains why the biomass from each one of the species is low and more equitatively shared. Graphically, the abundance curve comes out above the biomass curve, indicating higher biomass than numerical diversity in the distributions.
This index is treated extensively in Abundance Biomass Comparison Method.
W-statistic index. R. M. Warwick and K. R. Clarke in 1994 improved the ABC method with this index that consists of a conversion of the ABC method into a measurable index. Clarke's approach became the most commonly accepted one:
where Bi is the biomass of species i, Ai the abundance of specie i, and S is the number of species. The index can take values from +1, indicating a nondisturbed system (high status), to — 1, which defines a polluted situation (bad status). Values close to 0 indicate moderate pollution (moderate status).
This approach is specific for organic pollution and has been applied, with satisfactory results, to soft bottom tropical communities. The occurrence of dominant species in normal conditions, favored not by organic pollution but by other environmental factors, as in estuar-ine communities, may lead to confusing results. Problems may also arise from applying this method to marine vegetation, due to obvious difficulties in counting the number of individuals from vegetal species.
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