Problems and Their Solutions

Very often ¿-dominance curves approach a cumulative frequency of 100% for a large part of their length, and in highly dominated assemblages this may be after the first two or three top-ranked species. Thus, it may be difficult to distinguish between the forms of these curves. The solution to this problem is to transform the y-axis so that the cumulative values are closer to linearity, an appropriate transformation being the modified logistic transformation:

A potentially more serious problem with the cumulative nature of ABC curves is that their form is overdependent on the single most dominant species. The unpredictable presence of large numbers of a species with small biomass, perhaps an influx of the juveniles of one species, may give a false impression of disturbance. With genuine disturbance, one might expect patterns of ABC curves to be unaffected by successive removal of the one or two most dominant species in terms of abundance or biomass, and a solution is the use of partial dominance curves, which compute the dominance of the second-ranked species over the remainder (ignoring the first-ranked species), the same with the third most dominant, etc. Thus, if ai is the absolute (or percentage) abundance of the ,th species, when ranked

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Figure 2 Loch Linnhe macrofauna: Shannon diversity (H') and ABC plots over the 11 years, 1963 to 1973. Abundance, thick line; biomass, thin line.

in decreasing abundance order, the partial dominance curve is a plot of pi against log i ( i = 1,2,..., S — 1), where pi = i00«i/ Yj=1 a>

p2 = 100a2! Xj=2 aj;... ; PS -1 = 100as - ij (as -1 + as)

Earlier values can therefore never affect later points on the curve. The partial dominance curves (ABC) for undisturbed macrobenthic communities typically look like Figures 3g and 3h, with the biomass curve (thin line) above the abundance curve (thick line) throughout its length. The abundance curve is much smoother than the biomass curve, showing a slight and steady decline before the inevitable final rise. Under polluted conditions there is still a change in position of partial dominance curves for abundance and biomass, with the abundance curve now above the biomass curve in places, and the abundance curve becoming much more variable. This implies that pollution effects are not just seen in changes to a few dominant species but are a phenomenon which pervades the complete suite of species in the community. The time series of macrobenthos data from Loch Linnhe (Figure 3) shows that in the most polluted years, 1971 and 1972, the abundance curve is above the biomass curve for most of its length (and the abundance curve is very atypically erratic), the curves cross over in the moderately polluted years 1968 and 1970 and have an unpolluted configuration prior to the pollution impact in 1966 and 1967. Although these curves are not so smooth, and therefore not so visually appealing, as the original ABC curves, they may provide a useful alternative aid to interpretation and are certainly more

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Figure 3 Loch Linnhe macrofauna in selected years 1966-68 and 1970-72. (a-f) ABC curves (logistic transform). (g-l) Partial dominance curves for abundance (thick line) and biomass (thin line) for the same years.

Species rank

Figure 3 Loch Linnhe macrofauna in selected years 1966-68 and 1970-72. (a-f) ABC curves (logistic transform). (g-l) Partial dominance curves for abundance (thick line) and biomass (thin line) for the same years.

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robust to random fluctuations in the abundance of a small-sized, numerically dominant species.

In most cases where the presence of large numbers of small-bodied macrobenthic species in unperturbed situations has given a false impression of disturbance, those species have not been polychaetes. Prior to the Amoco Cadiz oil spill off the north coast of France in 1978, small ampeliscid amphipods (Crustacea) were present at the Pierre Noire station in relatively high abundance, and their disappearance after the spill confounded the ABC plots. The erratic presence of large numbers of small amphipods (Corophium) or mollusks (Hydrobia) also confounded these plots in the Wadden Sea. These small nonpolychaetous species are not indicative of polluted conditions. A taxonomic breakdown of the ABC response has shown that it results from (1) a shift in the proportions of different phyla present in communities, some phyla having larger-bodied species than others, and (2) a shift in the relative distributions of abundance and biomass among species within the Polychaeta but not within any of the other major phyla (Mollusca, Crustacea, Echinodermata). The shift within polychaetes reflects the substitution of larger-bodied by smaller-bodied species, and not a change in the average size of individuals within a species. In most instances the phyletic changes reinforce the trend in species substitutions within the polychaetes, to produce the overall ABC response, but in some cases they may work against each other. Indications of pollution or disturbance for marine macrobenthos detected by this method should therefore be viewed with caution if the species responsible for the perturbed configurations are not polychaetes, and the robustness of the plots should be tested using partial dominance curves.

Finally, a practical rather than a conceptual problem with the method is that it relies on a painstaking and time-consuming (and hence costly) analysis of samples in which all the species must be separated, counted, and weighed. Several groups of marine organisms are taxonomically difficult, for example (in the macrobenthos), several families of polychaetes and amphipods; as much time can be spent in separating a few of these difficult groups into species as the entire remainder of the sample, even in Northern Europe where taxonomic keys for identification are most readily available. Many taxa really require the skills of specialists to separate them into species, and this is especially true in parts of the world where fauna is poorly described. Identification to some higher taxonomic level, for example, family rather than species, is considerably easier and quicker, and the ABC method has proved to be encouragingly robust to analysis at the family level for both macrobenthos and fish; very little information appears to be lost.

See also: k-Dominance Curves; r-Strategist/ K-Strategists.

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