Partial Dominance Curves

A second problem with the cumulative nature of k-domi-nance curves is that the visual information presented may be over-dependent on the single most dominant species. The unpredictable presence of large numbers of a species, perhaps an influx of the juveniles of one species, may give a false impression of disturbance. With genuine disturbance, one might expect patterns of k-dominance curves to be unaffected by successive removal of the one or two most dominant species in terms of abundance, and the use of partial dominance curves has been recommended. These 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 a is the absolute (or percentage) abundance of the th species, when ranked in decreasing abundance order, the partial dominance curve is a plot of pi against log i ( i= 1,2,..., 5—1), where p1 = 100«1/X^_1 a,-.

_1a, p2 _ 100a2/SS_2a,, ..., ps-1 _ 100as-1 /(as-1 + as).

Earlier values can therefore never affect later points on the curve. Figure 2 shows partial dominance plots from the sand-flat study. In this case it does not lead to a novel interpretation of the data, instead showing a similar pattern to that revealed by simple k-dominance plots but in a different form.

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10 100 Species rank

Figure 2 Partial-dominance curves for samples of invertebrates from a marine intertidal sand-flat. Each curve is based on average abundances in four replicate samples as in Figure 1.

10 100 Species rank

1000

Figure 2 Partial-dominance curves for samples of invertebrates from a marine intertidal sand-flat. Each curve is based on average abundances in four replicate samples as in Figure 1.

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