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Fig. 4 (a) A single environmental gradient (thick black line) and three selected sites (black dots).
Each hypothetical branch/lineage, centred at a demand point, graphically is represented in the
figure by a point above its demand point, at a vertical distance equal to one-half of its distribution
extent on the gradient. Branch/lineage points in the figure are gray if no selected site overlaps with
the range-extent of the branch/lineage. Branch/lineage ‘a’ would be captured by the middle site
only, branch/lineage ‘b’ is not sampled by any sites as its extent is too small; it is therefore coloured
gray. Branch/lineage ‘c’ is captured by two sites. The height to the top of the gray area above any
demand point reflects the total number of branch/lineages centered at that point that are not over-
lapped by any selected sites. ED is the sum of the resulting triangular gray areas. When richness
varies along the gradient, the corresponding weights on demand points can be interpreted as if we
are calculating a volume when counting-up unrepresented branch/lineages to obtain the ED score.
(b) If the hollow-circle site is added to the selected set indicated by the black dots, the ED value
(number of branch/lineages not represented) will be reduced by the amount equal to the white- striped
area. This ED-complementarity equals x*y/2, where x and y are distances from the hollow circle
D. P. F a i t h