Proceedings of the Second Mediterranean Symposium on Marine

The average taxonomic distinctness (∆ + ) (Warwick & Clarke 2001) with <strong>the</strong> formula:
where is <strong>the</strong> species richness and ωij is <strong>the</strong> «distinctness weight» given to <strong>the</strong> path length
linking species i and j in <strong>the</strong> hierarchical classification.
The “variation in taxonomic distinctness” ∆ + can be expressed by:
where
Fig. 2: Two <strong>the</strong>oretical trees (a and b) with <strong>the</strong> same richness (7 species) but with a different taxonomy. The
mean path length between species is <strong>the</strong> same for <strong>the</strong> two trees and thus ∆ + is identical. The tree structure
has a greater unevenness or variability in (b) compared to (a), thus <strong>the</strong> variation in taxonomic distinctness
(Λ + ) is higher for (b). The «distinctness weight» given to <strong>the</strong> path length between each taxonomic level is
33.33 with a total <strong>of</strong> 4 levels. Step lengths are standardized so that <strong>the</strong> distinctness <strong>of</strong> two species connected
at <strong>the</strong> highest taxonomic level is set equal to 100. ∆ + ranges between 0 and 100, and Λ + is positive (Warwick
& Clarke, 2001).
To check for any stations deviating from <strong>the</strong> expected, <strong>the</strong> ∆ + and Λ + values for each
station were plotted in a probability funnel with 95% confidence limits. The simulated
mean against which <strong>the</strong>y were compared was <strong>the</strong> ∆ + and Λ + value <strong>of</strong> a “master list” from
all <strong>the</strong> species <strong>of</strong> green, brown and red algae found in <strong>the</strong> Greek seas. The taxonomic
structure <strong>of</strong> this master list came from <strong>the</strong> check lists <strong>of</strong> Gallardo et al. (1993) for <strong>the</strong>
brown algae, Ribera et al. (1992) for <strong>the</strong> green algae, and Athanasiadis (1987) for <strong>the</strong>
red algae, except <strong>the</strong> Ceramiaceae by Gómez Garreta et al. (2001).
ACTES DU DEUXIEME SYMPOSIUM MEDITERRANEEN SUR LA VEGETATION MARINE (ATHENES, 12-13 DECEMBRE 2003)
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