Abstract
Recently a new index has been proposed for the evaluation of biodiversity: taxonomic distinctness.
One of the positive features of this index is that it is neither sampling-effort nor sample-size
dependent. Until now, its application has been limited to the assessment of zoobenthos and fish
biodiversity. The main objective of this paper was to test the applicability of this index to the
macroalgal flora of the Gulf of Trieste (Northern Adriatic Sea). For this purpose the flora recently
censused in this area was compared with a checklist of the entire region compiled from the
literature. Two indices were mainly used for this study: average taxonomic distinctness based on
presence/absence data (AvTD), and variation in taxonomic distinctness (VarTD). Their relationship
with species richness was also assessed. The distinctness was compared with statistically significant
limits estimated using randomisation tests made on the local master species list. On the same data
set multivariate analysis based on a taxonomic similarity index was performed, and a 2nd stage
MDS was used to compare results at four taxonomic levels. The results confirmed that statistical
over-threshold situations can be highlighted only by one index (AvTD or VarTD) or only by their
relationship (AvTD vs VarTD) and not necessarily at the same time by both indices and their
relationship. While the average distinctness (AvTD) did not show values significantly different
from the expected ones, the variation (VarTD) in one site showed values significantly exceeding
confidence limits. This situation has already been described for zoobenthic communities and
explained as a consequence of a decrease in habitat diversity. In this case it might be the result of
the reduced presence of hard substrata suitable for macroalgal colonisation. The joint analysis of
both parameters (AvTD vs VarTD) revealed one site exceeding the 95% confidence limit, which
was not identified by analysing only one parameter at a time. This significant over-threshold pattern
in the relationship of the distinctness indices could be explained by a relative increase of
Rhodophyceae, attributable to the intensive grazing of the sea urchins at this site. The analysis of
taxonomic distinctness indices at each site compared with the 95% probability funnels or ellipses
derived from the regional species pool gave results consistent with the ones obtained using
multivariate analysis. The results of this study suggest that the method may have more general
validity.
