Estimating diversity of modular organisms may be problematic due to actual
difficulties in discriminating between ‘individuals’ and quantifying their abundances.
Quantitative data, when available, are collected through methods that
could preclude the application of classical diversity indices, making comparisons
among studies difficult. Taxonomic distinctness indices, such as the ‘Average
Taxonomic Distinctness’ (D+) and the ‘Variation in Taxonomic
Distinctness’ (K+) may represent suitable tools in investigating diversity beyond
the simple species number. The potential usefulness of such indices has been
explored almost exclusively on unitary organisms, neglecting modular ones. In
this study, we employed D+ and K+ to analyse patterns of diversity of epiphytic
hydroid assemblages living on Cystoseira seaweeds at a hierarchy of spatial
scales, along 800 km of rocky coast (SE Italy). ANOVA on species richness and
K+ showed no significant difference in sample diversity at the investigated spatial
scales. In contrast, there were significant differences at the scale of 10s of
km in D+. Analyses based on simulations detected significant variations at all
spatial scales in D+. Such findings underline the potential of D+ in highlighting
relevant spatial scales of variation in patterns of hydroid diversity. Our results
also suggest that the interplay between natural environmental variations and
the complex ecological traits of modular organisms might affect taxonomic distinctness
indices. We stress the need for further investigations focusing on
modular organisms before any generalizations on the use of taxonomic relatedness
measures in examining marine biodiversity can be made.
