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Taxonomic distinctness is a univariate (bio)diversity index which, in its simplest form, calculates the average 'distance' between all pairs of species in a community sample, where this distance is defined as the path length through a standard Linnean or phylogenetic tree connecting these species. It has some appealing properties: it attempts to capture phylogenetic diversity rather than simple richness of species and is more closely linked to functional diversity; it is robust to variation in sampling effort and there exists a statistical framework for assessing its departure from 'expectation'; it appears to decline monotonically in response to environmental degradation whilst being relatively insensitive to major habitat differences; and, in its simplest form, it utilises only simple species lists (presence/ absence data). Many of its practical characteristics remain to be explored, however, and this paper concentrates on the assumptions made about the weighting of step lengths between successive taxonomic levels (species to genera, genera to families etc.), which when accumulated give the overall path lengths. Using data on free-living marine nematodes from 16 localities/habitat types in the UK, it is shown that the relative values of taxonomic distinctness for the 16 sets are robust to variation in the definition of step length. For example, there is a near perfect linear relationship between values calculated using a constant increment at each level and a natural alternative in which the step lengths are proportional to the number of species per genus, genera per family, family per suborder etc. These weightings are then manipulated in more extreme ways, to capture the structure of phylogenetic diversity in more detail, and a contrast is drawn between the biodiversity of island (the Isles of Scilly) and mainland (UK) locations and habitats. This paper concludes with a discussion of some of the strengths and weaknesses of taxonomic distinctness as a practical tool for assessing biodiversity.

The taxonomic distinctness measure of biodiversity:weighting of step lengths between hierarchical levels