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

Taxonomic distinctness is a univariate (bio)diversity index which, in its s implest form, calculates the average 'distance' between all pairs of specie s in a community sample, where this distance is defined as the path length through a standard Linnean or phylogenetic tree connecting these species. I t has some appealing properties: it attempts to capture phylogenetic divers ity rather than simple richness of species and is more closely linked to fu nctional diversity; it is robust to variation in sampling effort and there exists a statistical framework for assessing its departure from 'expectatio n'; it appears to decline monotonically in response to environmental degrad ation whilst being relatively insensitive to major habitat differences; and , in its simplest form, it utilises only simple species lists (presence/abs ence data). Many of its practical characteristics remain to he explored, ho wever, and this paper concentrates on the assumptions made about the weight ing of step lengths between successive taxonomic levels (species to genera, genera to families etc.), which when accumulated give the overall path len gths. Using data on free-living marine nematodes from 16 localities/habitat types in the UK, it is shown that the relative values of taxonomic distinc tness for the 16 sets are robust to variation in the definition of step len gth. For example, there is a near perfect linear relationship between value s calculated using a constant increment at each level and a natural alterna tive in which the step lengths are proportional to the number of species pe r genus, genera per family, family per suborder etc. These weightings are t hen manipulated in more extreme ways, to capture the structure of phylogene tic diversity in more detail, and a contrast is drawn between the biodivers ity of island (the Isles of Scilly) and mainland (UK) locations and habitat s. This paper concludes with a discussion of some of the strengths and weak nesses of taxonomic distinctness as a practical tool for assessing biodiver sity.