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

Abstract

Taxonomic distinctness is a unlvanate (bioldiversity index which, In ~t s simplest form, calculates the average 'distance' between all pairs of species in a community sample, where thls distance is defined as the path length through a standard Llnnean or phylogenetlc tree connecting these species. It has some appealing properties: ~t attempts to capture phylogenetic diversity rather than simple richness of species and is more closely linked to functional diversity; it is robust to vanation in sampling effort and there e x ~s t s a statistical framework for assessing ~t s departure from 'expectation'; it appears to decllne monotonically in response to env~ronmental degradation whilst being relatively insensitlve to major habitat differences, and, in its simplest form, ~t utillses only simple species lists (presence/ absence data). Many of its practical characteristics remain to b e explored, however, and this paper concentrates on the assumptions made about the we~ghting of step lengths between successive taxonomic levels (species to genera, genera to families etc.), which when accumulated glve the overall path lengths. Uslng data on free-llving manne nematodes from 16 localit~es/habitat types in the UK, it is shown that the relative values of taxonomic distinctness for the 16 sets are robust to vanation 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 alternat~ve in w h c h the step lengths are proportional to the number of species per genus, genera per family, family per suborder etc. These weightlngs are then manipulated in more extreme ways, to capture the structure of phylogenetic divers ~t y in more detail, and a contrast is drawn between the biodiversity of island (the Isles of Scilly) and malnland (UK) locations and habitats. This paper concludes with a discussion of some of the strengths and weaknesses of taxonomc distinctness as a pract~cal tool for assessing biodiversity.