A further biodiversity index applicable to species lists: variation in taxonomic distinctness

A further biodiversity index is proposed, based on taxonomic (or phylogenet ic) relatedness of species, namely the 'variation in taxonomic distinctness ' (VarTD, Lambda (+)) between every pair of species recorded in a study. It complements the previously defined 'average taxonomic distinctness' (AvTD, Delta (+)), which is the mean path length through the taxonomic tree conne cting every pair of species in the list. VarTD is simply the variance of th ese pairwise path lengths and reflects the unevenness of the taxonomic tree . For example, a species list in which there are several different orders r epresented only by a single species, but also some genera which are very sp ecies-rich, would give a high Lambda (+) by comparison with a list (of equi valent Delta (+)) in which all species tended to be from different families but the same order. VarTD is shown to have the same desirable sampling pro perties as AvTD, primarily a lack of dependence of its mean value on the sa mple size (except for unrealistically small samples). Such unbiasedness is of crucial importance in making valid biodiversity comparisons between stud ies at different locations or times, with differing or uncontrolled degrees of sampling effort, This feature is emphatically not shared by indices rel ated to species richness and also not by properties of the phylogeny adapte d from proposals in other, conservation contexts, such as 'average phylogen etic diversity' (AvPD, Phi (+)). As with AvTD, the VarTD statistic for any local study can be tested for 'departure from expectation', based on a mast er taxonomy for that region, by constructing a simulation distribution from random subsets of the master list. The idea can be extended to summarising the joint distribution of AvTD and VarTD, so that values from real data se ts are compared with a fitted simulation 'envelope' in a 2 d (Delta (+), La mbda (+)) plot. The methodology is applied to 14 species lists of free-livi ng marine nematodes, and related to a master list for UK waters. The combin ation of AvTD and VarTD picks out, in different ways, some degraded locatio ns (low Delta (+), low to normal Lambda (+)) and the pristine island fauna of the Scillies (normal Delta (+), high Lambda (+)). The 2 indices are also demonstrated to be measuring effectively independent features of the taxon omic tree, at least for this faunal group (although it is shown theoretical ly that this will not always be the case). The combination of Delta (+) and Lambda (+) is therefore seen to provide a statistically robust summary of taxonomic (or phylogenetic) relatedness patterns within an assemblage, whic h has the potential to be applied to a wide range of historical data in the form of simple species lists.