Taxonomic distinctness and environmental assessment

1. The objectives of this paper are to test the performance of the taxonomi c distinctness index, Delta(+), in a number of environmental impact scenari os, to examine its relationship with functional diversity and to examine th e influence of habitat type and diversity on the index. 2. The index was applied to data on free-living marine nematodes from the c oasts of Britain and Chile. 3. The taxonomic distinctness of nematodes from environmentally degraded lo cations was generally reduced in comparison with that of more pristine loca tions, often significantly so. 4. Some habitat types may have naturally lower values of taxonomic distinct ness than others. However, unless the habitats are degraded in some way the Delta(+) values do not generally fall below the lower 95% confidence limit of the simulated distribution under a null hypothesis that the assemblages behave as if they are a random selection from the regional species pool. T his ameliorates the problem encountered with species richness measures of b iodiversity, which are much more strongly affected by habitat type and comp lexity, thus making comparisons difficult between data sets from different habitats or where habitat type is uncontrolled. 5. Taxonomic distinctness in marine nematodes is shown to be related to tro phic diversity: a reduction in trophic diversity will lead to a 1 eduction in taxonomic distinctness, although not necessarily to a reduction in speci es richness. Trophic composition itself is clearly affected by pollution, b ut is also strongly responsive to the major influence of habitat type. 6. These features of the taxonomic distinctness index, coupled with its lac k of dependence on sampling effort or differences in taxonomic rigour betwe en workers and a statistical framework for the assessment of the significan ce of departure from expectation, suggest that it may prove to be a biologi cally and ecologically relevant measure of biodiversity. 7. This paper demonstrates that the taxonomic distinctness index has a numb er of theoretical and logistical advantages over measures of species richne ss for the purposes of environmental assessment.