Taxonomic distinctness and species richness as measures of functional structure in bird assemblages

Most traditional "biodiversity" indices have an uncertain ecological interp retation, unfavourable sampling properties, and excessive data requirements . A new index of taxonomic distinctness (the average evolutionary distance between species in an assemblage) has many advantages over traditional meas ures, but its ecological interpretation remains unclear. We used published behavioural species data in conjunction with bird atlas data to quantify si mple functional metrics (the fraction of species engaged in non-competitive interactions, and the average between-species disparity in habitat prefere nces) for breeding-bird assemblages in Europe and North America. We then an alysed correlations of functional metrics with taxonomic distinctness and s pecies richness, respectively. All functional metrics had weak, positive co rrelations with species richness. In contrast, correlations between functio nal metrics and taxonomic distinctness ranged from slightly negative to str ongly positive, depending on the relative habitat heterogeneity, and on the resource involved in the between-species interaction. Strong positive corr elations between taxonomic distinctness and the fraction of interactive spe cies occurred for resources with few producer species per consumer species, and we suggest that taxonomic distinctness is consistently correlated with conservation worth. With its favourable sampling properties and data requi rements, this taxonomic distinctness measure is a promising tool for biodiv ersity research and for environmental monitoring and management.