Taxonomic distinctness as a measure of diversity applied over a large scale: the benthos of the Norwegian continental shelf

1. Data on soft-sediment macrobenthos from the Norwegian continental shelf (56-71 degrees N) was used to examine the use of average taxonomic distinctness (Delta(+), a measure of the average degree to which species are related to each other) as a diversity measure. 2. Delta(+) for all taxa combined decreased with both latitude and depth. In contrast Delta(+) for annelids and crustaceans (71% of the fauna) was positively correlated to latitude and depth, whereas molluscs (20.1% of the fauna) were not related to latitude or depth. Depth explained most of the variance in Delta(+) for annelids and crustaceans. Neither latitude nor grain size appeared to be important in explaining large-scale patterns in the taxonomic breadth of the three dominant phyla. In contrast latitude rather than depth explained most of the variance in Delta(+) of all taxa combined. 3. The finding that molluscs showed different patterns of Delta(+) than the two other dominant phyla shows that the biodiversity of different phyla responded differently to environmental gradients and that one phylum cannot be used as a surrogate for others in such studies. The most likely explanation for the inconsistency in Delta(+) is because the taxonomic classification system also varies between different phyla, and taxonomically related biodiversity measures might be more meaningfully applied to a single phylum than to all taxa combined. 4. Delta(+) and species richness (S) displayed different patterns of biodiversity. Thus Delta(+) is not a surrogate for species richness. Species richness of all taxa combined and annelids were highest in the middle depth range, and lowest in the shallowest and deepest parts of the shelf. Most of the variance in species richness is accounted for by depth, followed by latitude and grain size for all three dominant phyla and all taxa combined.