All animals are equal, but some animals are more equal than others

Abstract

If the number of animal species is to be used as a measure of ‘biodiversity’, or if distributions of species among taxa of higher rank are to be used to infer evolutionary or ecological patterns, then we need to know whether animal phyla are consistently subdivided in such a way that each species represents an equal division of life's diversity. It is widely assumed, intuitively, that the traditional Linnean classification of marine animals is inconsistent between different major groups. We demonstrate formally that this is the case. For this exercise we use a consistent taxonomic hierarchy for all marine phyla within a relatively large region, the UK. The value of average taxonomic distinctness Δ + is shown to vary considerably between phyla. There is a highly significant relationship between the number of species within a phylum and the average distance through the taxonomic hierarchy between those species. This implies that larger phyla are broken up into relatively small units at higher taxonomic levels. Interestingly, this occurs independently of the perceived taxonomic difficulty within phyla. Species number is therefore a poor unit of currency for evaluating biodiversity, and studies which infer patterns using distributions of, or ratios between, higher taxa through time should take phyletic differences into account.