Structural gradients in an intertidal hard-bottom community: examining vertical,horizontal, and taxonomic clines in zoobenthic biodiversity

Measures of biodiversity along environmental gradients have long been the focus of marine ecological research. However, a general lack of comparability between studies and under-appreciation of co-occurring, less obvious clines has often undermined any general conclusions. Latitudinal, vertical, horizontal, and taxonomic gradients in intertidal biodiversity were assessed directly and indirectly using a large data set from one locality in southwest Ireland. A total of 153 epifaunal species, from 106 families and 13 phyla, was recorded from over 200,000 individuals in a sample area of 63 m². Multivariate analysis revealed that the vertical gradient of immersion was the primary structuring factor in the community but that a flow-driven horizontal gradient was also important in structuring low- and mid-shore zones. Crustaceans, annelids, and molluscs were most numerous over the entire site, but bryozoans and sponges may have played the most significant role in driving the horizontal (flow-related) trend in species richness at lower levels on the shore. Comparison of species richness with sites from other locations around the world proved inconclusive in assessing a global trend in richness from the poles to the tropics. However, interpolation of this and similar studies may prove useful in future latitudinal investigations of hard-substratum intertidal biota. Higher-taxon surrogacy for species trends proved significant and useful at the scale investigated here, whereby analysis of data from family or even order level may prove an efficient method of future monitoring. More data are required to assess if the top-down taxonomic approach will sufficiently mirror species trends at larger spatial scales. Overall, the generality of effects of environmental gradients on intertidal organisms may be more conclusive where entire communities are studied and consideration is given to other, less dominant clines.Communicated by J.P. Thorpe, Port Erin