S H? : A similarity index based on shared species diversity, used to assess temporal and spatial relations among intertidal marine Gastrotricha

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Abstract
An index is proposed for measuring similarity between sets of biota over space and time. It integrates into a single measure hitherto separate and often incompatible procedures, based on species presence-absence and species composition. The index is demonstrated by a hypothetical set of data. When compared with the more usual indices of similarity, the proposed index is found to demonstrate greater rigor and hence greater powers of discrimination between data sets. The S H′ index is applied to one series of temporally and two series of spatially differentiated data sets, resulting from sampling the intertidal meiofauna subcommunity of Gastrotricha. All collections represent whole-beach transects, taken at low tide. The temporal series is derived from a boreal-silicious beach in Massachusetts, which was sampled 18 times during the period of a year. The two spatial series are derived from 16 borealsilicious beaches in Massachusetts and New York, and from 8 tropical-carbonate beaches in Florida (6) and the American Virgin Islands (2). Per cent composition by species and H′ D species diversity values were calculated for data from each site. H′ S similarity values based on shared species diversity were determined for permutation pairs of sites. These values then were converted into S H′ indices of similarity. The indices were clustered by an unweighted mean, pair-group method and, finally, similarity dendograms were constructed. Results allow the temporal series to be readily separated from both spatial series, and the boreal-silicious spatial series to be clearly distinguished from the tropical-carbonate spatial series. There is strong, though preliminary, evidence that the tropical-carbonate series does not comprise a single biogeographical unit, but that it is subdivided into a Florida subseries and an island subseries. Dendogram analysis indicates that the island effect is nearly as strong as the latitudinal-substratum effect in reducing shared species diversity between intertidal subcommunities of Gastrotricha.