Abundance – occupancy relationships in macrofauna on exposed sandy beaches: patterns and mechanisms

Abstract

We studied the relationship between abundance and extent of occupancy of 158 species of macrofauna inhabiting 66 sandy beaches around the coast of Great Britain. We also used these data to test the predictions of two hypotheses proposed to explain positive abundance‐occupancy relationships. We found a strong positive relationship between abundance and extent of occupancy; this pattern was apparent in taxonomic subsets of organisms which have contrasting reproductive and dispersal traits such as planktotrophic/lecithotrophic development in the plankton vs brood development under parental care. Moreover, the abundance‐occupancy relationships in these taxonomic subsets had statistically indistinguishable slopes, and elevation. We propose that this lends support to the notion that differences in population structure such as the tendency to form metapopulations may not be primary determinants of the abundance‐occupancy pattern in these taxa as proposed by the rescue/metapopulation hypothesis. To test the predictions of the niche‐breadth hypothesis we derived values describing the range of sediment grain‐sizes exploited by members of two taxonomic subgroups: amphipods and bivalves. We found a weak, statistically non‐significant relationship between this niche‐breadth measure and occupancy in bivalves which have been shown to respond to grain‐size in previous studies, however this was negated after correction for possible artefacts of sampling effort. All other relationships between abundance or occupancy and grain‐size range were non‐significant. The consistency of the demonstrated abundance‐occupancy relationship with those demonstrated in other studies of primarily terrestrial fauna indicates some shared mechanistic explanation, but our data fail to provide support for the two mechanistic hypotheses investigated.