Interpreting molluscan death assemblages on rocky shores: are they representative of the regional fauna?

Recent work has suggested that molluscan death assemblages in marine intertidal habitats are sufficiently representative of regional biodiversity to be used in rapid, comparative biodiversity assessments. If this can be shown to be a general property of death assemblages, they may be a valuable surrogacy tool, especially in countries such as Australia where comprehensive species lists are unavailable for many regions. To test this, I conducted surveys of death assemblages associated with 10 headlands within the Solitary Islands Marine Park, northern NSW, Australia. Species lists for each site were analysed to determine: i) average taxonomic distinctness (Δ⁺) - the degree to which species within a sample are related to each other; and ii) variation in taxonomic distinctness (Λ⁺) - the evenness of distribution of species across higher taxonomic levels. The values of these biodiversity indices were then compared to equivalent measures determined from lists of nearshore taxa and of taxa occurring more widely in the region. Species richness in death assemblages ranged from 99-161 species across the 10 sites. Analyses of representativeness indicated that species records from a single site were unlikely to adequately represent regional diversity but that a random combination of data from 2 or more sites was fully representative of nearshore diversity and of regional diversity of bivalves. Regional diversity of gastropods was poorly represented in these nearshore death assemblages; this was primarily due to under-representation of a number of dominant families. These patterns are most likely due to a combination of factors including recruitment processes and availability of suitable habitats, both of which differ over the cross-shelf gradient, and the influence of targeted removal of “collectable” species. Despite the disjunct geographical settings of this study and previous work (Isles of Scilly, UK), the proportion of the species pool contained in death assemblages was remarkably similar; further study from a range of locations, will help to determine the generality of such patterns.