Are there adequate data to assess how well theories of rarity apply to marine invertebrates?
Understanding the biology of rare species is a very important part of conservation biology. Most of our current understanding of rarity has, however, come from studies of terrestrial plants, birds, mammals and some insects. Freshwater and marine habitats are underrepresented in published studies of rare species or conservation biology. We therefore have little knowledge about how well our understanding of what makes particular species rare and how rare species persist applies to marine invertebrates which form a major component of coastal biodiversity. In this review, I examine some theories about rarity with respect to intertidal and shallow subtidal invertebrates to identify whether there are adequate data to apply these theories to marine invertebrates and how well such theories apply. The general conclusions are that the lack of quantitative data on abundances, ranges, habitat-requirements, dispersal and connectedness among populations for marine invertebrates means that their status as rare species cannot really be assessed appropriately. It is also unlikely that, without extensive sampling programmes and considerable expense, adequate data could be obtained for these small, cryptic animals, which typically have very patchy, variable and unpredictable patterns of distribution and abundance. Intertidal and subtidal assemblages are diverse, including species with many different life-histories from many phyla, occupying the same suite of habitats. It is therefore suggested that future research on rare organisms in marine habitats should build upon the long and successful history of experimental marine studies to test specific hypotheses about processes influencing rarity in the field. Such studies would not only add a new dimension to our current understanding of rarity, but would also provide badly-needed data on the status of rare marine invertebrates. abundances, invertebrates, marine, range, rarity