The population ecology of rare species

There is no general theory of rarity, although one is sorely needed both to understand population dynamics and to determine conservation priorities. Here we suggest some of the strands that might be woven into such a theory. They include relationships between local abundance, geographic range size and body size of species, and the determinants of minimum viable population sizes. In each of these areas much can still be learnt from the classical 'compare and contrast' approach using assemblages of species from a variety of taxa. Freshwater fish have contributed relatively little to the broad ecological literature in this respect. We perform some tentative analyses for this group of species, and speculate on how they might fit into our current understanding of rarity.