The claim by many authors that Spinitectus inermis (Zeder, 1800), a narrowly specific parasite of European eels Anguilla anguilla (L.), is a rare species is considered at three levels: its geographical range, its frequency of occurrence compared to other eel parasites and its relative abundance in component communities. The parasite is widely distributed in freshwater throughout the European range of the eel but its occurrence is erratic and unpredictable, being known from only 8 countries. Surveys of eel parasites in the United Kingdom and in Continental Europe show that it is present in only 13% of British and 29% of continental localities. This satisfies one of the criteria for rarity. When present, its prevalence ranges from 1.8% to 43.3%, so it can be considered rare in some localities but in a few it may be common and on occasion it may be the dominant species in the gastro-intestinal community. Populations of S. inermis are almost always characterised by high levels of overdispersion, even at low prevalence. The species also displays an ability to colonise a locality following introduction there. Overall it meets many of the criteria of a rare species including a restricted distribution and a low frequency of occurrence and so it can be considered to exhibit diffusive rarity.
Small mammals are just as likely to become extinct as larger species, although the latter receive disproportionate attention with respect to conservation activity and research. We focused on rarity, vulnerability to extinction and conservation status for small terrestrial mammals from the orders Soricomorpha and Rodentia occurring in the Balkans and Anatolia. Although these two regions have fewer mammalian species than Central Europe in very small biota areas (surface areas 4 km2), they accumulate species at a much faster rate with increases in surface area. The distribution ranges of fifteen species from a total of 88 (= 17%) are confined to this studied area, with eight species being endemic to Anatolia and six to the Balkans. High endemism is indicative of small ranges, i.e. of one form of rarity of Rabinowitz’s ‘seven forms of rarity’ model. The ranges of at least three species (Talpa davidiana, Myomimus roachi and Dinaromys bogdanovi) have declined since the Last Glacial Maximum. Although numbers of extinctions correlates strongly with the number of endemics, and species displaying both restricted distribution and low density are those most at risk of extinction, very little conservation activity and research is focused on small-range endemics.