How Island Geography and Shape may Influence Species Rarity and Biodiversity Loss in a Relict Fauna: A Case Study of Mediterranean Beetles

Abstract

In accordance with their relictual character, the tenebrionid communities of the Aegean Islands appear scarcely affected by present geographic characteristics. Three measures of rarity at community level were calculated: geographic distribution, habitat exploitation, and population size. Islands with greater habitat diversity and size are those with higher values of distributional rarity. Island shape appears to be correlated with habitat exploitation and population size rarities: the communities with highest proportions of ecologically specialized and scarce species are associated with islands with lower perimeter/area ratios (which possibly lack extensive 'inner' habitats, like forests), while an elongate island shape exerts a positive influence on the incidence of species with small populations (possibly because of an increasing habitat diversity in elongate areas which may support K-selected species). On the whole, islands with a more elongated shape are those with the most vulnerable species. A paleogeographical and paleoecological model is proposed to explain present patterns of rarity. As a result of relaxation after saturation, remote small islands retained generally the most common (less vulnerable) species, rare species surviving mostly on the larger islands, although ecologically specialized species can be concentrated on some small islands, if these species are associated with peculiar but locally common habitats. The multi- dimensional representation of community rarity presented in this study provided important insights into our understanding of the biogeographic mechanisms which may be involved in biodiversity loss.