Non-randomness in the species-area relationship: testing the underlying mechanisms

The species-area relationship (SAR) is one of the best documented patterns in ecology. However, interrelations between the SAR and species distributions are largely unexplored. This research was aimed (1) to assess if the SAR for a group of sedentary insects is associated to a random or non-random distribution of species across islands in a land-bridge archipelago, and (2) to investigate possible factors responsible for the non-randomness. Communities of tenebrionid beetles on the Aegean Islands (Greece) were studied as a case of a relict fauna. Three aspects of non-randomness were analysed: (1) non-random variation of species richness in the SAR, (2) degree of nestedness and (3) presence of special patterns of co-occurrence. Species co-occurrence and nestedness analyses indicated that historical aspects, as opposed to interspecific competition or distance-mediated colonization events, have moulded these species distributions.