Aim: To investigate responses of ecological specialization on area and environmental heterogeneity using a new metric.

Location: Aegean islands (Greece) as a case study.

Taxon: Terrestrial isopods as a case study.

Methods: The new metric, ‘ecorichness’, provides an estimate of an island’s species in the specialists-generalists spectrum. Species presences on each island are replaced with the numbers of habitats they exploit in the study system. Values are summed for each island and standardized by its species richness, giving the ‘ecorichness’ of each island. A data set with terrestrial isopods from 43 Aegean islands, and the respective description of habitats they exploit, was used as a case study. ‘Ecorichness’ was regressed on area, habitat diversity and the Choros model using linear and quadratic models, evaluated based on AICc. A reduced data set, without halophiles and coastal habitats, as well as an alternative description of habitat diversity, were also explored. The Small Island Effect (SIE) thresholds of a path analysis approach and piecewise continuous linear models were compared to the area of maximum ‘ecorichness’.

Results: ‘Ecorichness’ response to area and habitat heterogeneity fits best to quadratic models and peaks at an area similar to the SIE threshold of path analysis. The different habitat diversity description gives similar patterns. Exclusion of coastal species and habitats is informative on processes behind the patterns. The latter are mostly shaped by the increasing contribution of specialists in assemblages of larger islands.

Main conclusions: ‘Ecorichness’ is a useful metric to explore the role of ecological specialization in shaping community patterns. It can be applied on different community data sets, provided that information on habitats and habitat range exploitation by species is available. Results from the case study are in accordance with previous suggestions regarding the relative contribution of generalists and specialists in small and large island communities.

Data are based on species lists and habitat description given by Sfenthourakis (1994: PhD thesis, Univ. of Athens, 1996: Journal of Biogeography 23: 687-698) updated to current nomenclature. Each species occurrence has been replaced by its habitat range (number of described habitats it exploits on the archipelago). The sum of these values for each island, divided by its species richness, gives the 'ecorichness' value of the island, used to exploit its response to area and other parameters.