Island size is not the only consideration. Ranking priorities for the conservation of butterflies on Italian offshore islands

Ecological and historical factors virtually create a unique faunal assemblage on each island. From this perspective every island deserves protection. However, economic limitations usually restrict conservation efforts to particularly important areas. As part of the SLOSS issue (the relative importance of single large or several small areas), there is the long debated question of whether it is better to protect few large areas (islands) or several small areas (islands). Here, we assess the butterfly faunas of the Italian offshore islands, using several biodiversity measures, in order to highlight priorities for conserving butterfly richness, rarity and endemicity. First, the nested pattern of butterfly fauna was investigated to determine the relative importance of large and small islands. Then, residuals were assessed for the species-area relationship and for multiple regressions of richness, rarity and endemicity against geographic variables. Subsequently, two other indices were calculated: Biodiversity Conservation Concern and an index scoring islands in the order that maximizes the cumulative percentage of total, endemic, and rare species. The results clearly indicate that although greatest concern is for the island having the largest butterfly fauna in the sample (Elba), the importance of several small islands should not be ignored. This is primarily due to the substantial impact of source areas and consequently the occurrence of several rare and endemic species occurring on small islands as well as on large islands.