A resource-based conservation approach for an endangered ecotone species: The Ilex Hairstreak (Satyrium ilicis) in Flanders (north Belgium)

Ecotones (i.e., transition zones between two or more communities of considerable length that are narrower than the adjoining community areas themselves) are often species-rich and harbour specific resources and environmental conditions for invertebrates. Despite their functional significance for conservation, they are often not explicitly included in biotope typologies relevant to conservation policy and management (e.g., the European Habitats Directive). The Ilex Hairstreak (Satyrium ilicis) is a species of European conservation concern and a typical ecotone species. Its habitat covers gradients from open (e.g., heathland, grassland) to closed vegetation (e.g., woodland). Within the context of a regional Species Action Plan in Flanders (north Belgium), we investigated its occurrence and habitat use at different spatial scales. At a regional scale (Flanders), species distribution modelling predicted 1152 grid cells of 1 x 1 km² to be suitable of which 190 were presumed to occur within colonization capacity (± 2.5 km). At a local scale, adult butterflies were more abundant on sites sheltered by bushes and small trees and with nectar sources in the vicinity of tall oak trees (mate locating sites). For egg-laying, females preferred oaks of intermediate height (50-150 cm) with many low branches at some distance from the nearest woodland edge (12 m). Additionally, Alder Buckthorn (Frangula alnus) was abundant as well as a herb layer of 10-15 cm. 73% of the eggs were parasitized and parasitism occurred more often within sites where small oaks were very abundant. Making use of our results, we suggest conservation measures at different scales for this endangered ecotone species: policy measures at a regional level to delineate functional conservation units using species distribution models and local management measures using a resource-based approach.