Resource-based habitat definition, niche overlap and conservation of two sympatric glacial relict butterflies

The precise knowledge of ecological resources and conditions required by species threatened by rapidly changing environmental conditions is of prime importance for conservation biology. Transferability of this knowledge between species with similar ecological requirements is often assumed, but rarely tested. This is especially the case for glacial relict populations confined to climate-habitat traps from where they cannot move to rejoin areas with suitable environmental conditions. Using two glacial relict butterflies as model organisms, we first quantitatively define larval and adult resource-based habitat use of each species. Secondly, we test the transferability of ecological profiles (both habitat and ecological niche) between these two species that share both the same biotope and the same host plant. Our results show that both species have markedly different ecological requirements relating to differences in life history and behavioural traits (i.e. egg-laying strategies and mate-locating behaviour). Although the two species share many ecological features, they use different functional habitats within our study site. The high degree of interspecific niche overlap should indicate a high interspecific competition. However, we argue that their co-existence can be explained by the non-limiting abundance of some resources (e.g. host plants), by the partial separation in time of adult flight periods and by the territorial behaviour of one of the species. We discuss the following general messages: (1) functional habitat of a (threatened) species should be defined in a spatial context corresponding to individual station keeping, and (2) quick diagnosis based on similar ecological requirements may be misleading for the design of reliable conservation and restoration strategies. Detailed mechanistic and quantitative ecological understanding of resource-use and environmental tolerances across an organism's life cycle is essential for effective conservation in changing environments, like for glacial relict species.