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In contrast to several organisms that have already shown range shifts to the north as a response to climate change, southern populations of relict species are trapped in isolated altitudinal habitats. Therefore, there is a growing interest to better understand their habitat use, with particular attention to the thermal aspects and associated significance for their habitat management. We address this issue by a study of larval habitat use relative to vegetation structure and microclimate in a glacial relict butterfly of peat bog ecosystems, using a functional, resourcebased habitat approach. We analysed caterpillar presence and density relative to vegetation composition (reflecting gradients of humidity, temperature, and natural succession of the peat bog) and to the availability and quality of thermal refuges for caterpillars (i.e., structures provided by Sphagnum hummocks). We also tested caterpillar survival rates under different temperature and humidity treatments. We found that (1) Boloria aquilonaris was a specialist butterfly of early successional stages with very humid zones of peat bog, (2) the lack of Sphagnum hummocks reduced larval habitat suitability, and hence the population density, and (3) a reduction of the thermal buffering ability of Sphagnum hummocks was observed in less humid, degraded parts, or late-successional stages of peat bog. A larval rearing experiment showed a significant impact of temperature on caterpillar survival; survival being higher at lower temperature. Our field and laboratory results support the idea that the thermal environment exploited by caterpillars should be considered as a functional resource and included in a population-specific habitat definition. Appropriate management of the peat bog habitat of this glacial relict species should not exclusively focus on the larval and adult feeding resources, but also on the quality of thermal refuges provided by Sphagnum hummocks in humid zones of the peat bog, especially in the current critical context of climate warming.

Habitat loss and fragmentation are known to reduce patch sizes and increase their isolation, consequently leading to modifications in species richness and community structure. Calcareous grasslands are among the richest ecosystems in Europe for insect species. About 10% (1,150 ha) of the total area of a calcareous ridge region (Calestienne, Belgium) and its butterfly community was analysed over a timeframe of about 100 years. Since 1905 to present day (2005), the Calestienne region has undergone both calcareous grassland loss and fragmentation: not only did calcareous grassland size decrease and isolation increase, but also, the number of calcareous grassland patches within the landscape increased until 1965, and subsequently decreased, clearly reflecting the effects of fragmentation. These processes have had a profound effect on the butterfly community: extinction and rarefaction affected significantly more often specialist species, which means that generalist species are more and more overrepresented. This ecological drift, i.e. the replacement of specialists by generalists in species assemblages is likely to be a general effect of habitat loss and fragmentation on natural communities.

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