| Summary: | A core task in biodiversity conservation is to protect and restore populations of endangered species. The analysis of causes of decline based on data sampled in declining populations in sub-optimal habitats risks overlooking necessary environmental factors because they are no longer present at the sites. It may therefore be necessary to search for conditions that were present before the species began to decline. We used an historical ecology approach to identify critical ecological needs for the endangered butterfly Clouded Apollo, <em>Parnassius mnemosyne,</em> in Sweden in relation to historical and current land use. The most suitable habitat structure and management was transformed into a plan for restoration and management, applied at two <em>P. mnemosyne</em> sites. The populations of <em>P. mnemosyne</em> and its host-plant, <em>Corydalis solida</em>,<em> </em>were monitored annually 2004–2014. Based on previous knowledge about <em>P. mnemosyne, </em>the Swedish plan for conservation of the species recommends regular clearings of shrubs and trees in order to maintain a mosaic tree-grassland habitat, and a limited of removal of ground biomass. Our analysis suggested an alternative management approach, late and rather intense grazing. The populations of <em>P. mnemosyne</em> and its host plant increased in response to this management regime and the butterfly started dispersing to new sites. We suggest a structured method for combining ecological and historical knowledge, which can reveal habitat conditions difficult to detect in the field today, but that are of crucial importance for successful species conservation.
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