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Fragmentation of food resources is a major cause of species extinction. We tested the effects of habitat area, isolation and quality for the occurrence and population density of the endangered butterfly Polyommatus coridon. Polyommatus coridon larvae are monophagous on the plant Hippocrepis comosa, and both species are specialised on calcareous grassland, which is an endangered and highly fragmented habitat type in Germany.

In 2001 we surveyed all known calcareous grasslands (n=298) around the city of Göttingen (Germany) to map the population size of H. comosa in these habitats. Further, habitat isolation (between-patch distance: 70–7220 m) and habitat quality (cover of flowering plants, height of herb layer, percent bare ground, cover of shrub layer, wind protection, inclination) were quantified. Hippocrepis comosa occurred on only 124 fragments, which were then surveyed by 20 min transect counts for adult P. coridon in 2001 and 2002.

Occurrence and population density of P. coridon were largely determined by the population size of its larval food plant, which was correlated with grassland area. Effects of habitat isolation and habitat quality on P. coridon populations contributed only little to the explanation.

In conclusion, this monophagous habitat specialist depended on large habitats with large food plant populations to exist in viable populations. Habitat isolation and quality appear to contribute to occurrence and density patterns only in landscapes where these factors shift towards extremes, therefore general recommendations for conservation programs are difficult as they depend on regional distinctions.