Food plant density, patch isolation and vegetation height determine occurrence in a Swedish metapopulation of the marsh fritillary Euphydryas aurinia (Rottemburg, 1775) (Lepidoptera, Nymphalidae)

The occurrence pattern of the marsh fritillary was studied within a patch network on the Baltic island Oland, Sweden. Presence/absence was established for potentially suitable habitat patches (n = 158) on calcareous moist grassland and analyzed in a multiple logistic regression model where patch area, patch isolation and nine habitat quality variables were included as explanatory variables. Larval food plant density was positively, and patch isolation negatively, correlated to the presence of Euphydryas aurinia. Area did not contribute to the explanation of the occurrence pattern. Significant interactions between larval food plant density times patch isolation, and larval food plant density times vegetation height, show that with low food plant density the butterfly primarily occurs in patches with a vegetation height of 4-10 cm, within a distance of 250 m from nearest occupied patch. In patches with a high food plant density the butterfly occurs in patches where the vegetation height is higher, 4-16 cm, and the distance to nearest occupied patch can be longer, up to 1.4 km. This study supports earlier findings in other regions, suggesting that a network of adjacent patches with a high food plant density and a vegetation height within the preferred threshold, despite their size, is an apparent conservation goal.