THE UK STATUS AND SUSPECTED METAPOPULATION STRUCTURE OF A THREATENED EUROPEAN BUTTERFLY, THE MARSH FRITILLARY EURODRYAS-AURINIA

Results are presented of a partial quantitative survey of Eurodryas au rinia colonies in 1983 and a full review of all UK colonies in 1990. S ince records began, the butterfly has declined from 663 10-km grid squ ares to 253 squares in 1990: a reduction of 62 %. A total of 432 separ ate colonies were identified in 1990 (228 in England, 111 in Wales, 58 in N. Ireland, and 35 in Scotland), confirming that the UK is a major European stronghold of the species. However, the current loss rate of colonies in Britain is estimated to be at least 11.5 % per decade and is almost as high on protected as on unprotected sites. The butterfly usually breeds in damp, acidophilous grassland but spread onto calcic olous grassland during the present century, probably due to a general relaxation in grazing pressure combined with a switch from sheep to ca ttle grazing. Almost half the colonies surveyed in 1983 occupied very small habitat patches (<2 ha) and only one-sixth occupied patches larg er than 10 ha (max = 40 ha). The species is sedentary and colonial, bu t 149 single adults were reported from non-breeding habitats in 1971-1 990 and at least nine sites were colonised. Certain habitats appear to be transient or are intrinsically suboptimal and patch occupancy is o ften short, leading to a constant state of flux in the butterfly's dis tribution. The suspected metapopulation structure of E. aurinia and it s implications for conservation are discussed.