These changes in heathland cover must surely have had an immense effect on local fauna. However, widespread, open heathlands are not a natural habitat type in Denmark, and heathlands as we now know them are a direct result of Neolithic and Bronze-age agricultural practices, followed by more-or-less deliberate land management until the onset of the industrial and agricultural revolutions of the past two to three centuries. Strong decline and genetic isolation have been documented for the beetle Carabus arcensis Herbst and the butterfly Euphydryas aurinia (Rottemburg), while the iconic and once widespread grasshopper Bryodemella tuberculata (Fabricius) went extinct in the 1940s. This decline has naturally had dramatic effects on insect species dependent on heathland as their habitat. Heathland is one of the habitat types that have experienced the most severe decline, particularly in western Jutland. While forest cover has increased slightly, natural and seminatural open habitats have severely decreased because of intensification of agriculture and the spread of urban areas. Land use has changed dramatically in Denmark over the past few centuries. The unique genetic signature of the Læsø populations may be a result of the admixture of northern Jutland and western Swedish populations. We suggest that the two distinct clusters in western and northern Jutland indicate two temporally separated Holocene colonizations of Denmark, the latter of which may have been aided by changes in agricultural practice in the late Neolithic period. When including data from museum specimens, only a single locatSion showed a decline in heterozygosity between 19. Estimates of genetic diversity showed signs of inbreeding in several extant populations. Our results for the Bayesian population assignment of recent samples revealed three major current genetic clusters: western Jutland, northern Jutland, and the island of Læsø. We thus generated the most temporally and spatially comprehensive population genetic dataset for P. alcon specimens from 44 spatiotemporal locations in Denmark. To explore past and current patterns in population structure in relation to the decline, we analyzed DNA microsatellite data from 184 recent and 272 historical P. However, the population genetic consequences of this decline remain unknown. Phengaris alcon is an endangered, ant-associated butterfly found, amongst other places, in Denmark, where it has undergone a severe decline during the last century.
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