Climatic conditions, landscape, and habitat quality drive patch occupancy and larval density of a threatened mire butterfly

Mire ecosystems have severely suffered from land-use and climate change. As a result, many of the characteristic mire species are now threatened or even have become extinct. In this study, we aimed to identify the environmental drivers of habitat occupancy and larval density of the threatened Moorland Clouded Yellow (Colias palaeno) in one of its last refuges in Central Europe, the Black Forest (SW Germany). Our study revealed that climatic conditions, landscape and habitat quality drove patch occupancy and larval density. In generalized linear models, both patch occupancy and larval density were positively associated with summer sunshine duration and cattle grazing, and negatively associated with patch isolation. Under the cool and wet mountain climate, a warm macroclimate during summer as well as a warm and moist microclimate played a pivotal role for patch occupancy and larval density. Colias palaeno exhibited a metapopulation structure, depending on a network of well-connected but relatively small patches of high quality. Habitat quality was strongly improved by cattle grazing. The species benefited from grazing in three different ways by (i) fostering the nectar supply; (ii) enhancing rejuvenation of the host plant; and (iii) improving microclimatic conditions for successful development of the immature stages. Restoration measures should include the (i) re-wetting of drained mires and (ii) removal of shrub or tree encroachment. To secure high habitat quality over the long run, we recommend establishing cattle grazing with low stocking rates.