Contrasting trends of butterfly species preferring semi-natural grasslands, field margins and forest edges in northern Europe

Indicator classifications help us to focus on the most relevant groups of species in monitoring the effects of land use changes on biodiversity. We studied changes in distribution area of 74 butterfly species preferring one of the three common habitats of boreal agricultural landscapes: semi-natural grasslands (35 species), arable field margins (7) and forest edges (32). Using extensive atlas data from four time periods during the last 50 years in Finland, we quantified trends in the occupancy of the species in 10 km grid squares, and classified them into four classes: declining (23), stable (17), increasing (27) and fluctuating (7) species. Trends among the species favouring three habitats were different: 60% of the species of semi-natural grasslands had declined, whereas 86% of the species typical of open field margins had increased. An increase also predominated in species associated with forest edges. Declining and increasing species differed in three ecological characteristics: increasing species were more mobile, utilized a wider range of habitats and, based on their larval host plants, lived in more eutrophic habitats than declining species. Species overwintering as adults showed more positive trends in occupancy than species overwintering as eggs, larvae or pupae. Observed trends in occupancy are in good agreement with long-term changes in land use and habitat availability in Finland: a long-continued decrease in the area of semi-natural grasslands and an increased amount of open forest edges and clearings due to modern forestry during the past 50 years.