Directional temporal shifts in community structure of butterflies and ground beetles in fragmented oligotrophic grasslands of Central Europe

Oligotrophic calcareous grasslands are among the most species-rich habitats in temperate Europe. Land use changes caused a severe decline of these species-rich grasslands over the last decades. Today, these ecosystems exist as highly isolated and small remnants being threatened by abandonment, afforestation and the transformation into agricultural land. Local conservation activities caused changes in habitat structures within such oligotrophic calcareous grasslands and their often isolated character may have led to stochastic species extinctions due to population fluctuations. In this study we focus on potential changes of grassland biota over the past decades. We analyse retrospective species inventories (14 for carabid beetles and 7 for butterflies) from calcareous grasslands located in western Germany with adjoining parts of Luxembourg over a period from 1972 to 2011. Our data indicate two distinct temporal cohorts (past versus present) that differ strongly in the composition and structure of carabid beetles and butterfly communities. Patterns of species co-occurrences tended to be segregated in the past communities while the co-occurrences in recent communities were either random or aggregated. beta-Diversity increased with time. Our results indicate a temporal shift from past communities dominated by species interactions towards recent neutral assemblages. These changes might be caused by diverging conservation management being restricted to local sites. This resulted in a higher diversity of environmental structures combined with a loss of grassland habitats and declining matrix permeability. However, we detected no loss of rare carabid beetles, while butterflies showed severe extinctions of species with specific habitat demands, high endangerment and/or low dispersal behaviour. These trends let us argue that carabid communities mirror local changes in habitat structure (caused by conservation action), while butterflies show additional effects due to environmental stochasticity, local extinctions and a subsequent lack of re-colonization from adjoining populations.