Contrasting responses of multiple insect taxa to common heathland management regimes and old-growth successional stages

Maintaining heathlands in early successional stages to sustain heather (Calluna vulgaris) is a common, large-scale management practice in Europe. However, allowing patches of long-term natural vegetation development may increase habitat heterogeneity benefitting insects, but empirical evidence is sparse. We investigated how old-growth heathland (> 30 years abandonment) affect species richness and composition of bees (Anthophila), crane flies (Tipuloidea), ground beetles (Carabidae), hoverflies (Syrphidae) and rove beetles (Staphylinidae) in relation to their hygropreference. Adult insects, vegetation and edaphic explanatory variables were collected in old-growth, managed and wet sites and compared in four lowland heathland locations in Denmark. We found 299 species including 24 nationally red-listed. Species composition differed between managed, old-growth and wet heathland for all taxa. Indicator species and richness analyses showed a predominance of xerophilic bee species in managed heathland. Old-growth heathland showed a predominance of mesophilic indicator species, and higher richness of mesophilic crane flies and of hygrophilic ground and rove beetles compared to managed heathland. Wet heathland was generally dominated by hygrophilic species. Soil moisture, bare soil and vegetation height density were important drivers explaining the contrasting responses in richness and composition between heathland types. Our results demonstrate that heathland management focusing solely on early successional vegetation stages may homogenize insect communities. We suggest that management practices should focus on improving structural vegetation heterogeneity. This can be achieved through management regimes that reset the succession and expose bare soil, but also by allowing patches of old-growth vegetation stages to develop and by conserving existing ones.