Importance of habitat patch size for occupancy and density of aspen-associated saproxylic beetles

In Fennoscandian boreal forests, aspen (Populus tremula) is one of the most important tree species for biodiversity. In this study we explore how occupancy and density of beetles associated with dead aspen are related to habitat patch size and connectedness in a 45,000 ha boreal managed forest landscape in central Sweden. Patch size was estimated as amount of breeding substrate and connectedness as crown cover of living aspen in the surrounding landscape. The beetles were sampled by sieving of bark or by inspection of species-characteristic galleries in 56 patches with dead aspen. Six of nine aspen-associated species (Xylotrechus rusticus, Ptilinus fuscus, Mycetophagus fulvicollis, Cyphaea curtula, Homalota plana and Endomychus coccineus) showed a positive significant relationship between habitat patch size and occupancy. For all these species, except C. curtula, there was also a significant positive relationship between patch size and density. Connectedness was not retained as a significant variable in the analyses. Species not defined as aspen-associated constituted a significantly larger proportion of the total density of individuals of saproxylic beetles in smaller habitat patches than in larger patches. Richness of aspen-associated species was positively related to habitat patch size. Efforts in the managed forest should be directed towards preserving and creating larger patches of living and dead aspen trees and increasing the amount of aspen at the landscape level.