Explaining population size of the saproxylic beetle Prionus coriarius as a function of available dead wood resources

Abstract: Dead wood is an important substrate for many forest bound species including the large group of dead wood associated (saproxylic) beetles. Saproxylic beetles have species specific substrate requirements, that may vary according to factor such as tree species, object type, trunk diameter and decomposition stage. In modern production forests, the density of dead wood is much lower than in natural forests, which poses a threat to many saproxylic species that require higher densities of substrate. The minimum dead wood density at which species can sustain viable populations is referred to as the extinction threshold level. In this study, the red-listed saproxylic beetle Prionus coriarius was sampled at over 50 sites in Scania, south Sweden, using pheromone trapping. Surveys of dead wood densities were conducted in 38 sites where the beetle was present which allowed for a regression analysis between beetle catch and the dead wood density at a site. Deciduous stumps and damaged trees of minimum diameter 20 cm and intermediate decomposition state could best explain the variation in trap catches. No extinction threshold level of dead wood was detected. Dead wood at a spatial scale of 250 m radius could better explain abundances than a spatial scale at 50 m. GIS data of deciduous forest volume was tested as a proxy for dead wood, but such a relationship was not identified. Since the species can utilize cut stumps, the substrate requirements of P. coriarius alone cannot explain that the species is decreasing; dispersal limitation and fragmentation may be the reasons behind its decline in the past century. Several populations are low in number with low levels of substrate and should be boosted by creating more dead wood resources to avoid local extinctions. Clear cuts may serve as temporary habitat, but dead wood continuity must be ensured if the species is going to survive in production forests.