Forest dieback in a protected area triggers the return of the primeval forest specialist Peltis grossa (Coleoptera, Trogossitidae)

Forest set-aside is a commonly employed conservation strategy, but large-scale disturbances regularly evoke discussions on its utility for biodiversity conservation in former production forests. In this study, we reconstructed the return of a primeval forest specialist, the beetle Peltis grossa (Linnaeus, 1758) of the family Trogossitidae, to two national parks after more than a century of absence. To illuminate historical processes and the role of potential refuges and species' size, we compared the beetle's current distribution collected in a large-scale citizen science approach, with historical data and the current distribution of two closely related, smaller species. We quantified deadwood preferences and investigated the effect of benign neglect versus salvage-logging management practices on P. grossa abundances. Our findings support the view that P. grossa survived in the southern Bohemian Forest, which enabled its recolonization of the landscapes. However, a build-up of the population was dependent upon a massive supply of deadwood by bark beetles within the refuge area. In a large area in the north of the study region that >20 years ago contained ample amounts of deadwood, forest succession outpaced colonization by P. grossa. The current probability of the species' presence decreased with distance to the presumed refuge area from 70% to 5% at 40 km. In the present core distribution area of P. grossa, salvage logging reduced its abundance by the factor 20. These results provide three important lessons on the potential of natural disturbances as a biodiversity restoration tool in forest set-asides: First, large supply of deadwood by disturbance can trigger the return of locally extinct primeval forest specialists. Second, the species must survive within a critical distance (for P. grossa < 10-40 km). Third, salvage logging significantly reduces the population densities of forest specialists by reducing habitat amount.