Biodiversity conservation; Extinction debt; Saproxylic; Agroforestry; Patch size; Habitat connectivity

Background The hermit beetle (Osmoderma barnabita) is an internationally protected specialist of tree hollows considered a relic of primeval forests. The backbone of its distribution in eastern Czechia, however, is a system of pollard willows in intensively farmed lowlands. Pollarding, a traditional agroforestry practice inducing formation of tree hollows, was partly abandoned in the area during the twentieth century. To assess the state of the system of pollard willows and to investigate parameters of pollards and their stands which affect the beetle's presence. We inventoried pollard trees and beetle distribution across ca 2000 km(2) of lowlands along the Dyje, Morava and Odra rivers and their tributaries in eastern Czechia and westernmost Slovakia. We found 10 441 pollard willows in 324 stands: most stands contained trees of large diameters and poor health; young trees were rare. Probability of O. barnabita presence increased with number of trees in stand, decreased with distance to the nearest occupied stand, and was further affected by the health state of trees. Our results show that high hollow density in pollards allows for the existence of specialized, dispersal limited forest organisms in virtually deforested landscapes. They suggest stands should contain at least 200-300 pollard willows to sustain the beetle population. Although the beetle is still widespread over the study area, the existing populations are isolated and subjected to extinction debt. Implications for insect conservation Large-scale restoration of pollarding practice, planting and pollarding of numerous young trees, and increasing connectivity is vital to facilitate the survival of this hermit beetle population but also support other hollow associated organisms.