Metapopulation ecology of Osmoderma eremita - dispersal, habitat quality and habitat history
Sammanfattning: This thesis starts with a review of different biodiversity assessment methods in forests and the rest deals with the metapopulation ecology of Osmoderma eremita, a threatened beetle living in hollow trees. I concentrate on the effect of dispersal, habitat quality and habitat history for the presence, population size and future survival of this species. Dispersal was studied by using both capture-mark-recapture and telemetry. I also studied the pheromone system. The effect of habitat quality on population size and adult body size was analysed using population size per tree from three consecutive years, biometric data and habitat variables for each studied tree. Finally, I have studied if the present occupancy of O. eremita is best explained by the current number of hollow trees per group of hollow trees or by the historical amount of older and damaged trees in that area in 1820-1822. My results show that biodiversity assessment methods of boreal and nemoral forests should include the most species rich groups, for example insects and the natural disturbance regime of a region should be the basis for defining a suitable scale and the appropriate features. The two independent methods of studying dispersal of O. eremita give similar results. O. eremita has a restricted dispersal (< 200 m) and a low dispersal rate (12-18 %). This means that O. eremita may survive for long periods in a stand because of low emigration rate, long life-time of the hollow oaks and asynchronous population fluctuations. However, it will be unlikely to establish itself in any new stands arising at the distance of several hundred meters. The males attract females by emitting large amounts of a pheromone and this substance might be used in female colonization. The probability of occurrence of adult O. eremita increase with increasing wood mould volume inside the trees and is higher in hollows above ground. Both population size and adult body size is bigger in trees with a large wood mould volume. This highlights the importance of hollow oaks with a large wood mould volume for metapopulation survival.Finally, the occupancy of O. eremita in oak stands in a fragmented region of southern Sweden is best explained by the amount of older and damaged oaks in 1820-1822, while the present amount of hollow trees in the stands cannot explain occupancy. Following habitat destruction and fragmentation, metapopulations are often not in stochastic equilibrium with the structure of the present landscape. Instead the metapopulations are tracking a new equilibrium that can be local extinction. This delay creates an erroneous view of the present landscape with an excess of rare species that are often doomed to become extinct, the so called extinction debt. I argue that many small metapopulations of O. eremita can be seen as an extinction debt, that only can be avoided by a rapid increase of hollow trees in and near occupied stands.
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