Extinction risks in metapopulations of a beetle inhabiting hollow trees predicted from time series
Ancient and dead trees are declining habitats harbouring many threatened species. These habitats are naturally patchy, and inhabiting species might exhibit metapopulation dynamics at a small spatial scale. In this study, the demography and metapopulation dynamics was analysed for Osmoderma eremita, which is an endangered beetle species associated with tree hollows in Europe. Extinction risks of O. eremita populations were predicted using Monte Carlo simulations based on time series of population assessments. Predicted occurrence patterns were consistent with field observations from an area with many small stands in which the populations are believed to have been more or less isolated from each other during the last 150–200 yr. Population growth was found to be density dependent. Carrying capacity was proportional to the volume of wood mould (i.e. loose material of dead wood in the tree hollows), which varied widely between hollow trees. This generates large differences in local extinction risks between hollow trees. The predicted metapopulation extinction risk was much higher if the habitat dynamics (formation, gradual increase and deterioration of tree hollows) were taken into consideration than in predictions yielded by models in which the amount of wood mould was assumed to be constant over time. Thus, this system has features from both mainland-island metapopulations and habitat-tracking metapopulations, and is rather far from a classic metapopulation. For the long-term persistence of the species in hollow trees, the habitat dynamics seem to be more important than demographic processes. Since the formation and deterioration of suitable tree are partly stochastic processes, there is a considerable extinction risk for many O. eremita populations, because they mainly rely on only one or a few trees with large amounts of wood mould.
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Document Type: Research Article
Publication date: October 1, 2007