Importance of scale and density of hollow oaks for saproxylic oak beetles

• One of the key aspects in conservation management and in understanding species distributions is how they respond to habitat factors at different scales. Old-growth deciduous forests is severely fragmented in Europe and the most important substrates for saproxylic beetles in this habitat is hollow veteran trees. In this study, we used an extensive field survey data, mapping all large and/or hollow oaks (ca 33,000) in an area of 10,000 km² in south-eastern Sweden. With beetle occurrence data from 38 sites we did a multi-scale analysis of how different beetle species responded to oak density. A total of 16 species responded significant to substrate density from 52 m to 5200 m showing that conclusions made from data measured on a single scale may lead to wrong conclusions. We hypothesized that larger species should respond to larger scales and that both local and landscape scales should be important for several species. We found no evidence for that larger species responded to substrate density at larger scales. However, several species as e.g. Tenebrio opacus, responded to oak density at both small a scale (92 m) and a large scale (859 m). The reason for the importance of two scales is probably that several processes are acting on different time scales and therefore over different spatial scales. Individual oaks may act as static patches in the short term and the small-scale response may reflect the scale of metapopulation dynamics. However, as changes in the densities of old oaks over larger landscapes over several centuries occurs, long-term substrate dynamics are expected to act over larger areas, reflecting the larger scale. The variation in species response to substrate density at different scales means that habitat loss and fragmentation will have different effects upon different species.

Nyckelord

Saproxylic beetles

old oaks

Quercus

conservation

management

multi-scale

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