Fungus-infected trees as islands in boreal forest: Spatial distribution of the fungivorous beetle Bolitophagus reticulatus (Coleoptera, Tenebrionidae)

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Abstract
We studied the spatial distribution pattern of Bolitophagus reticulatus L. (Coleoptera, Tenebrionidae) in a boreal forest in south-eastern Norway. The beetle is monophagous and lives in basidiocarps of Fomes fomentarius (L.) Kickx. We collected 900 basidiocarps from all of the 452 birch trees infected with tinder fungus in the 225 ha study area. We then used logistic regression analyses to relate trees with and without beetle occurrence to patch size, isolation and other habitat characteristics. To understand why the beetle had disappeared from certain trees, we also did similar analyses on trees without present occurrence but with signs of earlier beetle presence versus trees with present occurrence of the beetle. The probability of B. reticulatus presence increased with tree diameter and with the number of dead basidiocarps on the tree. We interpret these variables as indicating the habitat patch size. The probability of beetle presence decreased as the distance to surrounding inhabited trees increased, even on a scale of less than one hundred meters. The fungus-infected trees were utilized to a much higher degree when they occurred in clusters. We discuss to what degree a metapopulation model is relevant in our case. Several other, less common forest insects depend on wood-rotting fungi or dead wood for their survival. If a similar response to increased isolation of habitat patches can be assumed for some of the rare species, guidelines for an ecologically sustainable forestry should include not only the need for more dead wood in the modern forest, but should also consider the spatial distribution of these key resources.