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

We studied the spatial distribution pattern of Bolitophagus reticulatus L. (Coleoptera, Tenebrionidae) in a boreal forest in south-ear;tern Norway. Th e beetle is monophagous and lives in basidiocarps of Fomes fomentarius (L.) Kickx. We collected 900 basidiocarps from all of the 452 birch trees infec ted with tinder fungus in the 225 ha study area. We then used logistic regr ession analyses to relate trees with and without beetle occurrence to patch size, isolation and other habitat characteristics. To understand why the b eetle had disappeared from certain trees, we also did similar analyses on t rees without present occurrence but with signs of earlier beetle presence v ersus trees with present occurrence of the beetle. The probability of B. re ticulatus presence increased with tree diameter and with the number of dead basidiocarps on the tree. We interpret these variables as indicating the h abitat patch size. The probability of beetle presence decreased as the dist ance to surrounding inhabited trees increased, even on a scale of less than one hundred meters. The fungus-infected trees were utilized to a much. hig her degree when they occurred in clusters. We discuss to what degree a meta population 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 fo restry should include not only the need for more dead wood in the modern fo rest, but should also consider the spatial distribution of these key resour ces.