Abstract. Signals of species interactions can be inferred from survey data by asking if some species occur more or less often together than what would be expected by random, or more generally, if any structural aspect of the community deviates from that expected from a set of independent species. However, a positive (or negative) association between two species does not necessarily signify a direct or indirect interaction, as it can result simply from the species having similar (or dissimilar) habitat requirements. We show how these two factors can be separated by multivariate logistic regression, with the regression part accounting for species-specific habitat requirements, and a correlation matrix for the positive or negative residual associations. We parameterize the model using Bayesian inference with data on 22 species of wood-decaying fungi acquired in 14 dissimilar forest sites. Our analyses reveal that some of the species commonly found to occur together in the same logs are likely to do so merely by similar habitat requirements, whereas other species combinations are systematically either over-or underrepresented also or only after accounting for the habitat requirements. We use our results to derive hypotheses on species interactions that can be tested in future experimental work.

Abstract. Signals of species interactions can be inferred from survey data by asking if some species occur more or less often together than what would be expected by random, or more generally, if any structural aspect of the community deviates from that expected from a set of independent species. However, a positive (or negative) association between two species does not necessarily signify a direct or indirect interaction, as it can result simply from the species having similar (or dissimilar) habitat requirements. We show how these two factors can be separated by multivariate logistic regression, with the regression part accounting for species-specific habitat requirements, and a correlation matrix for the positive or negative residual associations. We parameterize the model using Bayesian inference with data on 22 species of wood-decaying fungi acquired in 14 dissimilar forest sites. Our analyses reveal that some of the species commonly found to occur together in the same logs are likely to do so merely by similar habitat requirements, whereas other species combinations are systematically either over-or underrepresented also or only after accounting for the habitat requirements. We use our results to derive hypotheses on species interactions that can be tested in future experimental work.

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Preservation of woodland key habitats has become an integral part of biodiversity-oriented forest management in northern Europe. In Finland, brook-side spruce forests constitute the most important key-habitat type in terms of total area and timber volume. Our aim was to compare polypore diversity and the occurrence of red-listed species between brook-side key habitats and comparable sites of managed forest as controls. Furthermore, we assessed the importance of stand structural features to polypore communities, and contrasted regions with diVerent land-use history. Altogether 69 key habitats and 70 controls were inventoried in four regions across southern Finland. A total of 28,023 surveyed dead-wood units had 3,307 occurrences of 114 species of which 25 species were red-listed. Key habitats hosted on average 28% (about two species) more polypore species (mainly deciduous-specialists) than controls because of a larger average volume of dead wood and a higher proportion of dead deciduous wood. However, the average number of red-listed species was low and did not diVer between the two forest categories. We detected a landscape eVect connected with the intensity and length of forest management history, with the highest numbers of all and red-listed species per site in eastern Finland. Volume and diversity of dead wood were the most important variables explaining variation in species richness. Presently, key habitats in managed forests appear to contribute only little to the conservation of red-listed polypore species.

Summary1. Much of ecological research focuses on the responses of species and species communities to variation in the amount and quality of resources that are required for survival and reproduction. In such research, it is critical to measure the availability of resources in a manner that is relevant in relation to the ecological requirements of the species. 2. We have developed a measure for resource availability that integrates the contributions of the number, volume and diversity of resource units to quantify the amount of habitat that is available for a species community. We apply this measure to data on the occurrence of 116 species of wood-decaying polyporous fungi in 47 study plots of boreal forest within an area of 150 · 150 km. 3. We show that species richness and pooled abundance of common species is explained well by the number of downed logs, whereas the occurrence of 41 red-listed species is best explained by the total volume of logs and by the abundance of large logs in particular. The occurrence of common species is explained by the local availability of dead wood, whereas the occurrence of red-listed species is additionally affected by the spatial connectivity of the focal forest stand to the surrounding larger expanses of old-growth forest. 4. Our results elicit the contrasting ecologies of common and red-listed species in relation to how the number of logs, their size distribution and diversity, and forest connectivity affect species occurrences. The results suggest that the most cost-effective means of preventing further declines of threatened species is to increase the amount of large downed logs through restoration and biodiversity-oriented management in the vicinity of existing areas of natural-like forests. 5. Synthesis. Our results illustrate that the most relevant way of measuring resource availability can differ greatly even within a taxonomically coherent community seemingly sharing the same resources. Our approach for modelling resource availability applies to the resources that occur as discrete objects with variation in the size and quality of individual resource units.

Loss of old‐growth forests and greatly reduced volumes of coarse dead wood in managed forests are the main reasons for the decline of many wood‐inhabiting species in Europe and elsewhere. To assess the habitat requirements and extinction vulnerability of 13 polypore species associated mainly with spruce, their occurrences were recorded on 96 521 dead‐wood objects in 331 stands along a regional gradient of forest utilization history across southern‐middle boreal Finland. The substrates studied included a variety of tree species and dead‐wood qualities investigated in both unmanaged and managed stands at different successional stages. Hierarchical logistic regression models were constructed to analyze the relationships between the occurrence probability of individual species and variables at the substrate, stand and regional scales. The substrate preferences of the polypore species studied overlapped, since most of them favored large‐diameter spruce logs in mid‐decay stages. However, only a few species were restricted to this substrate. Other species were able to use a wider range of host tree species and qualities of dead wood, including man‐made substrates that are abundant in managed forests (logging residues and stumps). Species confined to logs had a significantly lower occurrence probability in regions with the longest and most intensive forest use history. Species less specialized in their resource use showed no decline or the opposite trend. Loss of threatened species is likely if the preservation of old‐growth forests is not combined with conservation measures in managed forests. Increasing extraction of logging residues and stumps for biofuel may cause non‐threatened species to decline by reducing substrate qualities utilized by them. The hierarchical models predicted a considerable part of the variation in Species' occurrence probabilities, and therefore provide powerful tools for setting quantitative targets for management.

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