A unified measure of the number, volume and diversity of dead trees and the response of fungal communities

1. 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 x 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.