Is the Eurasian Three-toed Woodpecker Picoides tridactylus a keystone species in boreal forest environments?

The Eurasian Three-toed Woodpecker Picoides tridactylus is a widespread species of the northern Palearctic forests. The species prefers mature coniferous and mixed forests, favouring forests with a good proportion of dead and dying trees. Three-toed Woodpecker has a close connection to forest habitats with natural dynamics and disturbances; it inhabits areas recently disturbed by fire, wind, snow, or other agents that increase the amount of suitable substrate of its insect prey. Its diet consists predominantly of spruce bark beetles (Coleoptera, Scolytidae), although phloem sap of conifer trees is also used especially in springtime. We present here results of both published and unpublished studies that are based on long-term population studies in boreal areas in southern and eastern Finland. The territory sites of Three-toed Woodpecker are detected to locate in structurally complex forests, and there is a strong positive correlation with the occupancy proportion of its territories and the quality of the forest landscape. The species is observed to indicate general species richness of forest birds and structural diversity of forest environment. Its breeding density increases along the conservation value of forest landscapes for breeding birds. The Three-toed Woodpecker is also considered a candidate of umbrella species for the bark beetle biodiversity given its requirement for ample dead wood and thereby susceptibility to forest management. The Three-toed Woodpecker is a primary cavity excavator, and its old nest cavities are used by various cavity-nesting bird species in its territories. The species frequently reuses its own cavities, but they have found to be very important for the Pygmy Owl Glaucidium passerinum. However, the general importance of the Three-toed Woodpecker as a cavity-producer in boreal forests is most probably much smaller than that of the Great-spotted Woodpecker Dendrocopos major. Available evidence suggest that the Three-toed Woodpecker may regulate tree-damaging bark beetle populations during an epidemic. This underlines the benefit of protecting the species and its habitats as a means to possibly limit beetle damages in surrounding managed forests. Based on the results and current knowledge, we evaluate the keystone species status and possible economic values of the Three-toed Woodpecker in boreal forest environments. 1. Fayt, P., Machmer, M. M. & Steeger, C. 2005: Regulation of spruce bark beetles by woodpeckers - a literature review. --- Forest Ecology and Management 206: 1--14. 2. Pakkala, T., Hanski, I. & Tomppo, E. 2002: Spatial ecology of the three-toed woodpecker in managed forest landscapes. --- Silva Fennica 36: 279--288. 3. Pakkala, T., Lindén, A., Tiainen, J., Tomppo, E. & Kouki, J. 2014: Indicators of forest biodiversity: which bird species predict high breeding bird assemblage diversity in boreal forests at multiple spatial scales? --- Annales Zoologici Fennici 51: 457--476.peerReviewe