Are the declining trends in forest grouse populations due to changes in the forest age structure? A case study of Capercaillie in Finland Saija Sirkiä a,b, * , Andreas Lindén a , Pekka Helle c , Ari Nikula d , Jonas Knape e , Harto Lindén b a Department of Biosciences, University of Helsinki, P.O. Box 65, FI-00014 Helsinki, Finland b Finnish Game and Fisheries Research Institute, P.O. Box 2, FI-00791 Helsinki, Finland c Finnish Game and Fisheries Research Institute, Tutkijantie 2 E, FI-90570 Oulu, Finland d Finnish Forest Research Institute, P.O. Box 16, FI-96301 Rovaniemi, Finland e Department of Environmental Science, Policy & Management, 137 Mulford Hall #3114, University of California, Berkeley, CA 94720, USA article info Article history: Received 17 December 2009 Received in revised form 23 March 2010 Accepted 31 March 2010 Available online 22 April 2010 Keywords: Forestry Grouse Spatial population dynamics Tetrao urogallus Time series analysis abstract In Finland, Capercaillie (Tetrao urogallus) populations have a history of serious decrease starting from the mid-20th century. The decline is temporally in line with the expansion of modern forestry practices that created major changes in the landscape. We used tetraonid route-censuses from 18 forestry board dis- tricts and Finnish forest inventories (data on forest stand structure) to analyze the decline in 1965– 1988. We used information theoretical model selection to evaluate a set of log-linear second order auto- regressive models, allowing for spatially correlated process errors. The average trend throughout the country corresponded to an annual decline of 4.01% (mean of local trends) ± 0.24% (SEM), parallel to a half-life of 17 years. The decline was surprisingly uniform throughout the country (SD = 1.01%) and most parsimoniously explained by a geographically constant log-linear trend. At the large scale of observation applied here, population trends could not be explained by the proportional increase of younger forest age classes (<40 years old and <80 years old, respectively). Our analysis does not support the hypothesis that the decline in Capercaillie numbers is due to changes in the forest age structure, but we cannot exclude the possibility that other factors behind the decline may have interacted with forestry in general. From a conservation point of view, we caution against over-emphasizing the role of forest age especially at large spatial scales, but leaning also on other research, we recommend that more management efforts would go into the preservation of the overall forest cover and the original physiognomy in single forest patches. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Human alteration of natural habitats is the largest single cause for biodiversity loss in the world (Millennium Ecosystem Assess- ment, 2005). Boreal forest being one of the world’s most extensive terrestrial ecosystems (e.g. Haila, 1994; Schmiegelow and Mönkkö- nen, 2002), the increasing pressure for land conversion in the bor- eal region is an important conservation issue. Forestry is largely acknowledged to be the driving force in the population declines of boreal forest species in Fennoscandia (e.g. Rassi and Väisänen, 1987; Esseen et al., 1997). The most affected are the species adapted to old forest (e.g. Hansson, 1992; Esseen et al., 1997; Mönkkönen, 1999; Brotons et al., 2003); a common example being the declining Capercaillie populations (e.g. Rolstad and Wegge, 1989a). In general, there is a profound consensus about the role of for- estry in the decline of Capercaillie in Europe (e.g. Storch, 2000, 2007; Miettinen et al., 2008). According to several studies, the neg- ative effects of forestry are mostly mediated through lekking site destructions (including the surrounding territories of the males, Rolstad and Wegge, 1987a; Lindén and Pasanen, 1987; Helle et al., 1994) and partly also through overall habitat loss and frag- mentation of forests (e.g. Storch, 2000; Miettinen et al., 2008). However, there is no clear analytical evidence of forest age being the number one cause in the decline, although traditionally, Caper- caillie has been thought to be dependent on older forests (e.g. Val- keajärvi and Ijäs, 1986; Storch, 1993a; Swenson and Angelstam, 1993), and the large-sized males in particular prefer large old- growth forest patches for their traditional lekking sites (Rolstad and Wegge, 1987b, 1989b). Other possible candidates for the decline include factors di- rectly or indirectly associated with forestry or other human land 0006-3207/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2010.03.038 Abbreviations: NFI, National Forest Inventory; AIC, Akaike information criterion. * Corresponding author at: Department of Biosciences, University of Helsinki, P.O. Box 65, FI-00014 Helsinki, Finland. Tel.: +358 9 191 57728; fax: +358 9 191 57694. E-mail addresses: saija.sirkia@helsinki.fi (S. Sirkiä), andreas.linden@helsinki.fi (A. Lindén), pekka.helle@rktl.fi (P. Helle), ari.nikula@metla.fi (A. Nikula), jknape@ berkeley.edu (J. Knape), harto.linden@rktl.fi (H. Lindén). Biological Conservation 143 (2010) 1540–1548 Contents lists available at ScienceDirect Biological Conservation journal homepage: www.elsevier.com/locate/biocon