Habitat thresholds and effects of forest landscape change on the distribution and abundance of black grouse and capercaillie

摘要

This study reports thresholds for the amount of habitat needed for individuals and local breeding (=lekking) populations of black grouse Tetrao tetrix and capercaillie T. urogallus in boreal forest. I also examine the effects of the forest age distribution on the abundance of the two species at local and regional scales. Using systematic surveys of displaying males and forest landscape data, the minimum habitat patch size requirements for presence of black grouse and capercaillie was found to be 0.2 and 0.8 km~2 for single males, and 0.9 and 2.2 km~2 for lekking males, respectively. In large patches of suitable habitat the density of lekking males was 5 km~(-2) for black grouse, and 2.5 km~(-2) for capercaillie. The local landscape scale habitat thresholds for presence of displaying black grouse, with a probability of 0.9, were 15% for solitarily displaying males, and 22% for leks, respectively. For lekking capercaillie the threshold was ca 34%. A 97% incidence in habitat patches exceeding the minimum required size, and asymptotic prevalence functions, suggest that available habitat patches were saturated with lekking males, and the grouse populations in equilibrium with their habirat. On the local landscape scale (16 km~2 plots), 84% of the density of lekking black grouse and 74% for lekking capercaillie were explained by the amount of the preferred habitat patches. Both on short-term (decades) in local landscapes, and on nation-wide long-term (60 yr) scales in Sweden, changes in the relative abundance of black grouse and capercaillie paralleled changes in the forest landscapes' age distribution. The results indicate that grouse species show close environmental tracking in terms of habitat amount, and that regeneration time after stand-replacing disturbance represents a resource axis subdivided by forest-living grouse in boreal Fennoscandia. Differences compared with the situation in the periphery of the distribution of these species are discussed, as well as their role as focal species for conservation.