LANDSCAPE FRAGMENTATION AND FOREST COMPOSITION EFFECTS ON GROUSE BREEDING SUCCESS IN BOREAL FORESTS

Abstract

We examined the breeding success of forest grouse in relation to anthropogenic forest fragmentation in Finland. Employing Geographic Information Systems (GIS) and grouse data derived from Finnish wildlife triangle censuses conducted during 1989–1994, we combined the locations of 2267 Black Grouse (Tetrao tetrix) and 1060 Capercaillie (T. urogallus) females after the breeding season in mid-August with landscape data. The indicators of breeding success were the proportion of grouse hens with a brood and brood size. Two study areas (each 45 000 km2) in the boreal zone were selected for investigation. The breeding success of grouse was negatively correlated with both fragmentation of forest area per se by farmland and the decreasing proportion of older forest as a result of clear-cutting. The extent of landscape accounting best for variation in nesting success was an order of magnitude larger (∼100 km2) than the area most probably used by a grouse female and her brood during the summer, which suggests that landscape-scale factors may override local factors such as track size and distance from edge. The proportion of grouse hens with a brood was lower in heavily fragmented landscapes than in more continuous forest landscapes whereas only minor differences in brood size were detected. We suggest that the most likely cause of the observed spatial correlation was higher nest predation by generalist predators in fragmented forest landscapes. The effects of landscape composition on the breeding success of grouse were more marked in northern than in southern Finland, probably because predator populations are more food-regulated in the north. The diminished breeding success of forest grouse as a result of increased forest fragmentation is a probable cause of population declines in forest grouse species during the past decades in Fennoscandia.