Landscape fragmentation and forest composition effects on grouse breeding success in boreal forests

We examined the breeding success of forest grouse in relation to anthropoge nic forest fragmentation in Finland. Employing Geographic Information Syste ms (GIS) and grouse data derived from Finnish wildlife triangle censuses co nducted during 1989-1994, we combined the locations of 2267 Black Grouse (T etrao tetrix) and 1060 Capercaillie (T. urogallus) females after the breedi ng season in mid-August with landscape data. The indicators of breeding suc cess were the proportion of grouse hens with a brood and brood size. Two st udy areas teach 45 000 km(2)) in the boreal zone were selected for investig ation. The breeding success of grouse was negatively correlated with both fragment ation of forest area per se by farmland and the: decreasing proportion of o lder forest as a result of clear-cutting. The extent of landscape accountin g best for variation in nesting success was an order of magnitude larger (s imilar to 100 km(2)) than the area most probably used by a grouse female an d her brood during the summer, which suggests that landscape scale factors may override local factors such as track size and distance from edge. The p roportion of grouse hens with a brood was lower in heavily fragmented lands capes than in more continuous forest landscapes whereas only minor differen ces in brood size were detected. We suggest that the most likely cause of t he observed spatial correlation was higher nest predation by generalist pre dators in fragmented forest landscapes. The effects of landscape compositio n on the breeding success of grouse were more marked in northern than in so uthern Finland, probably because predator populations are more food-regulat ed in the north. The diminished breeding success of forest grouse as a resu lt of increased forest fragmentation is a probable cause of population decl ines in forest grouse species during the past decades in Fennoscandia.