Decreasing biodiversity in small old-growth forest fragments with time since isolation



Raimo Virkkala, Finnish Environment Institute
Vesa Selonen, University of Helsinki


The occurrence of 13 species of birds specializing in mature or old-growth coniferous forests was compared between the different time-since-isolation classes. The number of observations was lowest in those fragments which have been isolated for the longest time (over 20 years, p<0.05). The mean size of the forest fragments was 9.2 ha (range 6-16 ha) in non- fragmented forest areas and 6.0-7.3 ha in the three isolation classes (range 4.3-10 ha). There were altogether 24 forest fragments. Birds were counted three times in each forest patch: in early, mid and late June (material for many other taxa under study).

Fig.1. Number of observations (per ha) of bird species specializing in old-growth/mature coniferous forests in the different time-since-isolation classes in forest fragments of ca 10 ha.