How Are Extensive, Human-Caused Habitat Alterations Expressed on the Scale of Local Bird Populations in Boreal Forests?

Abstract

We studied long-term population changes in 43 abundant species of forest birds using line transect censuses conducted in northern Finland from 1941 to 77. Previous studies have suggested that human-caused habitat changes are a major reason for regional, long-term changes of bird populations. Birds of old taiga forests have decreased in number because of clear-cutting and forest fragmentation. Many southern species have increased during the period studied, probably partly because of an overflow of individuals from increasing populations in southern Finland, partly as a consequence of habitat changes (increase of bushy young forests and forest edges) in northern Finland. While the median northern species decreased by about one-third, the median southern species increased 5-fold in northern Finland during the study period. How do the regional patterns translate to the local scale? An area characterized by mature forest and peatlands in Simo was censused in 1959 and 1977-81. Between these censuses the area changed drastically due to clear-cutting and draining. Species trends were similar to those observed regionally but more species declined, probably because, on average, the local habitat changes were more extreme than those on the regional scale. In contrast, another local study area, a luxuriant spruce forest (Tormivaara), remained intact from a first census in 1915 until censused again in 1981-83. In this area the local stability expected was overshadowed by the regional changes: in the 1980s the reserve-like Tirmiivaara was no longer characterized by the typical bird species of northern old forests, but the bird populations had changed largely as in managed forests in northern Finland. This result suggests that the bird populations in small forest reserves in northern Finland are not closed but affected by the dynamics of the regional populations.