We studied the effect of habitat fragmentation on the richness, diversity, turnover, and abundance of breeding bird communities in old, boreal mixed-wood forest by creating isolated and connected forest fragments of 1, 10, 40, and 100 ha. Connected fragments were linked by 100 m wide riparian buffer strips. Each size class within treatments and controls was replicated three times. We sampled the passerine community using point counts before, and in each of two years after, forest harvesting, accumulating 21340 records representing 59 species. We detected no significant change in species richness as a result of the harvesting, except in the 1-ha connected fragments, where the number of species increased two years after isolation. This increase was accounted for by transient species, suggesting that the adjacent buffer strips were being used as movement corridors. Diversity (log series alpha index) was dependent on area in the isolated fragments only after cutting, having decreased in the smaller areas. Turnover rates in the isolated fragments were significantly higher than in similar connected or control areas, due to species replacement. Crowding occurred in the isolated fragments immediately after cutting, but two years after fragmentation, the responses in abundance of species varied with migratory strategy. Numbers of Neotropical migrants declined in both connected and isolated fragments, and resident species declined in isolated fragments. Most species in these groups require older forest, many favoring interior areas. Abundance of short-distance migrants, most of which are habitat generalists, did not change. Overall, although there was no decrease in species richness from our recently fragmented areas, community structure was altered; maintaining connections between fragments helped to mitigate these effects. Nevertheless, the magnitude of the fragmentation effects we documented is small compared with those observed elsewhere. Birds breeding in the boreal forest, where frequent small- and large-scale natural disturbances have occurred historically, may be more resilient to human-induced habitat changes, such as those caused by forest harvesting. However, these results should be interpreted with caution. First, they are short-term and address only broad-scale community responses based on species richness and relative abundance. Second, the study area was embedded in a landscape where large areas of old, mixed forest are still available, potentially dampening any local-scale impacts of fragmentation.
Ecology – Ecological Society of America
Published: Sep 1, 1997
Keywords: boreal mixed-wood forest ; connectivity ; experimental fragmentation ; Neotropical migrants ; regional forests ; songbird communities
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