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Forest fragmentation and regrowth : use of riparian and upland forest by birds in managed and unmanaged mature coastal British Columbia rainfores Shirley, Susan

Abstract

Riparian ecosystems are known for their high diversity, yet they represent one of the most threatened habitats globally. I studied riparian and upland bird communities in undisturbed old-growth forest on western Vancouver Island, B.C. and in riparian buffer strips retained after harvesting. In undisturbed forest, species richness and diversity did not differ over the riparian-upland gradient. Except for riparian specialists and species associated with deciduous vegetation, there was broad overlap of species and abundances between riparian and upland habitats. To test if fragmentation of riparian habitats alters bird communities, I surveyed birds in buffers of varying widths 5-7 years post-harvest. Effects of fragmentation on species richness and overall abundance were weak to absent in these communities, which are dominated by several forest generalists. The few forestinterior species were less abundant in all but the widest buffers and were replaced by open-edge species in narrower buffers. Species assemblages in narrow buffers were also least similar to controls. Species richness and abundance increased in both buffers and adjacent clearcuts during the study period. Because populations able to use the forest matrix may persist better in buffers, I studied movements of birds across river and forestclearcut edges. Forest generalists, open-edge and ubiquitous species crossed riparian and clearcut edges most often. Movements were more frequent across clearcut edges than river edges and were positively related to densities in buffers rather than to buffer width. Movements were highest in narrow buffers, however, suggesting that birds move in and out of narrow buffers because they do not provide suitable habitat to support forestdwelling species. I suggest that narrow buffers function as foraging sites or travel corridors. Finally, I examined patterns of vegetation across varying buffer widths to test if habitat diversity better explains patterns of bird species richness and abundance than buffer width. Densities of faster-growing deciduous trees, and richness and cover of shrubs and forbs differed over the range of buffer widths. While species richness and abundances of several bird guilds and species were explained better by buffer width, three habitat specialist guilds: riparian specialists, forest interior and open-edge species and 4 of 8 individual species were best predicted by density and cover of deciduous vegetation. Deciduous vegetation was more common in wider buffers providing further evidence that retaining wide buffers enhances bird habitat.

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