Effects of forest harvesting disturbance on aquatic birds in the boreal forest of Alberta.

摘要

Effects of disturbances on some community characteristics may transcend habitat and taxonomic boundaries. For example, the intermediate disturbance hypothesis (IDH) purports that communities display their highest species richness at intermediate levels of disturbance. In the past, organisms sensitive to disturbance have been used as indicators of anthropogenic impacts on the environment. I examined aquatic bird communities in boreal Alberta, on 'harvested lakes' (forest harvesting closer than 450 m, within three years of my study) and 'unharvested lakes' (no harvesting within at least 450 m of the shore), to determine whether harvesting disturbance affected (i) species richness, following the IDH, (ii) community composition, and (iii) species densities. To investigate mechanisms linking harvesting disturbance to aquatic birds, I examined (i) community-environment and density-environment relationships, (ii) nest predation, and (iii) foraging patterns.; The composition of aquatic bird communities differed on harvested and unharvested lakes, although limnological variables (except conductivity) did not differ on these lake types. Species richness was higher on harvested lakes, because species that occurred sporadically tended to occur on these lakes. Densities of five of nine focal species (or species groups) differed before and after forest harvesting. However, only pre-/post-harvest decreases in scaup (Aythya affinis, A. marila) densities were maintained for two summers post-harvest.; Lake characteristics, including morphometry and water quality, were more important to communities and densities, than variation in lake characteristics attributable to harvesting. Artificial nests were depredated less in harvested areas than in intact forest, suggesting that harvesting may not negatively affect nest success of cavity-nesting waterfowl, and thereby community composition and bird densities. Finally, foraging behaviour of common loon (Gavia immer) and bufflehead (Bucephala albeola) did not differ between harvested and unharvested lakes.; Differences in community composition between harvested and unharvested lakes may conform to the IDH, although the level of harvesting disturbance necessary to decrease species richness is unknown. Community composition and scaup densities may be useful indicators of disturbance related to forest harvesting, however mechanisms causing post-harvest differences remain obscure. Documented patterns of changes in communities suggest that increased permeability of harvested areas may facilitate colonisation of harvested landscapes by bird species more typical of the prairie parkland region.