Avian Conservation and Ecology (Dec 2005)
Scale and Sensitivity of Songbird Occurrence to Landscape Structure in a Harvested Boreal Forest
Abstract
To explore the spatial scales at which boreal forest birds respond to landscape structure and how those responses are influenced by forest harvest, we quantified the relationship between amounts of forest in the landscape at multiple spatial scales and the occurrence of 11 common boreal songbirds in western Newfoundland. The habitat type was assessed at a local scale (25 m diameter area) and amounts of forest habitat were measured at neighborhood (300 m) and landscape (2500 m) scales. We further compared how these relationships differed, depending on whether the landscape had been harvested or not, i.e., the landscape context. Landscape-scale metrics were related to occurrence for 7 of 11 species. For five of these seven, landscape context was also important. Landscape context was not important in models that did not contain a landscape-scale term. In four of five of the models including landscape context, there was an interaction of the term with either landscape or neighborhood effects, indicating that, not only was there an effect of forest harvest at the broad scale, but that effect altered the response of the species to other metrics. For the majority of species, overall occurrence tended to be higher in natural than in harvested landscapes, especially at higher levels of forest cover. Interestingly, for some species, occurrence was relatively similar across levels of forest cover within harvested, but not natural, landscapes. The results suggest some scale-invariance in species' responses to landscape structure, and that some species respond to landscape structure at scales that are broader than those implied by our current knowledge of territorial or dispersal distances. Collectively, the results also suggest that forest management needs to consider not only how local-scale processes might be influenced by local-scale changes in amounts of forest, but also how the broader scale context might interact with those local-scale changes to produce counter-intuitive results. The complex nature of some of the relationships we observed suggests that generalized management policy for forests and songbirds will be elusive.
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