Woodlands fragmented by logging vary in their capacity to support wildlife (Lichstein, Simmons & Franzreb 2002; Betts et al. 2006), depending on biotic and abiotic interactions among adjacent forest and non-forest landscape components (Chen, Franklin & Spies 1995; Harper et al. 2005). Because distribution and fitness of wildlife can be influenced by the pattern of the surrounding landscape, conservation actions in woodland ecosystems should be informed by models predicting the landscape-level distribution and fitness of individuals (Fielding & Haworth 1995; McGarigal & McComb 1995). It is equally important to assess the performance of distribution models outside the training environment (Vaughan & Ormerod 2005; Graf et al. 2006).
The marbled murrelet Brachyramphus marmoratus Gmelin is an Alcid that forages inshore and nests in coniferous old-growth forests of the Pacific north-west of North America (Nelson 1997). Marbled murrelet nests are notoriously difficult to locate because of their occurrence high in old-growth trees and placement in rugged terrain and the birds’ secretive nesting behaviour (Bradley et al. 2004; Peery et al. 2004; Baker et al. 2006). Forest logging along the Pacific coast has produced substantial loss and fragmentation of murrelet nesting habitat (Raphael et al. 2002, 2006). Habitat loss, concurrent with perceived population declines, has precipitated habitat protection and listing of the species as threatened throughout its range, excluding Alaska (Nelson 1997).
Recent studies have focused on quantification of marbled murrelet populations (Bigger et al. 2006) and the impacts of deforestation upon them (Burger 2001; Meyer & Miller 2002; Raphael et al. 2002; Peery et al. 2004). Patch- and stand-level nest site selection has also been addressed (Burger & Bahn 2004; Baker et al. 2006; Zharikov et al. 2006). However, because of the paucity of known nest sites, effects of landscape pattern on the species’ breeding ecology remain poorly understood (Raphael et al. 2002, 2006; Ripple, Nelson & Glenn 2003). It is also not known whether distribution models developed for one nesting population can inform conservation actions elsewhere.
In British Columbia (BC), Canada, current guidelines for the conservation of marbled murrelet nesting habitat stipulate protection of patches of coastal forest structurally suitable for nesting. Suitability is primarily defined by patch-level characteristics (tree age, height, presence of epiphytes) derived from digital forest inventories and aerial surveys. Only tentative consideration is given to the overall landscape pattern (MWALP 2004). Landscape pattern can influence individuals above and beyond the effect of breeding habitat per se (Lichstein, Simmons & Franzreb 2002; Betts et al. 2006) and much of that influence in fragmented forest landscapes is the result of the increased amount and contrast of forest edges (Harper et al. 2005).
This study used the largest available sample of marbled murrelet nests to examine the effects of landscape pattern on the species’ breeding distribution and success in south-western BC. The same data set was used by Zharikov et al. (2006) to investigate the effects of patch size, location and topography on breeding distribution and success in the marbled murrelet. The current work focused on landscape-level effects, in particular of old-growth edge, on the species’ breeding ecology, with the aim of enhancing current conservation guidelines. This study also assessed the spatial transferability of distribution models with an independent data set. The following questions were addressed. (i) Are the choice of a nest site and the outcome of a nesting attempt predicted by the pattern of the surrounding landscape? (ii) How do artificial and natural edges of the old-growth forest influence breeding distribution and success in the species? (iii) How do landscape-level distribution models developed at one site perform elsewhere?