Multiscale impacts of forest degradation through browsing by hyperabundant moose (Alces alces) on songbird assemblages

Abstract

AbstractAimSongbirds are sensitive to changes in forest structure and composition at various spatial scales, particularly during the breeding season. Habitat degradation associated with herbivore browsing may contribute to declines in songbird populations. Here, we evaluate songbird responses to herbivore‐induced habitat change at multiple spatial scales.LocationIn Gros Morne National Park (GMNP), Newfoundland, Canada, browsing by hyperabundant moose (Alces alces) has changed forest structure by reducing understorey cover and converting regenerating stands to open areas dominated by grasses and shrubs.MethodsWe conducted point count surveys to measure bird occurrence throughout GMNP during the 2010 breeding season. Using vegetation information from ground plots and remote sensing, we characterized habitat at three scales: local, neighbourhood and landscape. Following a two‐step procedure to model species occurrence with habitat, the most important habitat factors within each scale were retained for cross‐scale modelling.ResultsCross‐scale models revealed patterns in the association of songbird habitat assemblages with moose‐altered habitats. Early successional species such as mourning warbler (Geothlypis philadelphia) were positively associated with moose‐browsed habitat at the landscape scale. Forest interior specialist (e.g. black‐throated green warbler, Setophaga virens) and generalist species (e.g. boreal chickadee, Poecile hudsonicus) were negatively associated with moose‐browsed habitat at the neighbourhood scale. Local songbird species richness was independent of moose‐browsed habitat at any scale.Main conclusionsThe influence of intense browsing on forest songbirds varies by species but has the potential to extend beyond the area of immediate impact. Continued intense browsing and resulting forest alteration could cause declines in forest specialists and generalists, but may increase populations of early successional species. To maintain bird assemblages characteristic of the region, we recommend management actions that lower moose density in areas with hyperabundant populations such as GMNP to maintain forest structure and regeneration comparable to that present prior to the introduction of moose.