Diversity patterns of bird assemblages in a post-glacial landscape of northern Wisconsin, USA

Background/Question/Methods

Virtually all species inhabiting the Great Lakes region of North America today have colonized the landscape following the last Laurentian glaciation, approximately 12,000 yr BP. Compared with unglaciated regions of North America, species diversities of most taxa in this region are low. Breeding birds, however, are an exception, thanks in part to their ability to migrate seasonally. In northern Wisconsin, approximately 80% of the breeding bird species are migratory, nearly half of which winter in Central or South America. Migration partly liberates birds from the constraints of severe winters, enabling species assemblages to exploit resources and ecological opportunities that are not effectively used by permanent residents.  This notion predicts that migratory species should represent a wider range of taxa and functional traits than year-round residents.  In this paper we compare species diversity, functional diversity, and phylogenetic diversity of breeding bird assemblages in 10 major habitat types in the eastern Chequamegon-Nicolet National Forest of northern Wisconsin, where we have led bird monitoring projects for more than 25 years. Our analysis is based on results from standardized counts at 312 permanent points representing the major habitat categories used by forest managers to administer harvesting programs and conservation strategies. 

Results/Conclusions

The most widespread habitat type in our study area, northern hardwood forest, exhibited the lowest average diversity of bird species, families, orders, and functional feeding types. Among closed canopy forest types, lowland conifers were consistently more diverse in all of the bird diversity metrics. Lowland hardwood forest and upland forests dominated by native conifers also supported relatively high diversities of breeding birds. Highest diversities overall were characteristic of open habitats such as sedge meadows and lowland shrubs, perhaps in part due to the greater detectability of birds in point counts. Small towns in the forested landscape also yielded relatively high breeding bird diversities in all dimensions (species, phylogenetic, and functional diversity). Multivariate analysis of bird point counts revealed species groupings that differed significantly from groupings associated with traditional forest classification systems. We conclude that bird assemblages in this region are not tightly associated with dominant canopy tree species, which themselves exist in a heterogeneous mosaic.  Conservation of breeding bird species in this post-glacial landscape therefore requires attention to habitat attributes that are not directly associated with management of forest trees.