Cross-scale interaction effects of socio-ecological conditions on forest bird abundance in the eastern United States.

Hierarchy theory explains that processes act at multiple spatial scales, and that broad-scale processes can influence fine-scale patterns. Many of these hierarchical processes are human-driven, and as anthropogenic pressure increases on natural systems, there is increasing urgency to assess such influences on biodiversity. I investigated the cross-scale interaction (CSI) effects of broad-scale climate and landscape-scale socioeconomic and ecological variables on forest bird abundance in the eastern United States. There were three distinct projects: Project I investigated effects of CSIs and within-scale interactions (WSIs) involving climate (regional and landscape breeding-season temperature) and landscape socioeconomics (percent female, percent college-educated, median age, and median income) on eight species’ abundances; Project II investigated effects of CSIs involving sub-regional climate (breeding- and pre-breeding season temperature and precipitation) and landscape forest patch size and percent exurban cover on eight species’ abundances; and Project III investigated effects of CSIs involving sub-regional climate (breeding-season temperature and precipitation) and landscape agriculture cover and clumpiness on five species’ abundances. For each project, I used Akaike’s Information Criterion to identify the best-supported model in a set of a priori competing negative binomial regression models. There were informative CSIs or WSIs in 10 of the 32 models in Project I, and informative CSIs in 9 of the 16 models in Project II and 4 of the 5 models in Project III. In some, but not in most models, CSIs were more influential than were the additive influence (main effects) of the interacting variables, similar WSIs, and other environmental variables considered. I did not find clear differences in CSI or WSI effects between forest-interior and forest-edge species, or between Neotropical-wintering and Nearctic-wintering species. In all projects, I found species-specific responses to CSIs. Responses of forest birds to CSIs were different from what would be expected from additive influences of the interacting variables, and therefore ignoring CSI influences may be misleading. These findings underscore the need to incorporate CSIs into analyses designed to inform conservation. Understanding how CSIs modify the effects of variables at different spatial scales may be crucial for the effective management of biodiversity in the face of limited conservation resources.