Temporal turnover of common species in avian assemblages in North America
We examine patterns of temporal turnover of common species in avian assemblages in North America to test the hypothesis that changes in avian diversity structure observed in these assemblages were associated with the colonization of common species. Location
The contiguous United States and southern Canada. Methods
We measured temporal turnover from 1968 to 2003 for 547 avian species at 1673 North American Breeding Bird Survey (BBS) routes. We used the Euclidian distance between expected and observed presence/absence vectors and randomization tests to place species into two categories, common and not-common, and into three categories for common species: (1) always common, (2) common and colonizing, and (3) common and extirpated. We used these categories to identify species experiencing extreme colonization and extirpation events and to examine changes in species composition at BBS routes. We also determined how these patterns were associated with changes in species richness and changes in similarity in species composition. Results
Nine of the 547 species represented outliers, where the number of BBS routes colonized greatly exceeded the number extirpated; no species showed extreme values for extirpation. The nine species colonized BBS routes primarily in the upper Midwest and north-eastern United States. Presence of the nine species at BBS routes was correlated with increasing net gain in common species (difference between common colonized and common extirpated), higher levels of species richness and increasing species richness over time, more similar species compositions and increasing similarity over time, and a greater prevalence of common species over not-common species. The literature indicates that all nine species experienced some form of geographical range expansion during the time of the survey involving four elements: (1) introduction and invasion; (2) the ability to use human-altered environments, including habitats associated with agricultural, suburban, or urban areas; (3) intensive management activities, including habitat improvements and reintroductions and (4) the ability to use habitats formed through forest regeneration. These factors in combination point to anthropogenic activities and related land use histories as the primary drivers of change. One of the nine species colonized regions well outside its historic geographical range and the remaining eight species were native within the regions they colonized. Main conclusions
Our results suggest that a combination of anthropogenic activities promoted, within certain regions of North America, the geographical expansion of a limited number of common species that were native to those regions. These colonization events were correlated with changes in diversity structure, implying that large-scale diversity patterns were being influenced by anthropogenic activities. These changes can be characterized primarily by gains in species richness, an increased prevalence of common species, and more similar species compositions. Thus, using simple large-scale measures of diversity could be problematic if recent biogeographical patterns of species diversity are not considered. Specifically, using species richness or an indicator species to assess diversity could bias assessments towards common species whose populations have recently benefited through anthropogenic activities.