Changes in avian diversity structure in North America: Patterns, causes, and implications

The aim of this research was to examine the major patterns of change in avian diversity structure over space and time in North America and to determine the primary participants and underlying drivers. I examined 1,673 North American Breeding Bird Survey (BBS) routes containing 547 terrestrial and aquatic species. The general analysis indicated the presence of one dominate pattern: a significant increase in species richness at BBS routes (12% increase from 1968 to 2003). I tested the hypothesis that common species, through the influence of anthropogenic activities, were associated with these patterns. I used the Euclidian distance between expected and observed presence/absence vectors and randomization tests to estimate commonality through patterns of temporal turnover. I also examined patterns of change in geographic range structure for avian assemblages at BBS routes to test the hypothesis that the spatial distribution of species richness was being partitioned between regions dominated by common species and regions dominated by rare species related, respectively, to the presence or absence of anthropogenic activities. Nine common species showed evidence of geographic expansion in the upper Midwest and northeastern United States. Eight of these species were native to these regions. Presence of the nine species at BBS routes was correlated with increasing species richness and a greater prevalence of common species over not-common species. The literature indicated all nine species experienced geographic range expansion during the time of the survey and suggested anthropogenic activities and related land use histories were the primary drivers. The analysis of geographic range structure indicated common species were becoming more common through geographic range expansion and more abundant within avian assemblages primarily at the expense of moderately rare species. These results suggest that 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. Overall, it appears human activities have simplified geographic patterns of biological diversity by promoting the geographic expansion and colonization of native common species.