Population dynamics, range size, and the risk of extinction

The abundance of a species through time and across space, and the variability of that abundance, determines the species' geographic range. I explore the population dynamics of bird species across their ranges to determine: (1) the temporal and spatial dynamics of populations; (2) differences and similarities in the dynamics of rare and common species; and, (3) the dynamic relationship between abundance and range-size when either is decreasing. I then apply these results to an analysis of the entire range of the Endangered Cape Sable seaside sparrow. Finally, I investigate the most restricted populations--those of island-bound endemic species. For each of these investigations I analyzed data that represented species' entire geographic ranges. My analyses show that, for common species, geographic ranges consist of high-abundance core populations that exhibit relatively little variation over time, and peripheral, low-abundance populations that are highly variable. Conversely, geographically restricted species exhibit a high degree of range turnover, such that their highest abundances can shift throughout their ranges over time. Declines in either geographic range or mean abundance across a species' range are not always synchronous or even of the same sign, contrary to predictions of the general positive relationship between range-size and abundance. The Cape Sable sparrow population currently consists of one highly productive area of no more than 400 km...