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 km² in extent. My co-authors and I investigate demographic and natural environmental factors that affect the sparrow's population dynamics. We then outline critical steps that must be taken to ensure the sparrow's recovery to its former range. For my analyses of island endemics, I applied island biogeography theory to Pacific island data to estimate the number of rails, pigeons and parrots that existed in this region before human colonization. I estimate that over 230 species of these taxa once existed, of which only half are accountable in the fossil record. The preservation of extant island endemics, and indeed of any species with a restricted range, will depend on a complete understanding of range-wide population dynamics. My results provide insight into these dynamics and suggest areas of research that need to be followed.