The spatial distribution of abundance: Analysis of the geographic range

The distribution of abundance throughout the range was studied in birds with data from the North American Breeding Bird Survey. Intraspecifically, species were rare at most sites where they occurred and were common in only a small proportion of their range. This resulted in a concentration of all individuals of a species in a small portion of the range. The abundance pattern was stable over short to medium time periods. The spatial autocorrelation of abundance was identical for all species, with high positive autocorrelation at short distances and rapidly decreasing correlation at larger distances. Abundance increased with increasing distance from the edge of range. These patterns are consistent with the multiple niche axis model proposed by Brown ten years ago.;In comparing different species, there was overlap in the locations of highest abundance. Species that had higher local abundances had both larger ranges and were absolutely rare in a smaller proportion of their range than species which were rare. This suggests that common species expand their range before increasing abundance within the range. An examination of population variability within the range showed that variability was strongly related to mean population size and that, after this effect was removed, many species tended to be more variable toward range edges than toward range centers.;An analysis of the abundance decline and subsequent recovery of three species after a series of late winters revealed that declines were greatest at sites with highest initial abundance, at sites closer to range edges, and at sites with a more severe winter. Abundance changes were accompanied by contraction and expansion of the range. Evidence was found of extinction and recolonization events that occurred closer to range edges.;All results suggest that abundance within the range deserves further investigation in other taxonomic groups. A major conservation implication is that central, high abundance populations that contain most of the individuals and fluctuate less in time should be protected. Further research needs to be conducted on the exact mechanisms by which local populations are tied together to create the entire range-wide distribution of abundance.