SPATIAL VARIATION IN ABUNDANCE

To quantify the magnitude and pattern of spatial variation in local po pulation density within a single species, we analyzed large numbers of samples, representing a large geographic area or a wide range of ecol ogical conditions. Our analyses focused on, but were not limited to, c ensuses of birds recorded in the North American Breeding Bird Survey. Birds and other organisms exhibited a common pattern: each species was represented by only a few individuals in most of the sample sites whe re it occurred, but was orders of magnitude more abundant in a few ''h ot spots.'' The highly clumped frequency distributions of intraspecifi c abundance among sample sites resemble distributions, such as the neg ative binomial, canonical lognormal, and broken stick, that have been used to characterize the distribution of abundances among species with in local ecological communities. We hypothesize that the spatial varia tion in abundance largely reflects the extent to which local sites sat isfy the niche requirements of a species. Several results are consiste nt with this hypothesis. First, a computer simulation model in which a bundance is determined by the multiplicative combination of several in dependent environmental variables produces ranked distributions of abu ndances similar to those observed empirically. Second, geographic patt erns of abundance of bird species have been relatively stable over sev eral decades, indicating that different abundances are associated with particular places on the landscape. Third, the abundance of bird spec ies varies in a systematic way over the geographic range, exhibiting p ositive spatial autocorrelation at small distances and a tendency to i ncrease from the edges toward the center of the range. The magnitude a nd pattern of spatial variation in local population density has import ant implications for basic ecology and biogeography, especially for th e dynamics and regulation of abundance in both space and time, the lim its and internal structure of the geographic range, and the interspeci fic variation in abundance observed within local communities. Patterns of spatial and temporal variation in abundance should be considered i n the design of nature reserves and the conservation of biological div ersity.