No abstract

For trees, mammals, birds, reptiles, insects, and amphibians, the species richness on mountaintops is generally less than that of lowland areas. Coincident with this decline in species richness with increasing elevation is an increase in the altitudinal range of species. This pattern is analogous to the relationship between the latitudinal range of species and latitude (Rapoport's latitudinal rule). Both of these Rapoport phenomena, the latitudinal and the new elevational rule discussed here, can be explained as being results of differences in the breadth of climatic conditions organisms experience along the geographical gradients. The influence of latitudinal or altitudinal range size on local species richness is poorly understood, but the tendency for range margins to fall in species-rich, rather than species-poor, areas may mean that species-rich communities contain many species that are maintained only through immigration. The presence of these persistent but locally non-self-maintaining populations may explain the increased number of species found in rich communities as compared to species-poor communities without the need to invoke other differences in local species interactions.

To quantify the magnitude and pattern of spatial variation in local population density within a single species, we analyzed large numbers of samples, representing a large geographic area or a wide range of ecological conditions. Our analyses focused on, but were not limited to, censuses 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 where it occurred, but was orders of magnitude more abundant in a few "hot spots." The highly clumped frequency distributions of intraspecific abundance among sample sites resemble distributions, such as the negative binomial, canonical lognormal, and broken stick, that have been used to characterize the distribution of abundances among species within local ecological communities. We hypothesize that the spatial variation in abundance largely reflects the extent to which local sites satisfy the niche requirements of a species. Several results are consistent with this hypothesis. First, a computer simulation model in which abundance is determined by the multiplicative combination of several independent environmental variables produces ranked distributions of abundances similar to those observed empirically. Second, geographic patterns of abundance of bird species have been relatively stable over several decades, indicating that different abundances are associated with particular places on the landscape. Third, the abundance of bird species varies in a systematic way over the geographic range, exhibiting positive spatial autocorrelation at small distances and a tendency to increase from the edges toward the center of the range. The magnitude and pattern of spatial variation in local population density has important implications for basic ecology and biogeography, especially for the dynamics and regulation of abundance on both space and time, the limits and internal structure of the geographic range, and the interspecific variation in abundance observed within local communities. Patterns of spatial and temporal variation in abundance should be considered in the design of nature reserves and the conservation of biological diversity.

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