Print PageThe nested subset pattern is described by the most species-rich site containing all species, while less species-rich sites contain a proper subset of the species observed at all richer sites. The nested subset pattern is of interest because of the generality of the pattern, which has been observed for a variety of regions and taxa. Based on the expectation that the nested subset pattern is produced by extinction and colonization, three assumptions were postulated. These assumptions are that sites must have a single species source pool with a common history, sites must have similar environmental conditions, and there must be a “hierarchical organization of niche relationships”. A further assumption that rare species should be observed at the most species-rich site is implicit in the definition of nestedness, which has caused the nested subset pattern to be used for conservation. Using Breeding Bird Survey (BBS) data from the contiguous United States, we examined nestedness at several scales to test the assumptions of a single species pool and sites with similar environmental conditions. We also examined significantly nested matrices to determine if species were observed at the most species-rich site.
Results/Conclusions
We observed nestedness at all spatial scales, indicating that the assumptions of a single species pool and sites with similar environmental conditions are not necessary for nestedness. We also observed that species with low incidence are not necessarily observed at the most species-rich site, indicating that the pattern does poorly at predicting the occurrence of species with increasing rarity. Because the assumptions of a single species pool and similar sites are not necessary to observe nestedness, the processes thought to cause the nested subset pattern may not cause the nested subset pattern. Further research needs to be conducted in order to understand what processes are important in producing the nested subset pattern.
