Ecology of Parapatric Distributions

Parapatry describes a distributional pattern in which pairs of taxa have separate but contiguous distributions, abutting along common boundaries. Smith first proposed the word (81, 122) to differentiate situations where the ranges of two taxa are in contact, both from sympatry where ranges overlap, and from allopatry where ranges are separate. The condition of parapatry was known much earlier. For instance, Darwin (38) wrote that the location of a species border was often determined by the border of a related species. More recently numerous examples of parapatry, from a wide range of taxonomic groups and geographic regions, have been described (53, 61, 64, 71, 82). Parapatry might previously have been more common since con­ tinuous patches of habitat where species ranges could have been in contact have now been fragmented by clearance. It may still be more common than is realized, because parapatry is difficult to detect. Detection usually involves taxonomic separation of closely related, morphologically similar taxa, and collection on a fine geographic scale (71). Parapatry has been most frequently reported in birds (40, 46), perhaps because their high visibility makes fine­ scale surveys relatively easy. Even among birds, detailed collection may reveal parapatry where sympatry has previously been reported (51). Initial discussion of parapatry revolved around speciation mechanisms, and