Riverine Barriers

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Science  22 Dec 2000:
Vol. 290, Issue 5500, pp. 2213
DOI: 10.1126/science.290.5500.2213b

Patterns of species distribution and diversity are influenced heavily by geographical barriers. In his accounts of Amazonian biogeography, Alfred Russel Wallace observed that the ranges of closely related primates would sometimes abut at large rivers; nearer the source of a river, the differences between the faunas of opposite banks would diminish. This observation gave rise to the hypothesis that the presence of numerous large rivers was a factor that significantly limited gene flow and contributed to the high levels of general species diversity seen in lowland Amazonia.

More recent primate studies have largely vindicated Wallace's idea. For marsupials and amphibians—and perhaps for diversity in general—the story might be different. Gascon et al. compare species assemblages of these animal groups on opposite banks of the Juruá river in Brazil. If Wallace's hypothesis were to hold, there should have been greater similarity between the faunas of the seasonally flooded forests on opposite banks than between those of the terra firme forests above the flood zone. However, they found no such relationship at any point along the river, suggesting that riverine barriers do not account for the patterns of diversity in marsupials and amphibians, and that other, more cryptic, geomorphological features might have played a significant role. — AMS

Proc. Natl. Acad. Sci. U.S.A.97, 13672 (2000).

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