Most species are not limited by an Amazonian river postulated to be a border between endemism areas

Abstract

At broad scales in the Amazon, it is often hypothesized that species distributions are limited by geographical barriers, such as large rivers (river-barrier hypothesis). This hypothesis has been used to explain the spatial-distribution limits of species and to indicate endemism areas for several phylogenetic lineages. We tested the ability of the river-barrier hypothesis to explain patterns of species diversity and spatial-distribution limits for 1952 easily-detected species in 14 taxonomic groups that occur around the Madeira River, and our results indicate that the hypothesis that the Madeira River is the border between endemism areas and explains much of the diversity found in the region is inappropriate for >99% of species. This indicates that alternative hypotheses should be proposed to explain the limits of distributions of species around the Madeira River, as well as a revision of the criteria that are used to determine species-endemism areas.

Figure 1

Estimates of the proportion of species with detectability >50% in each taxonomic or functional group that had their distributions limited by the Madeira River (Dark gray). Light-gray bars show the proportion of species for which the Madeira River was not a geographic barrier. Numbers in parentheses denote the number of species in each taxonomic or functional group.

Figure 2

Evidence suggesting that the Madeira River could have functioned as a vicariance barrier for Callicebus brunneus and Callicebus dubius. (a) Phylogenetic hypothesis of small, large and non-flying mammals (72 spp); (b) Vicariance hypothesis; and (c) Species distributions along the Madeira River; black squares represent known occurrence of C. brunneus, and gray squares represent known occurrence of C. dubius; the black solid line represents the Madeira River; the red solid line represents the Madre de Dios River in Bolivia and the dashed line represents the Amazon River. See Supplementary Fig. S7 for detailed phylogenetic hypotheses associated with species distributions along Madeira River (right or left bank of the river). Map generated using QGIS v2.18 (http://www.qgis.org).

Figure 3

Evidence suggesting that the Madeira River could have functioned as a vicariance barrier for Psophia viridis and Psophia leucoptera. (a) Phylogenetic hypothesis of Aves (446 spp); (b) Vicariance hypothesis; and (c) Species distributions along the Madeira River; black squares represent known occurrence of P. viridis and gray squares represent known occurrence of P. leucoptera; the black solid line represents the Madeira River; red solid line represents the Madre de Dios River in Bolivia; and the dashed line represents the Amazon River. See Supplementary Fig. S8 for detailed phylogenetic hypotheses associated with species distributions along the Madeira River (right or left bank of the river). Map generated using QGIS v2.18 (http://www.qgis.org).

Figure 4

Location of study area (maps generated using QGIS v2.18, http://www.qgis.org). (a) Section of the river investigated (red square); and (b) Location of sample grids (black dots) along the Madeira River (see sample-grid details in Fig. S8).