Testing the Wallace’s riverine barrier hypothesis based on frog and Squamata reptile assemblages from a tributary of the lower Amazon River

Abstract

ABSTRACT The classic Wallace’s riverine barrier hypothesis (RBH) has been widely invoked to explain species distribution and evolutionary divergence throughout Amazonia. Dispersal and gene flow limited by the inability to cross a river restricts geographic distribution to an interfluvial zone, which causes different assemblage compositions between opposite river banks. In this study, we sampled frogs and Squamata reptiles along the Jari River, a tributary of the lower Amazon River, to test the RBH at assemblage level. We used non-metric multidimensional scaling based on species abundance to compare assemblage composition between river banks. We found that 63.6% of the frogs sampled are absent or occur in low abundances on one of the river banks, which resulted in distinct assemblages between banks. This finding supported the barrier-effect as a predictor of the regional frog diversity, which is remarkable, since the Jari River is considerably narrower than all the Amazonian rivers for which RHB has been supported. However, although some reptile species were restricted to one of the river banks, assemblage composition did not differ between banks. This finding suggests that the barrier-effect should not be extrapolated to other groups of organisms, since good dispersers can homogenize assemblages between river banks.