River Network Rearrangements in Amazonia Shake Biogeography and Civil Security

Abstract

The scene for regional biogeography and human settlements in Central Amazonia is set by the river network, which presumably consolidated in the Pliocene. However, we present geomorphological and sediment chronological data showing that the river network has been anything but stable. Even during the last 50 kyr, the tributary relationships have repeatedly changed for four major rivers, together corresponding to one third of the discharge of the Amazon. The latest major river capture event converted the Japurá from a tributary of the Rio Negro to a tributary of the Amazon only 1000 years ago. Such broad-scale lability implies that rivers cannot have been as efficient biogeographical dispersal barriers as has generally been assumed, but that their effects on human societies can have been even more profound. Climate change and deforestation scenarios predict increasing water levels during peak floods, which will likely increase the risk of future river avulsions. This may have disastrous consequences for the local human societies, especially in those areas where the current floodplains are at only marginally lower elevations than the nearest water divide. We suggest that the prevailing paradigm of rivers as principal structuring elements of Amazonian biogeography needs to be re-evaluated, and that land use planning and civil risk assessment should take the possibility of river avulsions into account.

Hidden in the DNA: How multiple historical processes and natural history traits shaped patterns of cryptic diversity in an Amazon leaf‐litter lizardLoxopholis osvaldoi(Squamata: Gymnophthalmidae)

“…Major changes in the Madeira river course were already elucidated (e.g. Rossetti et al, ), and new data reveal more dramatic changes, supporting the hypothesis that the Madeira and Purus rivers were connected between 13 and 25 Kyr (Ruokolainen et al, ). Due to erosion, evidence of avulsion events older than 50 Kyr in Central Amazonia may be unavailable.…”

“…Despite a possible scenario of turbulent drainage dynamics as predicted by Ruokolainen et al (), the restricted distribution of lineage Am on the northern bank of Solimões–Madeira River indicates some stability in the course of this river in Central Amazonia since at least the Pliocene, when the Am lineage diverged from its sister group Bm. Considering that the evidence of rearrangement in paleo‐courses of Amazon tributaries is still limited to tens of thousands of years (Ruokolainen et al, ), testing the above hypothesis for species complexes, which have originated during the Miocene–Pliocene, is a great challenge for Amazonian biogeographers.…”

“…Due to erosion, evidence of avulsion events older than 50 Kyr in Central Amazonia may be unavailable. However, a similar dynamics of drainage capture could have occurred at different times, provoked by sporadic floods since the Miocene–Pliocene (Ruokolainen et al, ). Considering the small differences in elevation that separates most of the drainage basins in the Amazonia, we argue that river capture caused by floods could have led to deep changes in the landscape, and carry with them small animals living in the forest floor, favouring passive dispersal, isolation and dramatically altering their distributions.…”

“…osvaldoi lineages originated between 10 and 2.5 Mya, which ca. 100 times older than the Madeira River avulsion (Ruokolainen et al, ). In this context, we hypothesize that lineage distribution within this species may reflect changes of paleo‐river courses dating back to the Miocene and Pliocene, and that those spatial patterns have remained until today due to the poor dispersion capacity of the species.…”

“…Additionally, new lines of evidence suggest that river configuration within the Amazon Basin was far more dynamic in the Neogene than previously thought (e.g. Hoorn et al, ;Hoorn et al, ;Rossetti et al, ;Ruokolainen, Moulatlet, Zuquim, Hoorn, & Tuomisto, ), potentially mixing up populations from opposite banks over thousands of generations.…”