Amazonian rivers don’t often drive the creation of new species, but do help maintain distinct populations, according to a study published in the journal Science Advances last month.
Amazonian rivers create stark barriers through the landscape and form the edges of many species’ range. But a large-scale genetic analysis in the Rio Negro basin suggests that while rivers have not played a direct role in giving rise to most new bird species, they may be crucial to maintaining them.
The Guiana Shield is a region of the South American tectonic plate that includes Guyana, Suriname, French Guiana, Venezuela and parts of Colombia and Brazil, which together harbor high levels of biodiversity, including over 250 endemic bird species. Of these, 86 pairs have range boundaries that meet but never overlap, many of them coinciding with one of two rivers: the Rio Negro and the Rio Branco.
Using the most comprehensive genetic and geographical database compiled for any Amazonian river basin to date, Luciano Naka and Robb Brumfield, both from Louisiana State University in Baton Rouge, Louisiana, determined that the majority of pairs were unlikely to have speciated as a direct result of the formation of the two rivers. Most species evolved before the rivers formed, and did not show the pattern of evolutionary relationships expected of species separated suddenly by a physical barrier. But once formed, rivers act as significant barriers that isolate species, a finding that may also apply to man-made barriers such as roads.
“This is a major advance towards understanding the role of rivers as agents of speciation in Amazonia,” says Angelo Capparella, a zoologist at Illinois State University in Normal, Illinois, who was not involved in the study. Previous research has investigated this phenomenon for one or a few species, but large-scale studies have been hampered by a lack of specimens; it took Naka, who is also affiliated with the Federal University of Pernambuco in Recife, Brazil, over a decade to collect the necessary data for birds in the Rio Negro Basin. “The combination of [the] large sample size of paired taxa with the latest in phylogenetic methods … makes for this being a robust study,” says Capparella.
For many species of birds, mammals and invertebrates, Amazonian rivers mark the start and end of their range. The pattern is so noticeable, renowned naturalist Alfred Wallace wrote about the phenomenon in his 1852 article, “On the Monkeys of the Amazon”: “the native hunters … always cross over the river when they want to procure particular animals, which are found even on the river’s bank on one side, but never by any chance on the other.”
To understand how this pattern arose, Naka and Brumfield used mitochondrial DNA sequence data from specimens supplied by U.S. and Brazilian ornithological collections to estimate the likely divergence times for 74 pairs of bird species that replace one another along the stretches of the Rio Branco and Rio Negro. They found that the species arose between 240,000 and 8 million years ago. “The ages of each pair span through millions of years,” says Naka, making it very unlikely that a single event could be responsible for separating all pairs.
Next, the authors reconstructed the birds’ evolutionary tree to determine patterns of relatedness between pairs. If two species are created by the formation of a river, or other physical barrier, then they would be likely to be each other’s closest relatives, known as “sister taxa.” But this is not the case for birds living in the Guiana Shield. “There are other Amazonian populations that are more closely related to either one of the two taxa studied than its cross-river replacement,” Naka says, indicating that the species probably evolved elsewhere and later met at the river barrier.
Further support for this interpretation came from published geomorphological data for the area, which indicates that both rivers are in fact much younger than most of the species included in the study. The Rio Negro formed just 1.2 million years ago, and the Rio Branco is younger still, with evidence that its course changed from northeast to south, reaching its current route through the Guiana Shield just 19,000 years ago, making them far too young to have been the driving factor in the evolution of most bird species pairs.
However, 12 taxa pairs that are separated by the upper Rio Negro had diverged around the same time that the river was formed, making it possible that they speciated as a direct result of its formation. “We cannot rule out their common origin,” Naka says.
Amazonian rivers, they conclude, can play two distinct roles in species evolution: their formation may separate populations and create new species directly, and their presence can prevent hybridization or competition between related species that evolved independently and meet at the river.
In his 1852 paper, Wallace hypothesized that the width of a river would determine the extent to which species differed on either side. Naka says their results show some tentative support for this hypothesis. “The Rio Negro is a good example showing how the lower and very wide reaches of the river is a barrier for about 80 pairs of avian taxa, and the upper and much narrower reaches only bound the distribution of about 20 pairs,” he says. However, the Rio Branco bucks this trend: it is just 2 kilometers (1.2 miles) wide but acts as a barrier to 40 pairs of birds and several species of primates.
Understanding how the size of a barrier influences its ability to isolate populations genetically will have major implications for how conservationists try to mitigate the effects of habitat fragmentation caused by human activities. “Features such as major roads … could be fragmenting bird populations to the point that it will either unleash a new wave of speciation or, more likely, force populations into remnant habitat that is too small for long-term persistence,” Capparella says.
This type of study can underpin conservation approaches known as Conserving Nature’s Stage (CNS) that take into account the physical factors that generate and maintain biodiversity when designing nature reserves that will offer a habitat environment in the long term.
Banner image of the Rio Branco by Thiago Orsi Laranjeiras.
Citation: L. N. Naka, R. T. Brumfield, The dual role of Amazonian rivers in the generation and maintenance of avian diversity. Sci. Adv. 4, eaar8575 (2018).