Dynamic rivers helped the Amazon become rich in bird species

A new study considered the movement of rivers as a cause for the diversity of birds found in the Amazon. The lowland rainforests of the Amazon River basin are the most biologically diverse terrestrial ecosystems in the world. Eighteen percent of all trees found on earth make up this biome, carrying more freshwater than the seven largest river basins combined. Researchers have long wondered the root cause of the Amazon’s rich biodiversity. 

“Early evolutionary biologists like Alfred Russel Wallace noticed that many species of primates and birds differ across opposite riverbanks in the Amazon, and ornithologists now know that rivers are associated – in one way or another – with the origin of many avian species,” said study lead author Lukas Musher.  

“Moreover, accumulating geological evidence has suggested that these rivers are highly dynamic, moving around the South American landscape over relatively short time periods, on the order of thousands or tens of thousands of years.”

The research, led by the American Museum of Natural History, found that changes in small river systems over time spur the evolution of new species. To understand how the movement of rivers influenced the accumulation of birds, the researchers looked at the genes of six species of Amazonian birds.

“Even though birds can fly, our study confirmed that current rivers across the Southern Amazon rainforest, even relatively small ones, are highly effective at isolating populations of these six species, which leads to genomic divergence and ultimately speciation,” said study senior author Joel Cracraft.

However, because these rivers move around the landscape at different time scales, their movements can have varying outcomes for bird species: when rivers rearrange quickly, populations of birds on each side can merge before they’ve had time to differentiate; when river changes happen slowly, species have a longer time to diverge from one another; and when rivers change at intermediate rates, bird populations diverge and then join back together and co-occur when a river moves.

A startling discovery was also found when the researchers came across a previously unknown bird species at high risk of imminent extinction. In addition, birds that were considered a single species were identified as distinct populations that should be described as separate species. 

“Though we know Amazonian biodiversity is unmatched by any other terrestrial ecosystem, we demonstrated that its species richness may be greatly underestimated even in well-studied groups such as birds,” said Musher. 

In particular, the Amazon region is threatened by rapid and ongoing deforestation. Sadly, the diversity of species relying on this region is generally unrecognized. “Many of the distinct populations are relatively young and endemic to a small Amazonian region, meaning that a large portion of the Amazon’s birds may be threatened with loss to imminent extinction.”

The study is published in the journal Science Advances.

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By Katherine Bucko, Earth.com Staff Writer