Dynamic rivers contribute to Amazon's rich bird diversity

One of the most contentious questions in evolutionary biology is: how did the Amazon become so rich in species? A new study focused on birds examines how the movements of rivers in the Amazon have contributed to the area’s exceptional biological diversity.

The U.S. National Science Foundation-funded team, led by American Museum of Natural History scientists, found that as small river systems change over time, they spur the evolution of new species. The findings also reveal that previously unknown bird species found only in the small areas next to these dynamic river systems are at high risk of imminent extinction. The study is detailed in the journal Science Advances.

The lowland rainforest of the Amazon River basin harbors more diversity than any other terrestrial ecosystem on the planet. It is also a globally important biome containing about 18% of all trees on Earth and carrying more fresh water than the next seven largest river basins combined. Researchers have long wondered how the Amazon's rich biodiversity arose and accumulated.

"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 the study's lead author, Lukas Musher at Drexel University's Academy of Natural Sciences. "Geological evidence has suggested that these rivers are dynamic, moving around the South American landscape over relatively short time periods, on the order of thousands or tens of thousands of years."

To investigate how the movement of rivers has influenced bird species in the Amazon, the researchers sequenced the genomes of six species of Amazonian birds.

"Our study confirmed that rivers across the Southern Amazon rainforest, even relatively small ones, are effective at isolating populations of six species, which leads to genomic divergence and ultimately speciation," said the study's senior author, Joel Cracraft.

Because the rivers move around on different time scales, they can have varying effects on bird species. When river rearrangements occur 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.

The scientists also identified distinct populations of birds that should be described as separate species but have been considered a single species until now.