River Rapids Boost Species Diversity

Research posts

Museum researchers working in the lower Congo River have long been impressed by the remarkable biodiversity of the region—and curious as to what drives it.

“In this very short section of the Congo, we find a tremendous diversity of fishes,” said Melanie Stiassny, Axelrod Research Curator in the Museum’s Department of Ichthyology and an author on the study. “So what is it about this system that makes it such a pump for species?”

 

Two chichlids hover close to one another along the pebbly river bottom, larger rocks on view in the background.

The lower Congo River region is home to many distinct species of cichlid fishes. 

© O. Lucanus


A new genomic study conducted by Stiassny and her colleagues and published this month in the journal Molecular Ecology suggests that the area’s rapids—some of the fastest and most turbulent in the world—are driving fishes in the area to develop into new species more swiftly than expected.

Scientists have long understood that geographic separation is a driver for evolution. Isolating one population of a species—on an island, for instance—can cause it to take a development path distinct from other members. Over the course of many generations, this can lead to the rise of new species. 

The new research shows that geographical isolation is responsible for driving species diversity in the lower Congo as well—just not with traditional barriers, like islands or mountain ranges.

 

Map of the Kasai River shows it's many branches, and images of cichlids are placed on the map to designate their locations.

This detail from a table in the paper shows the distribution of two cichlid species in the region.

© M. Stiassny


“The genetic separation between these fishes show that the rapids are working as strong barriers, keeping them apart,” said lead author Elizabeth Alter, from The City University of New York’s Graduate Center and York College. “What's particularly unique about the lower Congo is that this diversification is happening over extremely small spatial scales, over distances as small as 1.5 kilometers. There is no other river like it.”

There are important conservation implications to this work, as well, since about 25 percent of the fish in the lower Congo are endemic, or only found in this particular location. But the area is currently being proposed as a site for major dam development.                

“Activity like that would majorly interrupt the evolutionary potential of this system,” Stiassny said.