Waterfalls promote freshwater biodiversity in rivers, new study

A new study by BioFresh members shows that waterfalls may promote biodiversity creation in river sub-drainages by acting as natural barriers to migration over an evolutionary time-frame.

Angel Falls, Venezuela. The tallest waterfall in the world is located within the Orinoco river basin, where the study was conducted. Photo: Creative Commons

A recent scientific paper by BioFresh colleagues at the Muséum National d’Histoire Naturelle in Paris and France’s interdisciplinary Institute of Research for Development (IRD) makes an important contribution to the science of freshwater ecology and biodiversity. Their study on the effect of natural habitat fragmentation in river systems, which was conducted in the Orinoco river basin in South America, found that highly fragmented sub-drainage ecosystems have higher neo-endemic species richness. This suggests that natural habitat fragmentation caused by waterfalls drives speciation for freshwater fish in sub-drainages.

The Orinoco river basin is located in Venezuela and Columbia. Photo: Creative Commons.

Speciation is a key evolutionary process that generates different species. The paper shows the role that natural fragmentation in river systems plays in this process, and therefore in promoting freshwater biodiversity, but it also highlights the importance of river sub-drainages as areas of neo-endemic species richness. This has important implications for conservation planning and the protection of biodiversity, which is central to the Aichi Biodiversity Targets under the Convention for Biological Diversity (CBD) and the EU’s biodiversity strategy to 2020. Investments in biodiversity informatics – the assembly of larger data sets by projects such as GBIF and BioFresh – are key to achieving these goals by assisting effective conservation planning through improving the data and knowledge about freshwater biodiversity, notable hotspots, and endemic areas.

Without such datasets, which are able to provide researchers and policy-makers with bigger and better data, many important studies, such as this one, would not be possible. By using a published dataset of fish species occurrence from the Orinoco drainage basin the researchers were able to highlight the evolutionary role that natural habitat fragmentation plays in the promotion of freshwater biodiversity, as well as drawing attention to sub-drainage basins as important areas of neo-endemic species richness.

Thierry Oberdorff, BioFresh member and co-author of the paper, explains that they “found that the number of waterfalls within a sub-drainage positively influences the number of neo-endemic fish species inhabiting this sub-drainage.” He adds, “this result leads us to make the hypothesis that habitat fragmentation generated by natural waterfalls drives speciation by promoting and maintaining (in the long-term) population divergence (barrier to gene flow).”

In short, both history and natural fragmentation (i.e. waterfalls) play important roles as biogeographic barriers that promote freshwater biodiversity in river drainage basins.

Orinoco river. Photo: Creative Commons.

But habitat fragmentation is also widely known to lead to extinctions. So a question that may arise is if natural fragmentation of freshwater habitats can lead to the creation of species, can human-induced habitat fragmentation, such as that caused by dams, also have the same effect?

In fact, the opposite is actually true. This is because speciation and extinction processes for fish act at very different time scales. As Oberdorff explains, “while speciation is a rather slow process (generally of the order of hundred thousand years – million years), the process of extinction can be much faster (hundred of years). As all anthropogenic barriers are usually ephemeral (around 200 years for dams) the only contemporary process acting is extinction.”

This is one of the reasons why the construction of large-scale dams (such as the Xayaburi mega-dam in Laos) are of such concern for freshwater biodiversity scientists and conservationists, because the fragmentation of river ecosystems (such as the Mekong) caused by damming may lead to the extinction of freshwater species crucial for ecosystem functioning upon which millions of people rely.

For more information see the full paper: Dias M et al. 2012, ‘Natural fragmentation in river networks as a driver of speciation in freshwater fish’, Ecography, vol. 35, pp. 001-007.