Flowing waters are richer in species

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Still waters may run deep, but they certainly do not promote biodiversity, according to research to be published in a forthcoming issue of the journal Ecology Letters.

James Palardy and Jon Witman of Brown University found that the stronger the water flow in an area, the greater the biodiversity of benthic marine communities in three geographically disparate regions (Palau, Maine and Alaska).  

The objective of the study was to test the following two hypotheses:
(1) increased water flow velocities lead to increased local species density and species richness and (2) water flow generates increased species richness by promoting the increased recruitment of rare species.

The authors first surveyed benthic invertebrate communities on vertical walls at six sites in Palau and four sites in Alaska, measuring the flow rates at the surveyed sites at the same time. 

They then experimentally manipulated water flow at sites in Alaska and Maine by deploying flow enhancers. The flow enhancers were essentially metal rectangular channels fitted with settlement plates.The flow enhancers narrowed in the middle and were shaped like a bow tie. This forced water flowing through it to speed up by simple physics. 

A series of control setups were also installed, consisting of channels that were of uniform width throughout. The experimental units were periodically surveyed until their removal after 20 weeks, with the exception of one site in Alaska, where the experiment was allowed to continue for two years.

The results of the study showed that the assembly and maintenance of diversity within the benthic invertebrate communities is strongly affected by flow. In all cases studied, the communities exposed to higher water flow had greater species densities and richness. The experiments also showed that areas with greater water flow had a higher proportion of rare species.

According to the authors, the reason for the increased species richness with greater water flow is deceptively simple: a stronger current would increase the number of species whose larvae would land and settle on a particular area, much like how some plants would use the wind to disperse their seeds.

"It totally blew us a way that we got almost identical results in two marine regions of the world separated by 4,000 miles with completely different regional diversities, and no species shared in common," co-author Jon Witman said. "It's a wake-up call saying that water flow is a really strong predictor of how many species are present in a particular area of the ocean."

For more information, see the paper: Palardy, JE and JD Witman (2010) Water flow drives biodiversity by mediating rarity in marine benthic communities. Ecology Letters DOI: 10.1111/j.1461-0248.2010.01555.x