Ignacio Rodriguez-Iturbe of Princeton University, and colleagues built a computer model of the entire Mississippi-Missouri river basin, which stretches across about half of the US. Without knowing details of fish ecology, the team were able to create a realistic map of fish species diversity.

The simulations show how the flow through the various branches of the waterway plays a major role in determining the species richness. For example, in moving through one sub-basin in Louisiana, the models estimates of the fish diversity at various distances from the river's outlet matched actual biodiversity measurements remarkably close.

Rodriguez-Iturbe and another set of colleagues then used a similar but slightly modified river model to predict how cholera would spread through waterways, and tested it against data from a two-year-long outbreak of the disease in South Africa.

Once again the scheme was remarkably good at predicting actual cholera outbreaks. The structure of the network determines how the disease spreads, often in a patchy way, similar to how fish species spread or migrate along rivers.

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