MODELING THE EVOLUTION OF AQUATIC ORGANISMS IN DYNAMIC RIVER BASINS

Rivers are conduits for aquatic organisms and host an exceptional number of species. However, over geologic time, rivers and the aquatic organisms that live in them are subject to changes in topography that can alter where rivers flow. Differences in erosion rates across drainage divides cause some river basins to grow and others to shrink. Occasionally, rivers are abruptly rerouted by river captures that create new dispersal corridors and barriers for aquatic organisms. These changes in habitat connectivity can lead to the evolution of new species, which has prompted suggestions that river captures may produce high biodiversity in river networks. We test this hypothesis using a computational approach. We couple a landscape model that simulates river basin reorganization with a macro-evolutionary model that simulates the dispersal, speciation, and extinction of organisms throughout a river network. We first show that the distribution of drainage area among river basins exerts a primary control on species richness. Species richness is higher in landscapes that are subdivided equally into a larger number of separate river reaches or river basins. Next, we describe how river capture affects the evolution of aquatic organisms. Following river capture, our model produces a temporary increase in species richness. In landscapes with frequent river capture, such as active orogens, rates of speciation and extinction are both elevated, but the speciation rate increases more than the extinction rate, yielding a positive diversification rate. River captures have a greater effect on diversification rates of aquatic organisms that speciate slowly or have more restricted dispersal ability. Although our results suggest that river capture increases aquatic species richness only transiently, the record of elevated speciation and extinction rates may be a more persistent evolutionary marker of river basin reorganization.