In the computer model developed by the research team of MIT plankton species are dispersed throughout the oceans by currents. Their model assumes, however, that all plankton species are equally strong competitors. Consequently, their model predicts high plankton biodiversity in stable environments, such as tropical oceans, where none of the species can outcompete the others. Their model further assumes that plankton species differ in maximum growth rate. Accordingly, biodiversity in areas with strong seasonal fluctuations, such as the polar seas, is predicted to be low as species with high growth rates rapidly gain dominance in spring. With this model, the MIT researchers claim to have explained the global diversity of plankton from high species richness in the tropics to low species richness in the polar regions.
However, Huisman indicates in his commentary that this study should never have been published in a top journal like Science. He used the same mathematical equations as the MIT team, but instead assumed that species differ in their competitive strength. This led to completely different model predictions. In this case, the strongest species excludes all other species under constant conditions, resulting in low biodiversity in tropical oceans. By contrast, biodiversity is high under fluctuating conditions.
Which model is then correct? In his commentary, Huisman points to a large number of existing studies. In laboratory experiments plankton species are seldom equally strong competitors; in most cases one species is stronger than the other. In addition, previous experiments have shown that fluctuating conditions increase the biodiversity of the plankton. In such dynamic environments, dominance relationships shift continuously among the species, so that no species gain the upper hand.