How do plankton species coexist in an apparently unstructured environment?

In a paper entitled The paradox of the plankton, Hutchinson asked 'how it is possible for a number of species to coexist in a relatively isotropic or unstructured environment all competing for the same sorts of materials' (Hutchinson 1961 Am. Nat. 95, 137-145 (doi:10.1086/282171)). Particularly relevant for phytoplankton, this paradox was based on two implicit, and perhaps naive, postulates, i.e. (i) that all plankton species have similar requirements and (ii) that the marine environment is relatively homogeneous in space and time. A number of hypotheses, based on purely theoretical or experimental studies, have been proposed to solve this conundrum, ranging from spatio-temporal environmental heterogeneity to biotic chaotic variability. Here, we characterize the ecological niche of 117 plankton species belonging to three different taxonomic groups and show that all species have a niche sufficiently distinct to ensure coexistence in a structured marine environment. We also provide evidence that pelagic habitats are, unsurprisingly, more diverse in space and time than Hutchinson imagined, the marine environment being neither unstructured nor stable in space and time. We, therefore, conclude that the niche theory, and its corollary the principle of competitive exclusion, apply as much for the plankton as for other forms of life, be they terrestrial or marine.