The Mechanisms of Coexistence and Competitive Exclusion in Complex Plankton Ecosystem Models

The biodiversity of plankton ecosystems may no longer be a paradox, but the mechanisms that determine coexistence of explicit competitors in ecosystems remain a mystery. This is particularly so in ecosystem models, where competitive exclusion remains the dominant process. Climate and fisheries models require plankton ecosystem sub-models that maintain competing plankton functional types extant, but coexistence can be reproduced in only a few ‘just so’ theoretical models. This limits our ability to predict the impacts of climate change and fisheries on ocean biota. We consider ecosystems of Kolmogorov form that conserve mass (CK systems). These systems describe a general class of ecosystem models that includes many theoretical and applied models. We develop heuristics that illuminate the key mechanisms that allow the coexistence of explicit competitors in these systems. These heuristics facilitate the identification of a large class of models with the structural property that all species coexist for all time. Our approach unifies many theoretical and applied models in a common biogeochemical framework, providing a powerful tool with the potential to generate new insights into the properties of complex ecosystems.