ECOLOGICAL STOICHIOMETRY, PRIMARY PRODUCER–DECOMPOSER INTERACTIONS, AND ECOSYSTEM PERSISTENCE

Primary producers and decomposers—the two most important groups for the functioning of ecosystems—have complex, indirect interactions. They are indirect mutualists through nutrient cycling, but also competitors for inorganic nutrients due to stoichiometric constraints in decomposers. We examine the conditions under which they are able to coexist, and hence ecosystems are able to persist, using a stoichiometrically explicit minimum model for an ecosystem. The model takes into account the coupling of carbon and a nutrient in the biomass and detritus, the nutrient limitation and the energy‐providing role of primary producers, the recycling role of decomposers, and the stoichiometric constraints leading to indirect competition for the nutrient. The model shows that two conditions must be met to ensure coexistence of primary producers and decomposers: (1) decomposers must be limited by the carbon provided by plant detritus, and (2) the difference between the carbon : nutrient ratios of primary producers and decomposers must be sufficiently small. Condition (1) is fulfilled if decomposers are better competitors than primary producers for nutrient uptake. When nutrient uptake by plants and decomposers has a Lotka‐Volterra form, these results are robust whether the nutrient cycle is closed or open. When nutrient uptake is donor controlled, however, coexistence is facilitated by an open nutrient cycle. We conclude that ecosystem persistence is not a trivial issue when stoichiometry is taken into account in ecological processes. Strict conditions on the carbon : nutrient ratios and competitive abilities of plants and microorganisms may be required. Given these theoretical results, we highlight the lack of experimental data concerning primary producer and decomposer coexistence conditions, and we suggest that more research has to be performed.