Ecological stoichiometry, primary producer-decomposer interactions, and ecosystem persistence

Primary producers and decomposers-the two most important groups for the fun ctioning of ecosystems-have complex, indirect interactions. They are indire ct 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 a ble to persist, using a stoichiometrically explicit minimum model for an ec osystem. The model takes into account the coupling of carbon and a nutrient in the biomass and detritus, the nutrient limitation and the energy-provid ing role of primary producers, the recycling role of decomposers, and the s toichiometric constraints leading to indirect competition for the nutrient, The model shows that two conditions must be met to ensure coexistence of p rimary producers and decomposers: (1) decomposers must be limited by the ca rbon 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 th an primary producers for nutrient uptake. When nutrient uptake by plants an d decomposers has a Lotka-Volterra form, these results are robust whether t he nutrient cycle is closed or open. When nutrient uptake is donor controll ed, however, coexistence is facilitated by an open nutrient cycle. We concl ude that ecosystem persistence is not a trivial issue when stoichiometry is taken into account in ecological processes. Strict conditions on the carbo n: nutrient ratios and competitive abilities of plants and microorganisms m ay be required. Given these theoretical results, we highlight the lack of e xperimental data concerning primary producer and decomposer coexistence con ditions. and we suggest that more research has to be performed.