Emergent competition resolves the paradox of stable microbial mutualisms

Oliver J Meacock , Sara Mitri 

What is the role of cooperation in determining ecosystem structure? This question is central to ecology, yet remains controversial; cooperative mechanisms such as plant-insect pollination and microbe-microbe cross-feeding are widespread, but ecological theory suggests that cooperative communities should be unstable. Here, we resolve this apparent contradiction by deriving a precise and general mapping between mechanistic interaction models and the generalised Lotka-Volterra (gLV) equation. By avoiding the common assumption that environmental and population dynamics occur on different timescales, we obtain a framework that is built on the rate of change of growth rates, i.e. growth accelerations. We apply our findings to a model of an obligatory mutualism between auxotrophic bacteria and obtain a counter-intuitive result: increasing the strength of cooperative mechanisms by increasing amino acid production rates causes competition to become stronger in the corresponding gLV model. At the same time, effective intrinsic growth rates become increasingly positive. These coupled effects allow enhanced cross-feeding to increase overall population sizes while maintaining ecosystem stability. Finally, we provide mechanistic insights into the resulting coexistence outcomes by exploiting an exact correspondence between mechanistic models and Modern Coexistence Theory (MCT) revealed by our framework. Our findings show that cooperative and competitive interaction mechanisms do not necessarily translate into positive and negative interaction strengths in phenomenological models, highlighting the importance of mechanistic insights for developing a correct understanding of ecosystem structure.