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The balance of interaction types determines the assembly and stability of ecological communities


What determines the assembly and stability of complex communities is a central question in ecology. Past work has suggested that mutualistic interactions are inherently destabilizing. However, this conclusion relies on the assumption that benefits from mutualisms never stop increasing. Furthermore, almost all theoretical work focuses on the internal (asymptotic) stability of communities assembled all at once. Here, we present a model with saturating benefits from mutualisms and sequentially assembled communities. We show that such communities are internally stable for any level of diversity and any combination of species interaction types. External stability, or resistance to invasion, is thus an important but overlooked measure of stability. We demonstrate that the balance of different interaction types governs community dynamics. A higher fraction of mutualistic interactions can increase the external stability and diversity of communities as well as species persistence, if mutualistic interactions tend to provide unique benefits. Ecological selection increases the prevalence of mutualisms, and limits on biodiversity emerge from species interactions. Our results help resolve long-standing debates on the stability, saturation and diversity of communities.

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Fig. 1: The balance of interaction types determines the species richness at which communities saturate under the UIM.
Fig. 2: External stability is determined by the proportions of interaction types in the UIM.
Fig. 3: The probability of extinction is determined by the proportions of interaction types in the UIM.
Fig. 4: Species persistence in the UIM is determined by the proportions of interaction types.
Fig. 5: Selection acts to increase Pm and sometimes increase Pe and decrease Pc in the UIM.
Fig. 6: The IIM shows some notable differences from the UIM.

Data availability

Data sharing is not applicable to this article as no datasets were generated or analysed during the current study. All simulation results can be reproduced using the code provided.

Code availability

The model of community assembly was implemented in Python. The simulation code is available at


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We thank A. Tilman, J. Van Cleve, L. Stone and G. Barabás for helpful comments regarding the manuscript. J.J.Q. was funded by the Roy and Diana Vagelos Scholars Program in the Molecular Life Sciences at the University of Pennsylvania.

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Both authors designed the study. J.J.Q. constructed the model, and both authors provided the analysis. Both authors contributed substantially to writing the manuscript. Both authors gave the final approval for publication.

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Correspondence to Erol Akçay.

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Supplementary Figs. 1–39, Table 1 and Sections 1 and 2.

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Qian, J.J., Akçay, E. The balance of interaction types determines the assembly and stability of ecological communities. Nat Ecol Evol 4, 356–365 (2020).

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