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Co-extinctions may exacerbate the current biodiversity crisis. Yet, we do not understand all the factors that shape the robustness of communities to the loss of species. Here we analyse how coevolution influences the robustness of mutualistic and antagonistic communities. We find that coevolution increases robustness in mutualism but reduces it in antagonism. These differences are due to coevolution altering the density of interactions in communities. The largest changes to robustness occur when coevolutionary selection is strong. Yet, the effect size of coevolution on robustness depends on the size of the community. Our results may broaden the suite of mechanisms affecting the resilience of ecological communities. These insights may inform efforts to reduce the risk of species loss in the face of global change.

Species interactions have evolved from antagonistic to mutualistic and back several times throughout life’s history. Yet, it is unclear how changes in the type of interaction between species alter the coevolutionary dynamics of entire communities. This is a pressing matter, as transitions from mutualisms to antagonisms may be becoming more common with human-induced global change. Here, we combine network and evolutionary theory to simulate how shifts in interaction types alter the coevolution of empirical communities. We show that as mutualistic networks shift to antagonistic, selection imposed by direct partners begins to outweigh that imposed by indirect partners. This weakening of indirect effects is associated with communities losing their tight integration of traits and increasing their rate of adaptation. The above changes are more pronounced when specialist consumers are the first species to switch to antagonism. A shift in the outcome of species’ interactions may therefore reverberate across communities and alter the direction and speed of coevolution.