Competitor-induced plasticity modifies the interactions and predicted competitive outcomes between annual plants.

The competitive effect of one individual on another can have impacts beyond just reductions in performance. Because species plastically respond to their environment, competition can also induce changes in species traits, and in turn, these modified traits can then affect interactions with yet other individuals. In this context, plasticity is often argued to favor species coexistence by increasing the niche differentiation between species, though experimental evidence for this hypothesis that explicitly projects competitive outcomes is largely lacking. Here, we transiently subjected four annual plant species to early-season intraspecific or interspecific competition to explicitly induce plastic responses and then examined the response of these individuals to competitors faced later in life. Competing with nearby individuals early in the growing season tended to amplify the sensitivity of individuals to competition, and particularly so for interspecific competition, but the strength of this effect depended on the identity of the focal species. This increase in interspecific relative to intraspecific competition caused plasticity to decrease the predicted likelihood of pairwise coexistence. By combining recent theory with a new experimental approach, we provide a pathway toward integrating phenotypic plasticity into our quantitative understanding of coexistence.