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

Species can modify their traits in response to changes in the environment - a process known as phenotypic plasticity. Because species traits can plastically respond to competition, the competitive effect of one individual on another involves not only reductions in performance, but also changes in morphology, behavior, phenology or physiology that affect interactions with other individuals. In this context, plasticity is often argued to favor species coexistence by increasing the niche differentiation between species, though experimental support that explicitly considers 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 other competitors. By measuring the interactions between the species with and without early-season competitors, we isolated the impact of plasticity on species interactions and coexistence. Growing with nearby competitors early in life impacted plant traits including height and morphology. These plastic responses tended to amplify the sensitivity of individuals to competition, and particularly so for interspecific competition. This increase in inter-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 towards integrating phenotypic plasticity into our quantitative understanding of coexistence.