Although the effects of variation between individuals within species are traditionally ignored in studies of species coexistence, the magnitude of intraspecific variation in nature is forcing ecologists to reconsider. Compelling intuitive arguments suggest that individual variation may provide a previously unrecognised route to diversity maintenance by blurring species-level competitive differences or substituting for species-level niche differences. These arguments, which are motivating a large body of empirical work, have rarely been evaluated with quantitative theory. Here we incorporate intraspecific variation into a common model of competition and identify three pathways by which this variation affects coexistence: (1) changes in competitive dynamics because of nonlinear averaging, (2) changes in species' mean interaction strengths because of variation in underlying traits (also via nonlinear averaging) and (3) effects on stochastic demography. As a consequence of the first two mechanisms, we find that intraspecific variation in competitive ability increases the dominance of superior competitors, and intraspecific niche variation reduces species-level niche differentiation, both of which make coexistence more difficult. In addition, individual variation can exacerbate the effects of demographic stochasticity, and this further destabilises coexistence. Our work provides a theoretical foundation for emerging empirical interests in the effects of intraspecific variation on species diversity.

| INTRODUC TI ONTo quantify the influence of competition on species presence, absence and abundance, ecologists need to be able to identify the species that will win and the species that will lose in competition over the long term (i.e. over multiple generations). The ecological literature on competition is vast, covering decades of empirical research, many reviews of both empirical methods (Brooker , 1992;Schoener, 1983), and the development of over 50 different metrics of competitive ability based on measurements of individual performance (Weigelt & Jolliffe, 2003). Despite the tremendous research attention, however, the question of how to identify the ultimate winners and losers in competition from empirical studies remains unclear (Trinder, Brooker, & Robinson, 2013). For example, the most recent comprehensive review of interspecific competition between plants (Aschehoug et al., 2016) concludes that the hundreds of studies that have examined the impacts of interspecific competition on the performance of individual plants "…have told us little about how such competitive effects ramify to populations." This observation matches messagesfrom influential reviews of empirical studies from previous decades Abstract 1. Understanding the role of competition in structuring communities requires that we quantify competitive ability in a way that permits us to predict the outcome of competition over the long term. Given such a clear goal for a process that has been the focus of ecological research for decades, there is surprisingly little consensus on how to measure competitive ability, with up to 50 different metrics currently proposed.2. Using competitive population dynamics as a foundation, we define competitive ability-the ability of one species to exclude another-using quantitative theoretical models of population dynamics to isolate the key parameters that are known to predict competitive outcomes.3. Based on the definition of competitive ability we identify the empirical requirements and describe straightforward methods for quantifying competitive ability in future empirical studies. In doing so, our analysis also allows us to identify why many existing approaches to studying competition are unsuitable for quantifying competitive ability. Synthesis.Competitive ability is precisely defined starting from models of competitive population dynamics. Quantifying competitive ability in a theoretically justified manner is straightforward using experimental designs readily applied to studies of competition in the laboratory and field. K E Y W O R D S competitive ability, competitive dominance, competitive effect, competition model, competitive response, interspecific competition, population dynamics, response surface | 1903 Journal of Ecology HART eT Al.

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Increasing evidence for rapid evolution suggests that the maintenance of species diversity in ecological communities may be influenced by more than purely ecological processes. Classic theory shows that interspecific competition may select for traits that increase niche differentiation, weakening competition and thus promoting species coexistence. While empirical work has demonstrated trait evolution in response to competition, if and how evolution affects the dynamics of the competing species—the key step for completing the required eco-evolutionary feedback—has been difficult to resolve. Here, we show that evolution in response to interspecific competition feeds back to change the course of competitive population dynamics of aquatic plant species over 10–15 generations in the field. By manipulating selection imposed by heterospecific competitors in experimental ponds, we demonstrate that (i) interspecific competition drives rapid genotypic change, and (ii) this evolutionary change in one competitor, while not changing the coexistence outcome, causes the population trajectories of the two competing species to converge. In contrast to the common expectation that interspecific competition should drive the evolution of niche differentiation, our results suggest that genotypic evolution resulted in phenotypic changes that altered population dynamics by affecting the competitive hierarchy. This result is consistent with theory suggesting that competition for essential resources can limit opportunities for the evolution of niche differentiation. Our finding that rapid evolution regulates the dynamics of competing species suggests that ecosystems may rely on continuous feedbacks between ecology and evolution to maintain species diversity.