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Abstract The ability of species to form diverse communities is not fully understood. Species are known to interact in various ways with their neighborhood. Despite this, common phenomenological models of species coexistence assume that per capita interactions are constant and competitive, even as the environment changes. In this study, we investigate how neighbor density‐dependent variation in the strength and sign of species interactions changes species and community dynamics. We demonstrate that incorporating these sources of variation significantly improves predictions of ecological dynamics compared to the outcomes of typical models, which hold interaction strengths constant. We compared the performance of models based on different functions of neighbor density and identity in describing population trajectories (i.e., persistence over time) and community dynamics (i.e., temporal stability, synchrony, and degree of oscillation) in simulated two‐species communities and a real, diverse annual plant system. In our simulated communities, we observed the highest level of coexistence between species pairs when species interactions varied from competitive to facilitative, depending on neighbor density (i.e., following a sigmoid function). Introducing within‐guild facilitation through a nonlinear bounded function allowed populations, both simulated and empirical, to avoid extinction or runaway growth. In fact, nonlinear bounded functions (i.e., exponential and sigmoid functions) accurately predicted population trends over time within the range of abundances observed over the last 10 years. With the sigmoid function, the simulated communities of two species exhibited a higher probability of synchrony and oscillation compared to other functional forms. These simulated communities did not always show temporal stability, even when they were predicted to coexist. Overall, varying species interactions lead to realistic ecological trajectories and community dynamics when bounded by asymptotes based on neighbor density. These findings are crucial for advancing our understanding of how diverse communities are sustained and for applying ecological theory to real‐world studies. 

ABSTRACT With many species interacting in nature, determining which interactions describe community dynamics is nontrivial. By applying a computational modeling approach to an extensive field survey, we assessed the importance of interactions from plants (both inter‐ and intra‐specific), pollinators and insect herbivores on plant performance (i.e., viable seed production). We compared the inclusion of interaction effects as aggregate guild‐level terms versus terms specific to taxonomic groups. We found that a continuum from positive to negative interactions, containing mostly guild‐level effects and a few strong taxonomic‐specific effects, was sufficient to describe plant performance. While interactions with herbivores and intraspecific plants varied from weakly negative to weakly positive, heterospecific plants mainly promoted competition and pollinators facilitated plants. The consistency of these empirical findings over 3 years suggests that including the guild‐level effects and a few taxonomic‐specific groups rather than all pairwise and high‐order interactions, can be sufficient for accurately describing species variation in plant performance across natural communities. 

Abstract While most studies of species coexistence focus on the mechanisms that maintain coexistence, it is equally important to understand the mechanisms that structure failed coexistence. For example, California annual grasslands are heavily invaded ecosystems, where non‐native annuals have largely dominated and replaced native communities. These systems are also highly variable, with a high degree of rainfall seasonality and interannual rainfall variability—a quality implicated in the coexistence of functionally distinct species. Yet, despite the apparent strength of this variation, coexistence between native and non‐native annuals in this system has faltered.To test how variation‐dependent coexistence mechanisms modulate failed coexistence, we implemented a competition experiment between two previously common native forbs and three now‐dominant non‐native annual grasses spanning a conservative‐acquisitive range of traits. We grew individuals from each species under varying densities of all other species as competitors, under either wetter or drier early season rainfall treatments. Using subsequent seed production, we parameterized competition models, assessed the potential for coexistence among species pairs and quantified the relative influence of variation‐dependent coexistence mechanisms.As expected, we found little potential for coexistence. Competition was dominated by the non‐native grassAvena fatua, while native forbs were unable to invade non‐native grasses. Mutual competitive exclusion was common across almost all species and often contingent on rainfall, suggesting rainfall‐mediated priority effects. Among variation‐dependent mechanisms, the temporal storage effect had a moderate stabilizing effect for four of five species when averaged across competitors, while relative nonlinearity in competition was largely destabilizing, except for the most conservative non‐native grass, which benefited from a competitive release under dry conditions.Synthesis: Our findings suggest that rainfall variability does little to mitigate the fitness differences that underlie widespread annual grass invasion in California, but that it influences coexistence dynamics among the now‐dominant non‐native grasses. 

Identifying overarching processes that maintain biodiversity in natural communities remains a challenge in ecology. Although functional traits help explain regional species distributions, they often fall short at the local community level. We investigate whether traits can offer mechanism-based insights into local diversity maintenance due to associations with the sign and strength of plant interactions. We examine the effect of 12 plant functional traits on the sign and strength of pairwise species interactions across two Mediterranean annual plant communities. Results show that traits mediate a spectrum from facilitative to competitive interactions and are influenced by neighbor density and identity. At low densities, species with conservative resource-use traits are consistently stronger facilitators than acquisitive species. The traits of the focal species, however, explain facilitation at low densities better than the traits of neighbors or the pairwise differences in traits. When neighbor density increases, facilitation switches to competition, a pattern we hypothesize reflects a density-mediated reduction in how much neighbors’ traits drive interactions. Moreover, species frequently receiving heterospecific facilitation also display traits associated with self-competition and low intrinsic population growth rate. This study of community-level trait sorting provides insights into the persistence of co-occurring populations, showing trait-based generalizable insights into the local context dependency of species interactions.