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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. 

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 Spatial synchrony, the tendency for temporal fluctuations in an ecological variable to be positively associated in different locations, is a widespread and important phenomenon in ecology. Understanding of the nature and mechanisms of synchrony, and how synchrony is changing, has developed rapidly over the past 2 decades. Many recent developments have taken place through the study of long‐term data sets. Here, we review and synthesise some important recent advances in spatial synchrony, with a focus on how long‐term data have facilitated new understanding. Longer time series do not just facilitate better testing of existing ideas or more precise statistical results; more importantly, they also frequently make possible the expansion of conceptual paradigms. We discuss several such advances in our understanding of synchrony, how long‐term data led to these advances, and how future studies can continue to improve the state of knowledge. 

Abstract Climate models predict increases in the frequency and intensity of extreme‐weather events. The impacts of these events may be modulated by biotic agents in unpredictable ways, yet few experiments cover sufficient spatiotemporal scales to measure the interactive effects of multiple extreme events.We used 15 years of a 28‐year experiment spanning several significant droughts to investigate how rainfall, large herbivores, and soil‐engineering termites affect understorey vegetation in a semi‐arid savanna.Herbivory was the dominant influence on community structure—decreasing cover, increasing species richness, and favouring occurrence of annuals relative to perennials—but these effects were contingent on rainfall and termitaria in non‐additive (hence unpredictable) ways.A separate experiment showed that resource enrichment, mimicking the effects of termitaria, does not straightforwardly compensate for top‐down effects of herbivory.Synthesis. Our study highlights the potency of top‐down forcing in African savannas. It suggests impressive robustness to drought and underscores the value of multi‐decadal experiments for studying interactions among multiple drivers of ecosystem dynamics. 

Abstract Accompanying the climate crisis is the more enigmatic biodiversity crisis. Rapid reorganization of biodiversity due to global environmental change has defied prediction and tested the basic tenets of conservation and restoration. Conceptual and practical innovation is needed to support decision making in the face of these unprecedented shifts. Critical questions include: How can we generalize biodiversity change at the community level? When are systems able to reorganize and maintain integrity, and when does abiotic change result in collapse or restructuring? How does this understanding provide a template to guide when and how to intervene in conservation and restoration? To this end, we frame changes in community organization as the modulation of external abiotic drivers on the internal topology of species interactions, using plant–plant interactions in terrestrial communities as a starting point. We then explore how this framing can help translate available data on species abundance and trait distributions to corresponding decisions in management. Given the expectation that community response and reorganization are highly complex, the external‐driver internal‐topology (EDIT) framework offers a way to capture general patterns of biodiversity that can help guide resilience and adaptation in changing environments. 

Rapid climate change poses a fundamental challenge to seed sourcing in restoration. While local provenancing is a common practice in restoration, local seeds may not survive or persist under future climate conditions. Alternative provenancing strategies, such as climate‐adjusted provenancing, that mix local seeds with non‐local seeds aim to increase the buffering capacity of restored populations. We hypothesized that seeds sourced from warmer and drier sites have higher seedling performance under drought than seeds sourced from cooler and wetter sites. We conducted a common garden experiment in a Great Basin rangeland where more frequent, severe drought events are expected to increase in the future. We sourced Bottlebrush squirreltail (Elymus elymoides[Raf.] Swezey) seeds from six locations along an aridity gradient and sowed them under three rainfall scenarios: ambient, moderate drought, and severe drought. We found strong interannual variation in seedling recruitment. In 1 year, some provenances from warmer/drier sites had high emergence and subsequent seedling survival under moderate drought. In another, emergence was low across provenances and rainfall treatments. Two provenances that survived 2 years of moderate drought had divergent seedling traits. Specifically, one had a high germination temperature optimum and high water‐use efficiency, such that it likely avoided freezing and resisted drought, while another had a low germination temperature optimum and low water‐use efficiency, such that it likely tolerated freezing and escaped drought. We highlight that understanding these differences in recruitment and stress coping strategies across provenances is important for creating climate‐adaptive seed mixes in anticipation of future climate conditions.