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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.
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Asymmetric relationships and their effects on coexistence
Abstract Species coexistence attracts wide interest in ecology. Modern coexistence theory (MCT) identifies coexistence mechanisms, one of which, storage effects, hinges on relationships between fluctuations in environmental and competitive pressures. However, such relationships are typically measured using covariance, which does not account for the possibility that environment and competition may be more related to each other when they are strong than when weak, or vice versa. Recent work showed that such ‘asymmetric tail associations’ (ATAs) are common between ecological variables, and are important for extinction risk, ecosystem stability, and other phenomena. We extend MCT, decomposing storage effects to show the influence of ATAs. Analysis of a simple model and an empirical example using diatoms illustrate that ATA influences can be comparable in magnitude to other mechanisms of coexistence and that ATAs can make the difference between species coexistence and competitive exclusion. ATA influences may be an important new mechanism of coexistence.
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- Award ID(s):
- 2023474
- PAR ID:
- 10487871
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Ecology Letters
- Volume:
- 27
- Issue:
- 1
- ISSN:
- 1461-023X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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