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Dryad

Fitness differences override variation-dependent coexistence mechanisms in California grasslands

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Nov 13, 2024 version files 450.10 KB

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 species 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. It is therefore uncertain to what degree rainfall variation can offset average fitness differences between native and non-native annual plants in California grasslands to promote coexistence, nor what coexistence mechanisms are most relevant. To test these dynamics, we implemented a competition experiment between five annual species of native forbs and non-native grasses. 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. Overall, we found little potential for coexistence. Competition was dominated by the non-native grass Avena 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 most species, 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. 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 amongst the now-dominant non-native grasses.