Plant functional traits have often been used to make inferences about the processes structuring plant communities. For functional traits to explain community assembly, they must be able to explain variation in species’ demographic rates in a community context. However, teasing apart the influence of the abiotic environment, species interactions, and traits on demographic rates can be challenging, especially in natural settings. Furthermore, little is known about how variation in the species pool translates into variation in neighborhood-scale demographic rates. Here we present the results of a mesocosm experiment conducted in the annual grasslands of California designed to test how the functional composition of competitors impacts plant species’ demographic rates. Specifically, we ask whether (i) functional diversity of the species pool, (ii) functional diversity of the competitive neighborhood, and (iii) focal species’ traits relative to competitor’s traits can explain fecundity, a key demographic rate. To address these questions, we quantified the fecundity of five focal annual plant species grown both in the absence of competitors and within 20 different mixtures of 10 annual plant species. We then used an extensive suite of functional traits to explore how competitor functional composition impacts focal species’ fecundity and resulting population dynamics.
Results/Conclusions
All five focal species successfully germinated and produced seed in the experimental plots. Fecundity of the focal species varied substantially across experimental mixtures (e.g. average fecundity of Plantago erecta varied over 20-fold). Contrary to common assumptions in community assembly studies, preliminary analyses with a subset of species and traits found no clear relationship between the functional diversity of either the species pool or the competitive neighborhood and per capita fecundity. Nevertheless, fecundity appeared to be better explained by the combination of competitor density and functional diversity, as opposed to by only the functional diversity of the pool of competitors. These results suggest a potential indirect role of the functional diversity of the species pool in community assembly by contributing functional variation to local competitive neighborhoods, highlighting the importance of neighborhood-scale demography and population dynamics in community assembly.