Abstract
Community assembly experiments in ecology provide a means to contrast abiotic and biotic influences that structure plant communities. Competition and facilitation are two common biotic predictors of success in plant communities. Abiotic limitations can include, but are not limited to, space, light, and water. Concurrent examination of spatial ecology of biotic and abiotic drivers with these interactions is critical to better understand plant invasion. Here, we test the relative importance of seed aggregation on the net outcome of interactions between a native desert annual Phacelia tanacetifolia and an invasive exotic species Bromus rubens. Our study proposes that seed aggregation can play a major role in determining whether an exotic species will co-exist or outcompete natives. We tested the ecological heteromyopia hypothesis that proposes interspecific competition is greater over shorter distances relative to intraspecific competition and that distance between species can thus lead to co-existence. We used competition trials with seeds and mimic plants via clumped and dispersed arrangements to test this hypothesis for one invasive and one native annual species. Monoculture and mixed seed trials were done under controlled greenhouse conditions for both species. To ensure findings that were not solely driven by physical interference or space constraints in pots, a plastic exotic mimic plant was used in a mixture treatment with natives to enable a contrast between biotic competition and space effects. Both P. tanacetifolia and B. rubens has significantly greater establishment and final abundances in monocultures with spatially clumped seeds. Dispersed seeds of B. rubens in monocultures had significantly lower establishment than the clumped sowing. Bromus rubens experienced a significant negative response to P. tanacetifolia in mixtures of clumped seeds. Additionally, the biomass of B. rubens was significantly higher in monocultures of clumped arrangement and mixtures of dispersed arrangement. The native P. tanacetifolia did not respond to the physical interference of the 3-D-printed plastic mimic brome invader. Light was also not a significant covariate in any of the competition trials. This pilot study suggests that seed aggregation is a potential key predictor of success for both the native and exotic species studied here in monocultures. Thus, the heteromyopia hypothesis was supported for these two dryland annual plant species because interspecific competition decreased as space and thus distance between individuals increased. This study highlights the importance of spatial structure of seeds in the seedbanks and seedlings in the emergent plant community in describing potential community assembly. Restoration efforts can consider seed sowing designs to decrease invasion by exotic species and increase native plant abundance by aggregating seeds at fine spatial scales.
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Acknowledgements
This study was possible thanks to the National Sciences and Engineering Research Council of Canada (NSERC) grant awarded to CJL.
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CC and CJL designed the experiment; CC performed the experiments and wrote the manuscript; CJL analyzed the data; NG co-wrote and thoroughly edited the manuscript; MO and JB contributed critically.
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Ghazian, N., Braun, J., Owen, M. et al. Seed aggregation tips the scale in plant competition. COMMUNITY ECOLOGY 22, 403–412 (2021). https://doi.org/10.1007/s42974-021-00064-5
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DOI: https://doi.org/10.1007/s42974-021-00064-5