Submitted to: Plant Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2009
Publication Date: 12/25/2009
Citation: Leger, E.A., Espeland, E.K. 2010. The shifting balance of facilitation and competition affects the outcome of intra- and interspecific interactions over the life history of California grassland annuals. Plant Ecology. 208(2):333–345.
Interpretive Summary: Plant-plant interactions in the landscape have often been shown to be competitive, with some plants winning the battle for light, water, and nutrients, and depressing the growth of other plants. Because this has been shown to be a common phenomenon, some ecological theories posit that competitive interactions among plants are the driving force in determining the distribution and abundance of plant species. If competition structures plant communities, we would expect that plants that have been co-occurring longer would have fewer competitive interactions (because they have evolved to tolerate one another in order to succeed with these neighbors), and plants that have been interacting for shorter periods of time to have stronger competitive interactions (less time to evolve tolerance). Likewise, we would expect plants of other species to exert stronger competitive influence on plants than plants of the same species. This study examines plant-plant interactions in the growth phase of individual plants, but also looks at how plant neighbors affect germination and survival for plants of six commonly-found annual plant species of California. We found that plant-plant interactions were generally facilitative in terms of seedling survival (having plant neighbors increased survivorship) and competitive in the growth stage (having plant neighbors reduced growth). For four of the six species studied, intra-specific competition was stronger than inter-specific competition: these plants reduced the growth of others of their same species more strongly than reducing the growth of other species. In this study we show that, for these plants, within-species density dependent processes exert a stronger influence on distribution and abundance than interspecific competitive interactions.
Technical Abstract: Niche and neutral theories have been proposed to explain the maintenance of diversity in plant communities, and make contrasting predictions about the relative strength of intra and interspecific competition. We propose that they also make different predictions about the strength of interspecific interactions in historically co-occuring species vs. recently assembled communities of exotic plants. Specifically, niche theory would predict stronger negative density-dependent interactions in communities of exotic species, since they have had less time to evolve distinct niches, but neutral theory would predict no difference in the strength of interspecific competition in recently assembled vs. older communities. We tested these predictions at three life history stages (germination, survival, and juvenile-adult growth) for three native and three exotic California annual species in a greenhouse experiment. We varied plant density and the nature of plant communities in small pots, with communities either strictly intraspecific or a three species mix of intra and interspecific neighbors. We saw a range of facilitative, neutral, and competitive interactions that varied significantly by species, rather than by native or exotic status. There were more competitive interactions at the germination and juvenile adult growth stages and more facilitative interactions for survival. The relative strength of competition in monospecific vs. mixed species communties depended on whether we considered only the juvenile-adult growth stage or the entire life history. Using traditional analysis of juvenile-adult growth only, all species showed negative density-dependent interactions for final biomass production. However, when the net effect of plant interactions from seed to adult were considered, two native species ceased to show negative density-dependence, and the difference between intraspecific 38 and mixed species competition was only significant for one exotic species. Results were more supportive of the predictions of neutral, rather than niche, theory for five of six species.