Biotic and abiotic factors interact with dominant plants—the locally most frequent or with the largest coverage—and nondominant plants differently, partially because dominant plants modify the environment where nondominant plants grow. For instance, if dominant plants compete strongly, they will deplete most resources, forcing nondominant plants into a narrower niche space. Conversely, if dominant plants are constrained by the environment, they might not exhaust available resources but instead may ameliorate environmental stressors that usually limit nondominants. Hence, the nature of interactions among nondominant species could be modified by dominant species. Furthermore, these differences could translate into a disparity in the phylogenetic relatedness among dominants compared to the relatedness among nondominants. By estimating phylogenetic dispersion in 78 grasslands across five continents, we found that dominant species were clustered (e.g., co-dominant grasses), suggesting dominant species are likely organized by environmental filtering, and that nondominant species were either randomly assembled or overdispersed. Traits showed similar trends for those sites (< 50%) with sufficient trait data. Furthermore, several lineages scattered in the phylogeny had more nondominant species than expected at random, suggesting that traits common in nondominants are phylogenetically conserved and have evolved multiple times. We also explored environmental drivers of the dominant/nondominant disparity. We found different assembly patterns for dominants and nondominants, consistent with asymmetries in assembly mechanisms. Among the different postulated mechanisms, our results suggest two complementary hypotheses seldom explored: (1) Nondominant species include lineages adapted to thrive in the environment generated by dominant species. (2) Even when dominant species reduce resources to nondominant ones, dominant species could have a stronger positive effect on some nondominants by ameliorating environmental stressors affecting them, than by depleting resources and increasing the environmental stress to those nondominants. These results show that the dominant/nondominant asymmetry has ecological and evolutionary consequences fundamental to understand plant communities.
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CAA and MWC acknowledge the funds provided by TD Professor of Urban Forest Conservation and Biology chair and the Natural Sciences and Engineering Research Council of Canada (number 386151). CAA thanks the Connaught International Scholarship for Doctoral Students at University of Toronto, Scarborough (UTSC), particularly G. Arhonditsis for his support. This work was generated using data from the Nutrient Network (http://www.nutnet.org) experiment, funded at the site-scale by individual researchers. MN Bugalho acknowledges support from the Portuguese Science Foundation (FCT) through Principal Investigator contract IF/01171/2014. Coordination and data management have been supported by funding to E. Borer and E. Seabloom from the National Science Foundation Research Coordination Network (NSF-DEB-1042132) and Long-Term Ecological Research (NSF-DEB-1234162 to Cedar Creek LTER) programs, and the Institute on the Environment (DG-0001-13).
Data and codes are available at: https://doi.org/10.5061/dryad.pzgmsbcn7.
Arnillas, Carlos Alberto; Borer, Elizabeth T.; Seabloom, Eric W.; Alberti, Juan; Baez, Selene; Bakker, Jonathan D.; Boughton, Elizabeth H.; Buckley, Yvonne M.; Bugalho, Miguel Nuno; Donohue, Ian; Dwyer, John; Firn, Jennifer; Gridzak, Riley; Hagenah, Nicole; Hautier, Yann; Helm, Aveliina; Jentsch, Anke; Knops, Johannes M. H.; Komatsu, Kimberly J.; Laanisto, Lauri; and McCulley, Rebecca L., "Opposing Community Assembly Patterns for Dominant and Nondominant Plant Species in Herbaceous Ecosystems Globally" (2021). Plant and Soil Sciences Faculty Publications. 170.