Intransitive competition is common across five major taxonomic groups and is driven by productivity, competitive rank and functional traits

1. Competition can be fully hierarchical or intransitive, and this degree of hierarchy is driven by multiple factors, including environmental conditions, the functional traits of the species involved or the topology of competition networks. Studies simultaneously analysing these drivers of competition hierarchy are rare. Additionally, organisms compete either directly or via interference competition for resources or space, within a local neighbourhood or across the habitat. Therefore, the drivers of competition could change accordingly and depend on the taxa studied. 2. We performed the first multi-taxon study on pairwise competition across major taxonomic groups, including experiments with vascular plants, mosses, saprobic fungi, aquatic protists and soil bacteria. We evaluated how general is competition intransitivity from the pairwise competition matrix including all species and also for each possible three-species combination (triplets). We then examined which species were likely to engage in competitive loops and the effects of environmental conditions, competitive rank and functional traits on intransitive competition. 3. We found some degree of competition intransitivity in all taxa studied, with 38% to 5% of triplets being intransitive. Variance in competitive rank between species and more fertile conditions strongly reduced intransitivity, with triplets composed of species differing widely in their competitive ranks much less likely to be intransitive. 4. Including functional traits of the species involved more than doubled the variation explained compared to models including competitive rank only. Both trait means and variance within triplets affected the odds of them being intransitive. However, the traits responsible and the direction of trait effects varied widely between taxa, suggesting that traits can have a wide variety of effects on competition. 5. Synthesis. We evaluated the drivers of competition across multiple taxa and showed that productivity and competitive rank are fundamental drivers of intransitivity. We also showed that not only the functional traits of each species, but also those of the accompanying species, determine competition intransitivity. Intransitive competition is common across multiple taxa but can dampen under fertile conditions or for those species with large variance in their competitive abilities. This provides a first step towards predicting the prevalence of intransitive competition in natural communities.

TRY is currently supported by DIVERSITAS/Future Earth and the German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig. S.S. was supported by the Spanish Government under a Ramón y Cajal contract (RYC-2016-20604). A.L. and M.C.R. acknowledge funding from Deutsche Forschungsgemeinschaft (DFG, grant no: RI 1815/16-1). F.A. has been supported by the Swiss National Science Foundation (grants no. 31003A_135622 and PP00P3_150698). M.D.-B. acknowledges support from the Marie Sklodowska-Curie Actions of the Horizon 2020 Framework Program H2020-MSCA-IF-2016 under REA grant agreement no. 702057. E.A. received financial support from the Swiss National Science Foundation (grant number 31003A_160212). S.B., E.A. and S.S. were partly funded by the DFG Priority Program 1374 “Infrastructure-Biodiversity-Exploratories” (Fi-1246/6-1). Fieldwork permits were issued by the responsible state environmental offices of Baden-Württemberg. B.K.S. is supported by Australian Research Council (DP170104634).