Functional traits and environmental characteristics drive the degree of competitive intransitivity in European saltmarsh plant communities

Abstract

Competitive intransitivity, the existence of loops in competitive hierarchies, is one mechanism that can promote the local coexistence of competitors and maintain high local species diversity, although its prevalence and importance remain largely unknown. A full understanding of local community assembly needs knowledge of how transitive and intransitive competitive interactions are linked to species functional traits and the strength of biotic and abiotic filters. We apply a recently developed statistical tool to quantitative data on central European inland saltmarsh plant communities to infer causal relationships between soil characteristics, species occurrences and functional traits, and we estimated coefficients of competition. We found a predominance of intransitive competitive hierarchies. The proportion of such hierarchies was positively correlated with local species richness and compositional variability. Average soil characteristics were not correlated with competitive intransitivity, whereas high soil pH and the high variability in local pH and soil salinity decreased the overall impact of competition on community composition. In pairwise comparisons of species, dissimilarity in morphology, resource demand and reproductive phenology was significantly negatively correlated with differences in competitive performance, while higher environmental dissimilarity was particularly linked to intermediate degrees of competitive superiority. Our results suggest that habitat filtering for similar traits might intensify competitive interactions, but might also give rise to intransitive competitive loops that subsequently promote species coexistence and permit species’ functional equivalence. Intransitive competition appears to increase local diversity and small scale‐species turnover. The observed local differences in competitive structures suggest frequent competitive plasticity and context‐dependent competitive interactions. Finally, our results support the view that local abundance distributions can be used to infer the strength and outcome of competitive interactions. Synthesis. Our results confirm that intransitive competitive interactions might be a strong force structuring local plant communities. Intransitivity needs to be considered when studying plant community assembly and species co‐existence.

Everything you always wanted to know about intransitive competition but were afraid to ask

“…() suggest strong links between intransitive competition and the evolution of greater functional trait divergences in a given clade, which is supported by the empirical, bivariate relationships found by Ulrich et al. () in saltmarsh plant communities. Soliveres et al.…”

“…Most metrics of intransitivity consider competitive exclusion as a binary process: a species either wins or loses in competition. However, two of the studies in this special feature (Gallien, Landi, Hui, & Richardson, ; Ulrich, Kubota, Piernik, & Gotelli, ) show the need to consider more continuous measurements of competition (see also Gallien, Zimmermann, Levine, & Adler, ). If we take a more continuous view, there are almost endless possibilities for how a simple three‐species (A, B, C) competition network can organize, ranging from pure (or “strong”) hierarchy (A ≫ B ≫ C, A ≫ C) to strong intransitivity (A ≫ B ≫ C ≫ A) and everything in between (e.g.…”

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

“…If functional traits of species do affect their likelihood of engaging in intransitive competition then we might expect changes in intransitivity to also feed‐back to affect trait distributions. Previous studies have shown an increase in functional trait diversity under intransitive competition (Maynard, Bradford, et al., ; Ulrich et al., ), but these changes are not always expected (Gallien, ). Our results suggest that, if intransitive competition is driven by reciprocal competitive advantages (as seems to be the cases for fungi, mosses and bacteria), then it should strongly relate to functionally diverse communities.…”

“…C uptake vs. aggressiveness; see also Maynard, Crowther, et al., ). Assuming that functional traits were not only related only to competitive ranks, but also to the different ways by which different species compete for resources (Kraft et al., ; Kunstler et al., ; see also Ulrich et al., , Supplementary Material ), this could explain the positive effects of trait variation in the intransitivity level of triplets of fungi, mosses or bacteria (Figure ) even when the variance in competitive ranks had a negative effect. However, when competing species are too different in their ranks (i.e.…”

“…This limitation is particularly important when pairwise competition experiments are conducted without replication (such as in our vascular plants and fungi). To address this issue, we calculated a new metric ( Inest ) based on the “nestedness” of the matrix, which allows us to use all the information from the RYs (see figure S2b in Ulrich et al., ). By using the RYs directly, we downweight competitive reversals that arise from small competitive advantages in our estimation of intransitivity (e.g.…”