Using traits to assess nontransitivity of interactions among coral species

Simulations and experiments have shown that species coexistence can be maintained via nontransitive competition, of which a simple case is the rock-paper-scissors game. Reef-building corals exemplify high biodiversity competing for a few limiting resources via several mechanisms. Thus, corals represent fertile ground for exploring competition and nontransitivity. This article aimed to test hypotheses about the effects of species-level traits on competitive outcomes, specifically, that more upright growth, larger corallites, smaller ranges, and difference in commonness co-occur with competitive superiority. Further aims were to test whether closely related species show less predictable competitive outcomes and greater nontransitivity and to examine the level of nontransitivity among a large number of species. These goals were addressed by fitting a mixed-effects model to outcomes of 2,322 interspecific interactions. Among species-level traits, corallite width had the greatest impact on outcome, followed by geographical range size, growth form, and the typical commonness of conspecifics in assemblages. These fixed effects had smaller estimated impacts than a random effect associated with species pair, suggesting a primary role for idiosyncratic species-pair or other factors. Closely related species had more variable, less predictable interaction outcomes. Nearly a quarter of three-way species relations were nontransitive. The observed degree of competitive nontransitivity and extent of idiosyncratic species-pair effects together provide an empirical baseline for further investigations of mechanisms of species coexistence.