Volume 106.3 of Journal of Ecology includes a special feature on intransitive competition and coexistence. The special feature, guest edited by Santi Soliveres and Eric Allan, consists of 6 research articles and an editorial. Santi and Eric tell us more about their special feature below…

Whether to see who starts in hide and seek or who does the dishes, every one of us has at some point played rock-paper-scissors. What makes the game interesting is that there is no absolute winner, and rock, paper and scissors can all lose to one strategy and beat one strategy.

Ecologists have known for decades that some species coexist, as in the rock-paper-scissors game, because there is no competitive hierarchy between them. Two classic examples are the coexistence of different sexual strategies of Anolis lizards (Sinervo & Lively, 1996) and coexistence between strains of Escherichia coli bacteria (Kerr et al. 2002).

This intransitive competition, as rock-paper-scissors type competition is more formally known, could therefore be a powerful mechanism of coexistence and an important source of biodiversity. It could allow the coexistence of more species than there are limiting resources: for instance, three species might form a competitive hierarchy when competing for one resource, perhaps nitrogen, but this can be transferred into an intransitive loop if the weakest competitor produces an allelochemical that damages the strongest. However, despite its potential importance, intransitive competition has been largely considered to be merely a curiosity rather than a major mode of competition. This may be because the few studies on intransitivity have dealt largely with model systems of three species or with theoretical models.

Empirical evidence for intransitivity has therefore been rare. Few studies have looked at intransitive competition in multiple species, explored how it varies along environmental gradients or how it is linked to other important community attributes, such as diversity or functional trait competition. We therefore have very little information on what factors might make this type of competition more prevalent: is it more common in fertile or heterogeneous environments? Are certain types of species more likely to compete intransitively? Is intransitive competition more common in diverse communities?

Read all the papers online. (Image from Laird & Schamp, 2018)

In order to address some of these research gaps, we organized a special thematic session on intransitive competition and coexistence at the 2016 BES annual meeting in Liverpool. We were lucky enough to get many of the world´s experts on intransitive competition to attend and were able to convince many of them to contribute to this special issue. We hope to provide an overview of the state of field and to identify the major questions, with the aim to inspire more research on the generality and implications of intransitive competition in nature.

The special issue presents six research papers and an editorial. These deal with four major issues:

• How can we measure intransitivity? This is a surprisingly difficult issue, which has hampered unification in the field and Laird & Schamp provide an overview of the pros and cons of different approaches.
• How does intransitivity interact with other coexistence mechanisms? Papers by Matias et al., Gallien et al. and Stouffer et al. integrate intransitivity into the highly successful modern coexistence framework developed by Peter Chesson, in which all coexistence mechanisms can be grouped into stabilizing and equalizing processes.
• When is intransitivity common? Ulrich et al. use empirical approaches to understand the drivers of intransitivity in natural communities, while Soliveres et al. use multi-species experimental approaches to determine whether certain types of species or conditions lead to more intransitivity.
• How does intransitivity emerge over time? Stouffer et al. step away from the typical assumption in coexistence models that there is a fixed equilibrium and examine how population fluctuations generate intransitvitiy and coexistence. Gallien et al. consider even longer time scales and explore, theoretically, how intransitivity can evolve in sympatry.

Overall, the studies within this special issue help us to better understand what drives intransitive competition and what its consequences are for natural communities. More importantly though, they clearly suggest that we should keep investigating how general or important this type of competition is. It is our hope that this special issue will introduce intransitivity to plant ecologists who are unfamiliar with it and will ideally get them excited about it! Those already familiar with intransitive competition will surely find interesting research questions and methodological advances. We think there are many exciting developments ahead in research on intransitivity.

Santiago Soliveres, University of Alicante (Spain)

Eric Allan, University of Bern (Switzerland)

Read the special feature and the rest of issue 3 online: Volume 106 issue 3