Background/Question/Methods Contemporary niche theory is a structuring framework for understanding species coexistence, species distribution and ecosystem functioning. It highlights the reciprocal interactions between organisms and their environment: organisms respond to their environments through their requirements, but also modify it in return through their impacts. Initially developed in the context of resource competition, extensions such as the inclusion of shared predation under the term “apparent-competition” have broadened the scope of the theory.
Organisms, however, do not always compete for resources: sometimes they exchange them for their mutual benefits. Yet, a more general framework of contemporary niche theory that includes such positive interactions is still missing. Such a need is reinforced by the recent growing interest for positive interactions within and between plant and microbial communities. Building on recent advances on the subject, this presentation is an attempt to lay down the theoretical premises of such a niche theory of positive interactions, with a focus on mutualism. These ideas will be illustrated through a model of cross-feeding between bacterial species and indirect positive interactions in a small food web.
Results/Conclusions Using the example of the resource cross-feeding between two bacterial strains, I will show how we can adapt niche theory to situations that involve mutualists. I will revisit May’s “orgy of mutual benefaction” in the context of this resource explicit mutualism and discuss how it is avoided in practice by the presence of a third limiting resource. I will then synthesize these results around a general theory of the niche that connects competition to mutualism, which relies on two metrics, the impact and requirement niche differences.
I will then look at the indirect interactions between plants and carnivores mediated by herbivore abundance in a small trophic food web. I will show how these interactions are effectively mutualistic: the plants help the carnivores by feeding herbivores, while the carnivores help plants by eating herbivores. Importantly, the strength of that “apparent mutualism” can be quantified using the same metrics as in the previous example, enabling its comparison to other ecological situations and showing the broad applicability of the framework and its associated concepts.
