Nathan Humbert

Project summary

What makes a collection of interacting species (that is, a community) stable or not to an external perturbation? Part of the complexity of the question lies in what we mean by ‘stable’, and also by ‘a collection of interacting species’. This PhD aims to contribute humbly to answering this question, using a combination of analytical and numerical approaches. This relies on a number of challenges.
First, stability recovers many distinct meanings in ecology. Therefore, the stability of a community can be measured through multiple measures. In addition, these measures do not always work hand in hand: a community can become more stable according to one measure, while becoming less stable according to another one. For example, Pennekamp et al. (2018) showed in communities of aquatic ciliates that species richness increased temporal stability but decreased resistance to warming. Then, part of the challenge in uncovering the determinants of community stability is to disentangle the relationships among the numerous stability measures.
Second, formal representations of ecological communities are often incomplete, as they may omit key interactions or species that shape the community. For example, plant-pollinator studies often overlook the impact of herbivory on plants, despite evidence of its effect on community stability. Similarly, food web studies frequently neglect non-trophic interactions (that is, all interaction types but trophic), even though these interactions are known to influence community stability. Hence, this PhD seek to restore these overlooked elements and elucidate the mechanisms by which they operate on community stability.
Last, ecological communities themselves can vary greatly. A grassland plant community competing for resources is vastly different from an aquatic food web in terms of species composition and the structure of their interactions. This observation begs the question of whether there are general mechanisms underlying community stability that apply across diverse community types. This undoubtedly represents the most challenging issue, fraught with uncertainties.