Recruitments

The distribution of biomass among the different species forming food webs is the outcome of the population dynamics of each species, which are governed by bottom-up (resource availability) and top-down (predator-induced mortality) controls. However, direct intra-guild competition (i.e. within the same trophic level, such as territoriality) is also a major factor determining biomass distribution, as it leads to self-regulation of the abundance of each species. The relative strength between trophic interactions and direct intra-guild competition explains the distribution of biomass among species according to biomass pyramid or trophic cascade patterns. However, the parameters related to this direct competition are still very poorly understood, unlike those related to trophic interactions (e.g. consumption rates), which limits the predictions provided by food web models based on allometric parameters (biological rates calculated based on the body mass of organisms). We propose using eco-evolutionary approaches based on the theoretical framework of adaptive dynamics to predict and understand the values taken by these uncertain parameters depending on the ecological context : the already well-established trophic parameters, the trophic level of the species and resource availability. Such an approach is indeed useful to estimate realistic values for a biological parameters when the rest of the system is well known, as it seeks to optimise the growth rate in response to environmental conditions. We expect eco-evolutionary feedbacks between trophic levels, as intra-guild competition limits predator biomass and relaxes top-down control over their prey, which would then lead to cascading effects on lower trophic levels. The models considered will be food chain and food web models, which will be analysed analytically and numerically using computer simulations. Theoretical predictions will be compared with empirical measurements on macro- and meso-faunal soil invertebrates. These measurements will be carried out during micro- and mesocosm experiments designed to track resource consumption rates and mortality as a function of the number of individuals of detritivorous and carnivorous species. Direct intra-guild competition is characterised either by a mortality rate that increases with density or by a decrease in consumption rates. This project will significantly improve our general understanding of trophic dynamics by coupling both theoretical and experimental approaches and improve our description of trophic dynamics in soil communities.

This thesis aims to provide answers to the following question: what is the impact of biological actors (e.g. roots, soil meso- and macrofauna), as agents of soil stabilisation and restructuring, on soil functioning? This thesis project will be carried out as part of the PEPR Sols Vivants (Living Soils) programme, whose mission is to generate knowledge on the role of soil organisms and their multiple interaction networks in soil multifunctionality, for a variety of soil types, uses and management methods, in order to meet the challenge of preserving and improving soil health.