Description and purpose
Decomposition of organic matter in ecosystems, i.e. its transformation leading to the production of carbon dioxide and nutrients, is the second most important ecosystem function for the maintenance of life on Earth (Gessner et al. 2010). It can be divided into three processes: physico-chemical, microbial and animal transformations. Most animal transformations are carried out by a huge diversity of invertebrates, called detritivores. In soils, their presence increases the decomposition by 50% on average, especially large detritivores (macrodetritivores: e.g. earthworms, isopods). However, their contribution may vary greatly and remains poorly understood (García-Palacios et al., 2021). Indeed, due to the inherent complexity of soil trophic networks (Wolkovich, 2016), a major paradigm claims that detritivores are generalist consumers, preferring to consume litters that are easily accessible, soft, highly colonized by microorganisms and of high nutritional quality (Gessner et al., 2010). Past research has therefore mainly studied the detritivore-litter interaction through nutritional and digestive constraints only, often measuring just litter characteristics.
Recently, a conceptual framework for detritivore-litter pairwise interaction was built, using a trait-matching approach (Marchand, Lecerf, et al., 2024). A trait is a measurable characteristic of an organism at the individual level. This framework is at odds with the previous paradigm since it assumes that the consumption of litter by a detritivore is governed by five stepwise interaction facets, distinct from each other’s but complementary (spatial, biomechanical, digestive, elemental and energy-related) and governed by traits of detritivores, traits of litters and their potential matching. Part of this framework has been experimentally tested. For example, the relative importance of mechanical and elemental facets has been demonstrated, as well as the importance of measuring detritivores and litter traits (Marchand, Estabes, et al., 2024; Marchand, Pey, et al., 2024). Independently from this new conceptual framework, it has also been shown that couple energy and elemental constraints improved predictions of detritivore-litter interactions (Ott et al., 2012; Ruiz et al., 2021). In the context of detritivore-litter interaction, it would therefore seem appropriate to simultaneously consider the elemental and energetic facets through a trait matching approach. Another major argument supporting this approach is that energy constraints are temperature dependent (Brown et al., 2016). In the context of climate change, increased temperatures could completely change these interaction rules (Ott et al., 2012), yet detailed studies on this are currently lacking.
The main objective of the present PhD project will be to establish the mechanistic rules of interaction between a detritivore and a litter, in relation to the elemental and energy constraints, by using a trait matching approach. These results will contribute to improve the understanding and prediction of detritivore-litter interactions and decomposition of organic matter at large.
Context
The beginning of the thesis is subject to the success of the candidate to the SDU2E Doctoral School competitive examination which will take place in May-June 2025.
The thesis will be supervised by Benjamin Pey (Maître de conférences, INP Toulouse), Mathieu Santonja (Maître de conférences, Aix Marseille Université) and Sophia V. Hansson (Chargée de recherche, CNRS). The position will be based at the « Centre de Recherche sur la Biodiversité et l’Environnement » (Campus AgroToulouse, Toulouse, France), with extended visits to the « Institut Méditerranéen de Biodiversité et d’Ecologie marine et continentale » (Marseille, France). The supervising team is specialized in understanding the role of organisms in the dynamics of organic matter within soils. All the operating costs of the thesis (missions, equipment, etc.) will be largely covered by credits already acquired by the supervising team. The thesis project will also benefit from a robust existing knowledge and an existing network of collaborators, stemming from a recent thesis (Theo Marchand, thesis SDU2E 2020-2023, director: B. Pey) and a recently completed EC2CO project (Elementary project, 2022-23). This context opens the door to this thesis, which aims to answer complex and unresolved questions about detritivore-litter interaction. A new project whose themes will be closely associated with this thesis topic will be submitted in September 2025. This will allow the doctoral student to benefit from formal and/or informal collaborations with the already existing network of collaborators, as well as from the programmatic framework of this new project and its funding.
The doctoral student will have to conduct laboratory and field experiments in various French forest sites. It will benefit from the scientific and technical environment of the two laboratories to set up its experiments (e.g. collaborations, technical assistance, experimental infrastructures). In particular, it will benefit from access to the analytical platforms and laboratories of (geo)chemistry of the Observatoire Midi-Pyrénées (Toulouse, France) which will enable it to acquire the necessary data for the thesis project. The doctoral student will also have the opportunity to supervise the research work of at least one Master 2 trainee.
Expected profile
Student at M2 level or equivalent, in ecology, biology or agronomy or environmental sciences. Experience in sampling and/or laboratory experiments recommended. Written and oral communication skills required, proficiency in the English language (B2 level) is required. Skills in data analysis under R. An international experience would be a plus. Specific skills: taste for handling living organisms, autonomy, teamwork, good physical health for field work, car driving licence. In CRBE and IMBE, we want to reflect the diversity of society and accept applications from all qualified candidates.
Agenda
Send as soon as possible your CV and a cover letter (2 pages maximum, expression of your motivations but also how you plan to achieve the thesis if you are recruited) to the following email:
benjamin.pey@toulouse-inp.fr
In case of questions, do not hesitate to contact us beforehand. Applications will be evaluated as they progress and interviews will be offered to the candidates that are most in line with the prerequisites of the thesis.
More information on the thesis project and the SDU2E Doctoral School examination:
http://sdu2e.obs-mip.fr/
Go to the section « Faire une thèse à l’ED » then « Propositions de sujets de thèse ouverts au concours pour l’attribution d’un contrat doctoral » to find more details on this thesis project.
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