Conférence de Théo Marchand

Mardi 10 octobre 2023

La concordance des traits des consommateurs et des ressources pour mieux quantifier et prédire la décomposition des litières : une approche inter-écosystème aquatique et terrestre

Lieu : Salle de réunion A60 (13h)

Abstract:

Detritivores play fundamental roles in aquatic and terrestrial ecosystems. They are a key component of “brown” trophic networks which are based on dead organic matter. They regulate litter decomposition by fragmenting, ingesting, and digesting detritus. They influence matter and energy flux at the ecosystem level. Detritivores are considered to feed preferentially on high-quality litter, characterized by high macroelement content and little refractory compounds. Yet, the great diversity of detritivores interacts with a great diversity of litter items. The interaction, as well as the resulting influence of detritivores on decomposition, is highly variable and remains difficult to predict as it depends on a complex trophic network which can be represented as the sum of multiple bipartite interactions. Rules of bipartite interactions can be understood by studying functional traits driving the interaction. Traits of both interacting agents may act in interaction, resulting in trait-matching.

In this thesis we aim to better understand the fundamental rules of the interaction between macrodetritivores and leaf litter through (1) identifying detritivores and litter traits associated with constraints driving consumption, (2) testing if trait-matching better explain the interaction, and (3) testing if these rules shape detritivores at the community level. Under a null hypothesis, we expect detritivores to be generalists and to interact with the full range of litter. Under the opposite hypothesis, we expect detritivores to be hyperspecialists and to have narrow feeding niches ruled by strong trait-matching.

To have a comprehensive understanding of the constraints and the associated traits, we built a conceptual framework about detritivore-litter interactions at the individual level (Chapter I). We identified 5 main constraints that can influence detritivore–litter interaction, namely spatial, biomechanical, digestive, metabolic, and elemental constraints. To identify the relative importance of constraints and test if trait-matching allows us to better explain these constraints, we performed laboratory consumption tests by feeding a range of detritivore species from aquatic and terrestrial ecosystems with a range of leaf litter (Chapter II). We identified mechanical constraints as being predominant over elemental constraints. Litter toughness was an especially important trait in preventing consumption. Elemental constraints were mostly due to constraints in phosphorus element. To test if the identified mechanical constraints rule the interaction throughout the decomposition process, we performed laboratory consumption tests by feeding a range of detritivore species from aquatic and terrestrial ecosystems with 2 leaf litter species under a gradient of natural decomposition (Chapter III). We showed that litter toughness has a non-linear influence on the consumption rate, with a detritivore-specific threshold effect. To test if the constraints identified in Chapter II result in an elemental match between detritivore communities and leaf litter, we characterized soil detritivore communities in 12 pairs of sites differing by their available litter (Chapter IV). Detritivore communities’ chemistry weakly matched local litter chemistry. This effect was better explained by changes in detritivore abundances than by changes in detritivore chemistry.

To conclude, our results show consistent rules in aquatic and terrestrial ecosystems: litter toughness stands as a major trait influencing litter consumption by macrodetritivores, even though mechanical constraints are traditionally neglected compared to elemental constraints. Plus, trait-matching only has a weak influence. Detritivores are mainly limited by litter properties but also have species-specific capacities to overcome these limitations. Detritivores then appear as generalist consumers with a trend toward specialization.

 

Keywords: Trait-matching, Functional ecology, Decomposition, Functional trait, Trophic interactions, Forests