An innovative analytical approach based on the distribution patterns of plant species traits - PRAIRIES MC CC

Nathan Rondeau defended his doctoral thesis on 18th december 2024

Climate change is altering the way living beings function and pushing ecosystems towards their ecological limits, beyond which they will no longer be able to maintain their functioning and provide ecosystem services at current levels. Our current knowledge suggests that this will have dramatic consequences for life on Earth (erosion of biodiversity) and for human societies (living conditions). One of the challenges of this thesis is to analyse and evaluate how biodiversity, from both an ecological and functional perspective, can be a major lever for adapting livestock farming systems to climate change and supporting their transition. We will focus on managed agro-ecosystems that are rich in biodiversity, such as permanent grasslands. In particular, we will be seeking to better characterise and understand the spatio-temporal dynamics of plant communities in a context of climate change and their impact on ecosystem functioning and its capacity to deliver ecosystem services, in relation to a diversity of management practices.

Summary 

Understanding and predicting the dynamics of biodiversity under global change is a major scientific challenge. However, biodiversity responses to global change are inherently complex.
Drivers of change not only affect species diversity and abundance but also alter biotic interactions between species, which may impact community assembly and dynamics. In this context, studying the diversity of functional traits within communities could lead to significant advances, as traits reflect how species respond to and influence their environment. To make the trait-based approach operational for the study of complex ecological systems, we developed an innovative analytical framework based on the study of the shapes of trait distributions. The shapes of trait distributions can be characterised by an inequality between the skewness and the kurtosis, the Skewness-Kurtosis Relationship (SKR). Using this inequality, we developed two key indicators. 

The TADeve, which characterises the evenness of trait distributions, and the TADstab, which characterises the stability of trait distributions. Using permanent grasslands as a study model, we highlighted the relevance of studying the evenness (TADeve) and stability (TADstab) of trait distributions in order to disentangle the influence of deterministic processes (e.g. habitat filtering, niche differentiation), while accounting for the inherent stochasticity of ecological systems (Chapter 1). 

Using a long-term dataset of managed permanent grasslands (17-years), we demonstrated that the temporal variability of trait distributions was not random, but depended on management practices (Chapter 2). Intensively managed grasslands (high levels of fertilisation) are associated with unstable and uneven trait distributions. These results are consistent with predictions of the “habitat filtering” theory and the occurrence of intense competition between plant species that limit local diversity. Conversely, extensively managed grasslands (no fertilisation) were linked to remarkably even and stable trait distributions over time. Furthermore, we also showed that the cessation of fertilisation in extensively-managed grasslands led to a rapid convergence towards even and stable trait distributions, which promoted the long-term recruitment and persistence of a rich and diverse grassland flora. These findings are consistent with a theoretical scenario of niche differentiation, which predicts a stable coexistence among functionally contrasting species. Finally, we found that the high evenness and stability of trait distributions, in extensively managed grasslands, are explained by a functional complementarity between dominant and subordinate species, facilitating the long-term stabilisation of the functional assemblage and of the entire plant community (Chapter 3).
Using observational data from various ecological context, we showed that semi-natural and natural plant communities shared a similar functional organisation. The observation of common functional patterns over space and time suggests the existence of general rules governing the assembly, diversity, and dynamics of plant communities.
 

In conclusion, the SKR approach appears to be a suitable tool to study complex dynamic systems, such as ecological systems in the context of global change. In the Anthropocene era, identifying general assembly rules based on functional traits could enable the design of management methods adapted to the conservation and restoration of biodiversity, as well as the maintenance of ecosystem multifunctionality.