Components of biodiversity and ecosystem services by birds in Cantabrian agroecosystems

Résumé

Biodiversity is essential for the stability and the maintenance of ecosystems. However, human impact is jeopardizing global biodiversity, affecting ecosystem functioning throughout the disruption of ecosystem processes, which can ultimately affect the human being. The positive link between biodiversity and ecological functions has become a paradigm in theoretical and applied ecology. Early studies have been focused on species richness as a measure of biodiversity. However, such taxonomic approach hinders the estimate of the quantity and quality of functional roles in a community. Recently, studies have proposed different biodiversity components based on other properties of species assemblages (e.g., traits, lineages) which may be more appropriate for understanding the functional role of biodiversity in ecosystem functioning. Nevertheless, little is known about how biodiversity components (e.g., trait-based and phylogenetic diversity) impact on the provision of ecological functions, and the underlying mechanisms of such effects. Moreover, how environmental heterogeneity influences biodiversity and functions remains unclear. This knowledge gap is particularly glaring in landscapes under anthropogenic pressures such as agroecosystems, where sustainable food production must be reconciled with biodiversity conservation. The present PhD thesis aims to gain insight into how environmental heterogeneity, imposed by ecological and anthropogenic factors, shapes avian diversity (e.g., abundance, traits, lineages) in farming landscapes, and how and why these effects cascade into ecological functions and their derived services. Using birds as a target group, this thesis assesses the magnitude of seed dispersal and insect predation in Cantabrian agroecosystems (woodland pastures and cider apple orchards) examining changes in the relationships between different biodiversity components (taxonomic, functional, and phylogenetic diversity) and ecological functions across gradients of environmental variability. The objective of the first study is to ascertain the relative importance of neutral and deterministic processes in determining the species-specific functional contribution to seed dispersal process. Specifically, this work evaluates the combined effects of species abundances and trait-matching between species driven the occurrence of pairwise plant-frugivore interactions. The second study analyses how functional diversity, especially trait and phylogenetic diversity, mediate in the response to disturbance and the effect on ecological functions. In particular, this work evaluates the effects of habitat loss (represented as a gradient of forest cover) on biodiversity components (i.e., trait and phylogenetic diversity) of plants and birds, and how these changes translate into functional changes that would affect seed dispersal function by estimating the degree of functional complementarity of both trophic levels in their interaction networks. The third study seeks to disentangle the relevance of trait diversity, phylogenetic diversity, and species abundance in two avian ecological functions across agroecosystems (woodland pastures and apple orchards). Additionally, this work evaluates the direct and indirect (biodiversity-mediated) effects of environmental factors in ecological functions. By means of the combination of empirical approaches based on the Biodiversity Ecosystem Functioning (BEF) theory, the results of this thesis suggest that there is a consistency in the mechanisms driving species interactions in spite of the compositional changes of the species across years (e.g., strong interannual variability of species abundances). In fact, the combined effect of both species abundances and trait-matching determined interaction frequency in plant-frugivore networks. Likewise, anthropogenic disturbances (e.g., forest loss) usually influence bird diversity components (i.e., traits, abundance, lineages), which ultimately affect how species contribute to the magnitude of ecological processes. Particularly, plants do not respond to forest loss, however, a decay of biodiversity within their interacting trophic level (e.g., frugivorous birds) may compromise the ecological function throughout the extinction of functional roles in their interaction networks. Finally, biodiversity components showed differential effects on two ecological functions provided by birds, whose effects were strongly influenced by the ecological context. Namely, environmental heterogeneity control biodiversity effects on ecological functions, firstly by filtering avian diversity, but also by regulating the strength of the relationship between biodiversity components and ecosystem functioning. Specifically, the occurrence of wide resource gradients could hamper the role of trait diversity role by triggering strong effects of species abundances. Overall, this thesis provides an integrative overview based on multiple components and trophic levels of the functional relevance of biodiversity on ecosystems across functions and agroecosystems, and the underlying mechanisms of that relationship. This work reinforces current trait-based approaches as promising tools for studying the impact of environmental factors on biodiversity, and the consequences of biodiversity decay on ecological functions. However, these results also highlight the need of including other biodiversity components, as well as considering the environmental heterogeneity for forecasting ecological functions under global changes.