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International audience; Land use changes rank among the highest threats to biodiversity, but assessment of their ecological impact is impaired by data paucity in vast regions of the world. For birds, land use changes may mean habitat loss or fragmentation, changes in resource availability, and disruption of biotic interactions or dispersal pathways. As a result, avian population sizes and assemblage diversity decline in areas subjected to urbanization, agricultural intensification, and land abandonment worldwide. This threat is especially sensitive in hotspots such as the Mediterranean basin, where avifaunas of several biogeographic origins meet, encompassing numerous endemic taxa, and ecological specialists with low resilience to habitat modifications. Here, we correlated several facets of bird taxonomic and functional diversity to a fine-grained land-use change classification, in order to identify priority areas in need for enforced protocoled bird sampling in a conservation prospect. For this, we computed the species richness, functional richness, originality and specificity of 211 bird assemblages based on bird extent-of-occurrence data for 279 species and 10 ecological traits. We used a spatialized regression model to correlate bird diversity patterns with bioclimatic gradients and land use change between 1992 and 2018, assessed from an unsupervised clustering on 2 km resolution data. We showed that species-rich bird assemblages are subjected to agricultural intensification, while functionally diverse assemblages are mainly undergoing desertification and land abandonment. Unfortunately, most of these changes occur in areas where protocoled bird surveys with sufficient spatial and temporal resolution are lacking. In light of these results, we urge for the setting of bird monitoring programs targeted mainly on parts of North-Africa and the Levant, in order to allow a region-level evaluation of the threat posed by recent land use changes on the exceptional avifaunistic diversity of the basin. Fostering such regional-scale evaluations of congruences between human threats and centers of diversity is a necessary preliminary step for a pragmatic response to data deficiencies and ultimately setting appropriate responses to avoid the collapse of avian assemblages.

International audience; Species introductions on islands result into heterogeneous assemblages with short common biogeo- graphic histories, yet potentially redundant ecological functions. How native and feral species interact over resource and habitat in the absence of long co-evolutionary histories presides to the deterministic part of assembly processes in these assemblages, in turn affecting strategies to preserve indigenous bio- diversity. Critical to this question are the ability of introduced species to cope with novel environments and their degree of niche separation with native species, which may or not permit local invasion. These processes can be investigated by studying shifts in alien species’ ecological trait space between their native and introduction ranges, and its degree of overlap with co-occuring native species on introduction grounds, accounting for the depth of their common phylogenetic histories. Previous results suggested that insular assemblages are driven either by past competitive exclusion, habitat filtering or niche dis- placement associated with post-introduction lability in species’ trait space. We attempt to disentangle these processes in New Zealand bird assemblages, which uniquely mix species from multiple biogeo- graphical origin and range from narrow endemism to colonizing alien birds. New Zealand thus hosts some of the world’s most peculiar bird assemblages, albeit without obvious conflicts among species with differing local histories. We use a multispecies hierarchical bayesian model fitted on 501 point counts to investigate niche separation between 21 alien and native species through their responses in abun- dance to habitat composition at landscape (proportion of native and man-modified habitats) and local scales (forest composition and vertical structure). Complementarily, we investigate the roles of species’ ecological trait space and biogeographical origins in these responses through an ordination framework constructed with literature data. Our results support habitat filtering operating simultaneously at multiple scales, associated with trait-based separation between native and alien species. We further present preliminary results on the contributions of biogeographic origin and phylogenetic relatedness to habitat-trait associations in New-Zealand bird assemblages, and eventually propose avenues to refine these results through novel curve analysis methods. Our framework aims to integrates functional and biogeographic perspectives on novel insular communities to the benefit of a better understanding and conservation of island biodiversity.

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Deciphering the processes by which human activities influence the diversity of ecological systems, at various temporal and spatial scales, is at the foreground of research in conservation biology. The ecological niche appears in that respect as a relevant conceptual interface. My work addresses the implications of the structure and variations of this interface for our interpretation of the consequences of changes in landscapes, habitats and climatic conditions on biotic communities. I examine this issue through the model of European common birds, for which large-scale data bases allow quantifying the niche in a multivariate and multiscale way. My reasoning holds with three main results. First, beyond the multiplicity of niche axes, exploring their relations and structures allow inferences on the processes that drive the segregation of species along environmental gradients. Second, crossing habitat and climatic niches reveal interactions between sources of vulnerability which seem at a first glance to operate at distinct scales. Last, the breadth of the habitat niche is not only a cause, but also a consequence of species' responses to environmental variations. I explore the consequences of these three major results on our understanding of processes that sustain the taxonomic and functional diversity of biotic communities. I suggest that, beyond the traditional view of the niche as a network of nested filters, accounting explicitly for interactions between these filters and their variations would sensibly improve our ability to explain and predict the ecological effects of global changes., Comprendre les processus par lesquels les activités humaines influent sur la diversité écologique, à des échelles spatiales et temporelles diverses, est une priorité en biologie de la conservation. La niche écologique constitue à cette fin une interface conceptuelle pertinente. Ma thèse traite de l'influence de la structure et des variations de cette interface, et de leur propension à affecter notre interprétation des conséquences des modifications des paysages, des habitats et du climat sur les communautés biotiques. A partir du modèle de l'avifaune commune, pour lequel de vastes bases de données permettent une quantification de la niche multivariée et multi-échelle, je développe un argumentaire en trois volets. Le premier propose qu'au-delà de la multiplicité des axes de la niche, leurs relations et leurs structures permettent d'inférer des processus qui conduisent à la ségrégation des espèces communes le long de gradients d'habitats. Deuxièmement, croiser la niche climatique et la niche d'habitat révèle des interactions entre des sources de vulnérabilité qui semblent de prime abord opérer à des échelles différentes. Enfin, la largeur de la niche d'habitat est non seulement une cause, mais aussi une conséquence de la réponse des espèces aux variations de leur environnement. J'explore les conséquences de ces trois résultats principaux sur notre compréhension des processus qui sous-tendent la diversité taxonomique et fonctionnelle des communautés. Je propose en conclusion de dépasser la vision classique de la niche comme un ensemble de filtres aux effets spatialement et temporellement hiérarchisés, pour prendre explicitement en compte les interactions entre ces filtres et leurs variations dans l'explication et la prédiction des effets écologiques des changements globaux.