Functional and phylogenetic diversity of island birds in the Anthropocene: A multidimensional analysis of dispersal, environmental and anthropogenic filters on island bird diversity

This dissertation examines how classical biogeographical and anthropogenic filters affect the functional and phylogenetic diversity of bird assemblages on islands. It focuses on how classical dispersal and environmental filters interact with anthropogenic filters, and on the role of in situ speciation within archipelagos in occupied trait space. To make the effects of these filters and in situ speciation comparable across spatial and temporal scales, analyses were conducted at global, archipelago, and regional levels, as well as over time. At the global level, numerous island bird assemblages were analysed to describe both dimensions of diversity, using functional characteristics and phylogenetic trees to show dispersion in trait space and functional range. Morphological characteristics were used to reflect the species' resource use, lifestyle, and mobility. Additionally, a global bird phylogeny was employed to identify common lineages and the extent of evolutionary divergence between species. Building on this, an archipelago-wide perspective was developed, distinguishing archipelago-endemic species and their radiations from non-endemic island species. This enabled the importance of in situ speciation for the structure of the trait space to be quantified. Additionally, the long-term monitoring of breeding birds, waterfowl and wading birds at regional, abundance-based levels was evaluated on six East Frisian islands in the Wadden Sea. Taxonomic diversity (Shannon index), functional richness and functional dispersion were correlated with weather and climate trends to analyse changes in functional community structure in relation to weather events and long-term climate developments. Globally, there is a positive but decoupled relationship between functional and phylogenetic diversity; both dimensions respond differently to filters. Functional diversity decreases with increasing isolation, while phylogenetic diversity increases with island size. The results suggest that dispersal filters primarily limit functional differentiation, while larger islands positively affect phylogenetic diversity. Anthropogenic filters are at least as influential as filters related to dispersal and environmental conditions. The strongest negative predictor for both diversity dimensions is the pressure of introduced predators, followed by land cover diversification, which promotes functional and phylogenetic clustering. Overall, the patterns indicate extensive functional and phylogenetic homogenisation due to anthropogenic influences. From an archipelago-wide perspective, endemic species predominantly occupy the central regions of trait space. Compared to all island birds, archipelago-endemic species and their radiations occupy a smaller and less dispersed portion of the global trait space. Within individual archipelagos, however, they often show reduced functional richness but increased functional dispersion. This is particularly evident among endemic passerines, suggesting that in situ speciation primarily results in fine-scale niche differentiation within an evolutionarily and ecologically constrained trait space, rather than expanding it globally. Abundance-based regional monitoring in the Wadden Sea reveals further decoupling. In both monitoring programmes, taxonomic diversity is increasing while functional diversity is decreasing. This indicates progressive functional homogenisation. The functional diversity of breeding birds is more strongly related to short-term weather fluctuations, whereas long-term climate trends influence the temporal changes in the functional diversity of waterbirds and waders. The synthesis anchors the results in a tiered filter framework that links island ontogeny and historical contingency with dispersal, the environment, and biotic interactions. Anthropogenic influences act as modulating filters that largely operate in parallel with classical filters, altering their selectivity along the island ontogeny. The Anthropocene is considered a contingency event that influences these processes further. This dissertation demonstrates that the effect of anthropogenic filters on functional and phylogenetic diversity can match or exceed that of classical filters. Furthermore, local functional differentiation, archipelago-internal diversification, and global functional and phylogenetic homogenisation can be understood as expressions of the same filter cascade, modified by anthropogenic influences. Functional homogenisation is also temporally decoupled from taxonomic changes. This necessitates considering functional and phylogenetic parameters in monitoring, assessment, and nature conservation planning. Overall, the dissertation illustrates that functional and phylogenetic diversity should be considered central components of an integrative, historically influenced filter concept in island biogeography.