One of the main goals in ecology is to understand the effects of climate change on biodiversity and ecosystem functioning. In marine environments the overexploitation of resources by human activities is another forcing factor on species assemblages. This PhD thesis suggested, within a multi-components biodiversity approach, to project expected changes in the structure of Mediterranean coastal fish assemblages under climate change but also under different fishing pressures.
The originality of this work is to consider the dynamics of size structure, diversity of species, but also lineages (phylogenetic diversity), functional traits (functional diversity) and interactions (diversity of trophic links) in fish assemblages at different scales within the Mediterranean basin, the latter three components being largely ignored in climate change projections whereas they are essential for maintaining the viability of ecosystems and associated services beyond the simple number of species.
We therefore projected future geographic ranges of Mediterranean coastal fish through the implementation of a new climate model (NEMOMED8) and based on bioclimatic envelopes models. Projected range shifts of Mediterranean coastal fish show that for the end of the century (i) 54 species would lose their climatic niche, (ii) species richness may decline on 70.4% of the continental shelf, particularly in the Western Mediterranean basin and in the Aegean Sea, and (iii) the average maximum size of fish assemblages would increase on 74.8% of the continental shelf. The small species, not targeted by fishing activities, would be the species most threatened by climate change, while larger species are most vulnerable to fishing effort.Our projections at the whole Mediterranean scale show a decrease of 13.6% for the phylogenetic diversity (PD) of coastal fish and 12.6% for their functional diversity (FD) by the end of this century and suggest a significant erosion of some lineages like the gobidae family. While erosion of PD and FD is partly due to the loss of species richness (fewer species implies less lineage and functions) we obseved, especially in the western basin, that the fish assemblages would loss more phylogenetic and functional diversity than expected simply due to the erosion of species richness.
These two components, were largely ignored in conservation of marine species assemblages and appear to be strongly impacted by global change.To understand the effects of climate change on food web structure we developed a new methodology based on the robust relationship between the size of preys and predators. We were able to highlight potential changes in food webs of fish assemblages under climate change for the Mediterranean continental shelf. We found that a significant portion of the Mediterranean continental shelf would face a reduction in the number of trophic links, vulnerability (number of predators per prey) and generality (number of preys per predator) of species on average, while connectance and trophic level within fish assemblages would increase by the end of the XXIth century.
Beyond changes in species richness, the Mediterranean coastal fish assemblages may be modified, in the functions that they play in ecosystem, in the amount evolutionary history they support, as well as in their interactions structuring food webs. This PhD thesis paves the way towards the biogeography of ecosystem functioning using parsimonious and hybrid models to integrate different components of biodiversity, physical oceanography, and the level of exploitation of ressouces to infer the futur of marine systems cfunctioning facing multiple pressures that are already there.
Keywords: Climate Change, Mediterranean Sea, coastal fishes, bioclimatic model, BIOMOD, NEMOME8, functional diversity, phylogenetic diversity, diversity of links, trophic network.
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