A half century of change in global marine fish biodiversity

Abstract

The aim of this PhD thesis is to understand how marine fish biodiversity is changing in space and time, and how, over the last half century, global fisheries are exploiting species that play diverse roles in ecosystem functioning. The biological diversity, including its spatial and temporal biogeography, has been reshaped by human activities. New methodological advances, and large databases, have allowed the development of approaches that can provide novel insights into these challenges, but some knowledge shortfalls remain. The classical facet of biodiversity, known as taxonomic diversity, draws on information about species richness and abundance. However, the ecological roles played by all species within ecological communities can now also be quantified using data on functional traits; this is known as functional diversity. This thesis uncovered new insights into the biogeography of functional traits by showing that 11,961 bony fishes and 866 cartilaginous fishes have general trends across latitudinal gradients. It also documented greater concentrations of species, rare in multiple facets of biodiversity, towards higher latitudes and in coastal systems, and showed that those concentrations are higher than expected by chance. A further key finding in the thesis was that the species contributing most to biodiversity change have clear differences in functional trait characteristics compared with the remaining species within assemblages. As these results make clear, the species that contribute the most to biodiversity change are also targeted by marine fisheries exploitation. Moreover, the functional diversity of exploited bony and cartilaginous species is changing in magnitude and direction. Taken together my work highlights the importance of considering multiple facets of biodiversity when examining biodiversity in exploited marine systems.