PhD Research

PhD research Oct. 2017 – Jan. 2022

A HALF CENTURY OF CHANGE IN MARINE FISH BIODIVERSITY

Thesis Abstract:

The aim of my 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. Pioneering ecologists and naturalists, such as Alexander Von Humboldt (1769-1859), Maria Martin Bachman (1796-1863), Charles Darwin (1809-1882), and Alfred Russel Wallace (1823-1913), laid the foundations for asking how biodiversity, 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 defined 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 13 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. Main Research Question Chapters:

• BIOGEOGRAPHY OF MARINE FISH:

The main goal of this chapter was to fill an important knowledge gap associated with the need to understand multiple facets of biodiversity in biogeographical studies at the macroecological scale, particularly taxonomic and functional diversity facets. This chapter therefore describes the patterns of trait distribution for the most speciose classes of marine vertebrates: bony fishes (Actinopterygii – with 11,961 species) and cartilaginous fishes (Elasmobranchii – with 866 species). It examined variation in seven continuous traits (length of first maturity, age, growth, food consumption, trophic level, depth max, temperature). In this chapter, two distinct spatial perspectives were adopted: (i) the global latitudinal gradient (latitudinal bands in ten-degree bins in the Northern and Southern hemispheres) and (ii) species geographical extent (measured as a proxy for common and rare species). Complementarily, this chapter also clarifies the main differences between traits from both classes of fishes. The main results show that functional trait biogeography varies with the latitudinal gradient and indicates that colder regions are associated with a predominance of slow-growth, late-maturity and late-age species in comparison with what occurs at lower latitudes. This could reflect a “fast-slow continuum” and the different metabolic rates found in colder and warmer regions. The comparison of functional trait distributions in common and rare species also uncovered distinct patterns. For example, the most restricted bony and cartilaginous fish species exhibited higher food consumption when compared with the most widespread species. Since food consumption is affected by temperature it is possible that the mechanism also influences this pattern. Finally, there were also marked differences between the two groups of fish, e.g. cartilaginous species reach a higher trophic level compared to bony fishes. The functional trait biogeographical patterns described in this chapter could be important for stakeholders’ decision-making, improving conservation and global fisheries management.

• BIOGEOGRAPHY OF RARE FISH IN THE WORLD’S OCEANS:

Rare species, which represent a large fraction of the taxa in ecological assemblages, account for much of the biological diversity on Earth. These species make substantial contributions to ecosystem functioning, and are targets of conservation policy. To date the emphasis has been on designating species as rare or common on the basis of their relative abundance, and/or spatial occurrence patterns. However, it is now clear that an additional facet of diversity, namely functional diversity, needs to be considered in these designations. In this chapter an integrated approach is adopted, combining information on the rarity of species trait combinations, and their spatial restrictedness, to quantify the biogeography of rare fish (a taxon with almost 13,000 species) in the world’s oceans. Concentrations of rarity were found, in excess of what is predicted by a null expectation, near the coasts and at higher latitudes. Mismatches were also observed between these rarity hotspots and marine protected areas. This pattern is repeated for both major groupings of fish, the Actinopterygii (bony fish) and Elasmobranchii (sharks, skates and rays). These results uncover global patterns of rarity that were not apparent from earlier work, and highlight the importance of using metrics that incorporate information on functional traits in the conservation and management of global marine fishes (see Trindade-Santos et al., 2022 - https:// 10.1038/s41467-022-28488-1).

• DIVERSITY CHANGE:

Ecological community structure is composed mostly of rare species, with abundances concentrated within a very few species. Over a number of decades a variety of metrics to quantify how abundance is distribute d between species have been developed. Those metrics focus mostly on translating the degree of evenness, or unevenness, in abundance distribution between species at the assemblage level. However, until now it has not been possible to understand how each species contributes to changes in evenness. A new framework, developed by Chao together with Ricotta, allows the investigator to understand how species are contributing to biodiversity change in space and time. This chapter employs this novel approach. It uses BioTIME, the largest compiled database with surveyed abundance data, and focuses on studies with assembled abundance data over ten or more years (≥ 10 years, continuous or interspersed years). Each case study that matched this criterion in BioTIME was assigned into a different Large Marine Ecosystem using the case study coordinates. Trends of change in evenness were detected at assemblage level (x = years, y = evenness metric), and the contributions of each species to biodiversity change were also computed. Subsequently, the top ten most exploited species within each of the nine Large Marine Ecosystems (LMEs) were identified. The analysis then asked if the top contributor (i.e the species that contributes most to biodiversity change) in each case study was one of these exploited species. It showed that the species that contribute the most to biodiversity change were exploited by marine fisheries in 73% of all 2194 case studies examined. The traits of these top contributors were also explored. The take home message from this investigation is that species that occupy certain regions of the trait distribution predominate in their contributions to biodiversity change, and these species are dominated by commercially important taxa in six out of nine LMEs.

• FUNCTIONAL DIVERSITY OF MARINE FISHERIES EXPLOITATION:

Global biodiversity is under threat, particularly with the marine ecosystems undergoing overexploitation. Marine fisheries catches (including subsistence, recreational, artisanal, industrial fisheries and discards) have grown from 30 million tons (m.t.) in 1950 to 110 m.t. in 2014, with a peak of 127 m.t. in 1996. Traditional fisheries management has focused on commercially valuable taxa, mainly using a population by population approach (i.e. focusing on the taxonomic diversity facet). This practice places less emphasis on the role that each species plays in the ecosystems, and as a consequence little is known on how global fisheries have been exploiting functional diversity. With this knowledge gap in mind, this chapter sheds light on the amount of functional diversity that fisheries exploited from the Large Marine Ecosystems (LMEs) of the world from 1950 until 2014. This was done using the composition of all species for each LME, then calculating the functional space occupied by all species in those environments and finally assessing the amount of functional space that was exploited by fisheries in each LME. The main results show widespread change in the functional diversity exploited by fisheries from the LMEs of the world in the last 65 years. The spatial and temporal trends of functional diversity exploited from the LMEs were calculated using global reconstructed marine fisheries catch data provided by the Sea Around Us initiative (including catches from all fisheries sectors and discards) and data for eleven functional traits available in FishBase. Exploited functional richness increased for both classes of marine fishes used in this chapter: ray-finned fishes (80% of LMEs) and cartilaginous species (sharks, rays and skates) (75% of LMEs). Considerable geographic and temporal variation in the patterns was detected for the functional evenness and functional divergence of these catches. The results found in this chapter provide evidence that global fisheries are increasingly targeting species that play diverse roles within the marine ecosystem and underline the importance of incorporating functional diversity in ecosystem management (see Trindade-Santos et al., 2020 - https://doi.org/10.1098/rspb.2020.0889).