Drivers of reef fish community trophic structure and function (2024)

The high biomass and productivity of coastal and shallow-water reef fishes support ecosystem services to billions of humans worldwide. However, many of the ecological mechanisms contributing to the production and maintenance of this biomass remain unclear. Fundamental to the production of fish biomass is the dynamic process of mass, nutrient, and energy transfer between primary producers, consumers and predators, described generally as ‘trophodynamics’. Body size is a key determinant of ecological interactions, leading to size-structured ecosystems where predators consume larger prey as they themselves increase in size. As body size is impacted to varying degrees by temperature, trophic interactions are also subject to change as temperatures increase. Understanding the ecological mechanisms underlying fish trophodynamics is therefore critical to managing ecosystem services into the future.
To investigate reef fish community structure and trophodynamics, this thesis combines size-based ecosystem theory, novel empirical methods, and a large database of fish community surveys. Along with the existing database of ~15, 000 underwater reef fish community surveys from Reef Life Survey program, I produced a novel dataset on feeding ecology from 996 fish specimens from 133 reef fish species, which were hand-sampled over ~30 degrees of latitude (ranging ~15 ^oC). From these specimens I recorded length, weight, gut content prey composition and stable isotope measurements.
Chapter 1 uses empirical gut content analysis and fish community survey data to investigate whether community-level predator to prey mass ratios (PPMRs) explain variation in the slope of reef fish ‘abundance size spectra’ (i.e., relative abundance or biomass of fish at large and small body sizes) across large spatial scales. I combined observations of 8,128 prey items in gut contents of 97 reef fish species with ~15, 000 reef fish community surveys from around the Australian continent. The study revealed that PPMR of coastal reef fish is nearly threefold higher than many previous estimates, and that PPMR trends differed with body size both between and within trophic guilds. We also found that community-level PPMRs (cPPMR) were positively related to size spectrum slopes in the tropics, but there was no clear relationship on temperate reefs. Lack of clear interaction between community size structure and PPMR on temperate reefs could suggest that size-based predation plays a weaker role in these communities, or that other factors may be at play in temperate reefs, which either interact with or obscure the role of cPPMR in determining fish community size spectrum slopes. Such factors could include human impacts, importance of invertebrate PPMRs (not measured in the present study), and other environmental differences. This study provides the first general and broad-scale estimates of cPPMR across Australian reef communities and trophic guilds, and advances size-based ecological knowledge and models by providing empirical evidence for the role of cPPMR in the partitioning of abundance and biomass across body size ranges in tropical fish communities.
Chapter 2 presents a detailed analysis of the primary production pathways supporting a tropical reef fish community. The relative contributions of distinct benthic or pelagic energy pathways in coastal ecosystems has important implications for ecosystem structure, function, and resilience, as well as resource management. However, it is still unclear to what degree coastal fish communities are supported by largely local benthic production, versus local and imported pelagic production. In this study I used carbon and nitrogen stable isotope analysis (SIA) from 274 individuals of 67 fish species (ranging from 1 – 7,878 g wet weight) and 44 non-fish organisms, along with quantitative observational data from 10 years of underwater visual surveys of fish community structure at the same reefs. We estimated that ~50% of the community fish standing biomass at Lizard Island, Great Barrier Reef, Australia, is supported by the pelagic pathway. This study is one of the first to estimate pathway contributions to whole fish communities in shallow?water Australian reefs. Its findings are consistent with a previous study (using a different approach), highlighting approximately equal importance of benthic and pelagic production pathways to reef fish biomass. Further, we show that the importance of the benthic or pelagic pathway in a species diet, described through a continuous pathway contribution index (PCI) metric, can be relatively well predicted by taxonomic relatedness — an important finding for diverse coral reef fish communities in which most species have limited trophic data available. In contrast, traditional categorical diet or behavioural classification schemes of fish do not always capture the range of pathways supporting species. I concluded that using these schemes to estimate pathway contributions to coral reef fish communities may considerably underestimate the importance of pelagic primary productivity.
Chapter 3 investigates macroecological patterns of fish community body size and trophic composition with temperature over a spatial gradient spanning ~14 – ~30 ^oC using the continental scale underwater visual survey data. To quantify temperature-size relationships at a community level, I estimated mean community level body size of fish observed through underwater fish surveys and assessed how this mean individual size correlated to the mean annual sea surface temperature. The analysis was done at both the community level and separately for trophic guilds (herbivores, invertivores, planktivores and piscivores). I found a clear and strong decrease in community-level mean individual fish body size with temperature over the temperature gradient through space, with a nearly 40% decrease in mean length or fourfold decrease in mean body mass over 10 ^oC. Such decrease was seen in all trophic guilds except for piscivores. I also explored how mean body size and abundance trends relate over the same spatial gradient to explain the contribution of trophic guilds to total community biomass. I found that herbivores contribute the most to community level biomass at warmer temperatures >20 ^oC, and invertivores at cooler temperatures <20 ^oC. In contrast, for planktivores and piscivores the relative contribution to the community level biomass across the spatial gradient remains similar, as temperature driven changes in body size at cooler temperatures were compensated by changes in relative abundances.
Finally, I bring together insights from my research of trophic pathways, body size and temperature, to improve our understanding of the ecological mechanisms that facilitate and sustain the reef fish biomass. In doing so, I critically assess the tools we have to answer research questions in complex reef ecosystems and highlight future research avenues. Methodologies which are often seen as universally suitable for exploring questions on trophodynamics may not apply to all systems or require more effort than is commonly done. This is especially true for stable isotope analyses, and I present alternative approaches, solutions, and key considerations.
Combined, my chapters reveal insights into the entangled roles of temperature, body size, and primary production in characterizing the trophic and size structure of complex reef ecosystems, and important knowledge gaps for future research.

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