Variation in structure and function of reef fish assemblages among distinct coral habitats

Richardson, Laura Elizabeth (2018) Variation in structure and function of reef fish assemblages among distinct coral habitats. PhD thesis, James Cook University.

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Anthropogenic disturbances are altering the abundance and distribution of organisms across biomes, disrupting the function and stability of ecosystems, and the goods and services they provide. On tropical coral reefs, global climate change and a range of local stressors are reducing populations of habitat-building corals, resulting in unprecedented coral loss and marked shifts in coral species dominance due to differential susceptibilities of coral taxa to disturbance. However, the extent to which shifts in coral species composition will alter the organization of associated organisms and undermine the resilience of coral reefs remains unclear. This thesis exploited a natural experiment on reefs surrounding Lizard Island, Australia, where multiple taxonomically distinct coral habitats existed, characterised by dominance of differing coral taxa, to assess the influence of coral species composition on the structure, function and resilience of reef fish assemblages. Specifically, the four data chapters of this thesis (2–5) addressed the following questions: (1) How does coral species composition affect the cross-scale structural complexity of coral reef habitats? (2) How does the functional diversity of reef fish assemblages vary among taxonomically distinct coral habitats? (3) To what extent does pre-disturbance coral species composition influence the susceptibility of reef fish assemblages to coral bleaching events? (4) Do critical herbivory functions (browsing and grazing) vary among distinct coral habitats?

Chapter 2 compares the cross-scale structural complexity of four coral habitats (i.e. branching Porites, massive Porites, Pocillopora, soft coral), with degraded habitats (characterized by low coral cover <10%, dead coral, rubble and macroalgae) across five ecologically relevant scales of measurement (4-64 cm). Results show that the structural complexity of habitats was underscored by coral species composition, and was not a simple function of total coral cover. However, among-habitat variation in structural complexity changed with scale. Importantly, the range of scales at which habitat structure was available also varied among habitats. Complexity at the smallest, most vulnerable scale (4 cm) varied the most among habitats, with inferences for as much as half of all reef fishes that remain small-bodied and refuge dependent for much of their lives.

Using an ecological trait-based analysis, Chapter 3 compares the functional diversity of fish assemblages among six distinct coral habitats (characterised by branching Porites, massive Porites, staghorn Acropora, Pocillopora, soft coral, and mixed coral assemblages). Despite comparable species richness and functional evenness of fish assemblages among habitats, functional richness and functional divergence varied significantly. Variation in both metrics of fish functional diversity were best predicted by the relative structural complexity among habitats, and were largely driven by the abundance of small-bodied, schooling planktivores in the Porites habitats. The findings suggest that differential structural complexity among coral habitats may act as an environmental filter, altering the distribution and abundance of associated species traits, particularly those of small, habitat-dependent reef fishes.

Chapter 4 compares temporal changes in five complementary trait-based indices of reef fish assemblage structure among six habitats (i.e. branching Porites, massive Porites, Pocillopora, soft coral, mixed coral assemblages, degraded habitats) exposed to a system-wide thermal stress event. The analyses revealed an increased taxonomic and functional similarity of previously distinct reef fish assemblages following mass coral bleaching, with changes characterized by subtle but significant shifts in dominant fish taxa towards small-bodied, algal-farming habitat generalists. The taxonomic and functional richness of fish assemblages did not change across habitats. However, an increase in functional originality indicated an overall loss of functional redundancy, and interestingly, pre-bleaching coral composition better predicted changes in fish assemblage structure, than the magnitude of coral loss.

Finally, Chapter 5 examines the relationships between coral and fish species composition and critical herbivory processes using in situ surveys and experimental assays to compare grazing and browsing functions among three distinct coral habitats (i.e. branching Porites, soft coral, and mixed coral assemblages). There was variation in browsing on the red macroalga, Laurencia cf. obtusa, among habitats, best predicted by differential benthic condition and composition but displaying little relationship with visual estimates of herbivore species biomass. Conversely, there was no variation in the grazing of algal turfs, suggesting that different mechanisms may be driving browsing and grazing processes.

Overall, this research develops a more comprehensive understanding of the role of coral species composition in partitioning available habitat structure, the spatial ecology of reef fishes, and the susceptibility of tropical reef ecosystems to disturbance. The resilience of coral reef ecosystems can hinge upon a range of factors including those considered here: the extent of physical habitat provisioning, the functional diversity and redundancy of associated reef fish assemblages, and the capacity of herbivorous fishes to remove macroalgae following extensive coral loss. However, managers face new governance challenges as species shifts transform reef assemblages with unknown implications for their capacity to maintain key ecosystem functions. This thesis highlights the likely ecological consequences of shifts in species configurations, and provides strong support for assigning greater concern for the composition of habitat-building corals in efforts to promote ecosystem resilience, particularly where they remain present but have undergone taxonomic shifts.

Item ID: 54754
Item Type: Thesis (PhD)
Keywords: assemblage structure, beta diversity, climate change, coral composition, coral reefs, coral species composition, environmental filtering, fish assemblage structure, functional diversity, functional redundancy, scales, traits
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For this thesis, Laura Richardson received the Dean's Award for Excellence 2019.

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Richardson, Laura E., Graham, Nicholas A.J., and Hoey, Andrew S. (2017) Cross-scale habitat structure driven by coral species composition on tropical reefs. Scientific Reports, 7 (1).

Chapter 3: Richardson, Laura E., Graham, Nicholas A.J., Pratchett, Morgan S., and Hoey, Andrew S. (2017) Structural complexity mediates functional structure of reef fish assemblages among coral habitats. Environmental Biology of Fishes, 100 (3). pp. 193-207.

Date Deposited: 25 Jul 2018 02:52
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050102 Ecosystem Function @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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