The Functional Ecology of Coral Rubble on Tropical Coral Reefs

Abstract

Tropical coral reefs continue to experience widespread coral mortality at local and regional scales, driven primarily by rising ocean temperatures that trigger mass coral bleaching events and disease outbreaks. These global stressors are compounded by local human pressures such as overfishing and coastal pollution, which further erode reef resistance and resilience to our warming ocean. As a result, coral rubble, the fragmented remains of dead coral skeletons, has become increasingly prevalent on many reefs and is expected to continue accumulating in the coming decades. Despite having a seemingly barren and featureless appearance, coral rubble habitats support a diverse suite of motile cryptofauna, often more than an order of magnitude greater than that found within live coral colonies. Modelling projections predict an initial increase in secondary productivity of benthic invertebrates on degraded reefs may temporarily support reef trophodynamics through enhanced resource availability. Therefore, it is increasingly important to identify the motile cryptofauna communities and the reef fish communities that can successfully exploit the rich cryptic resources provided by rubble habitats.

This thesis aimed to address key knowledge gaps in the ecology of motile cryptofauna and reef fish communities associated with coral rubble habitats, and to identify the drivers underpinning these patterns. Specifically, I sought to 1) quantify the spatial variability of motile cryptofauna diversity and community structure (total density, biomass, and community composition at three taxonomic resolutions) in coral rubble across scales (m to km) and test whether variability at smaller scales could be explained by gradients in microhabitat features; 2) quantify the quantity, quality, and origin of organic matter in seawater, algae growing on individual rubble pieces, and sediment beneath the rubble as proxies for food availability and test the extent to which gradients in organic matter parameters explained variability in cryptofauna community and functional structure, and 3) combine 28 years of reef fish and benthic community monitoring data with targeted rubble disturbance and fish foraging experiments on reefs in the inner Seychelles Islands to test whether invertivorous reef fish community structure on coral rubble habitats is influenced by benthic regime (rubble regime, hard substrate regime, macroalgal regime) and opportunistic reef fish feeding mechanisms.

In my first data chapter, I found that coral rubble cryptofauna communities were most variable at intra-site scales (m) rather than inter-site scales (100s m) or between reef zones (km scales), suggesting that these communities are largely structured by small-scale ecological processes. A substantial amount of variation in cryptofauna density and phyla-level community structure was explained by small-scale habitat characteristics, specifically the substrate type below the rubble and the variability in macroalgal cover on individual rubble pieces. This suggested micro-habitat complexity and food availability as key drivers of cryptofauna communities. This was supported in my second data chapter, which demonstrated that the availability of food resources to cryptofauna, specifically organic matter in seawater, algae growing on rubble pieces, and sediment beneath rubble patches, was a key driver of community structure. Fresh, high-quality benthic-derived organic matter across these sources explained substantial variation in taxa biomass, taxonomic evenness, and breadth of functional strategies represented within communities. In contrast, increasing carbon enrichment in seawater and sediment correlated with reduced taxa biomass and species richness, and communities dominated by a few taxa rather than maintaining a more balanced distribution. Finally, I found that while invertivorous reef fish community structure was highly distinct amongst benthic regimes, this does not have an overall effect on the community structure of invertivorous reef fish utilising coral rubble habitats. Instead, invertivorous reef fish communities on coral rubble habitats are influenced by opportunistic mechanisms linked to the physically dynamic nature of coral rubble. Static undisturbed coral rubble habitats offer limited foraging opportunities for invertivorous reef fishes, however these constraints are significantly reduced when unconsolidated rubble is physically disturbed, expanding foraging opportunities across a broader range of species and body sizes.

Overall, this thesis contributes to our understanding of ecological processes structuring motile cryptofauna and reef fish communities associated with coral rubble habitats. My findings highlight that the ecological processes underpinning rubble-associated motile cryptofauna are strongly scale-dependent, emphasising the need to consider spatial and temporal context when studying reef trophodynamics. Cryptofauna communities are tightly coupled to dynamic and heterogeneous organic matter pools, making them particularly sensitive to alterations in nutrient regimes driven by climate and local environmental change. Furthermore, reef fishes that exploit rubble-associated prey are constrained by the limited accessibility of cryptic resources, requiring disturbance or specific habitat conditions to benefit from them. My results are directly applicable to natural resource managers by showing that, although coral rubble supports dense and diverse cryptofauna communities, these thrive under specific nutrient conditions. If these conditions are not maintained, the ecological processes they provide become constrained. Further, alongside efforts to restore structural complexity on coral reefs to support fish communities, maintaining or creating rubble patches under appropriate conditions can sustain natural ecological processes facilitated by coral rubble. However, although motile cryptofauna and rubble-associated reef fishes may be sustained or even enhanced on rubble habitats, rubble generation ultimately depends on live coral framework production. Thus, any benefits of reef erosion for rubble-dwelling taxa, rubble-foraging fishes, and the functions they support may diminish as coral production, and therefore the supply of new rubble, declines.

Date of Award	3 Dec 2025
Original language	English
Awarding Institution	Bangor University
Sponsors	Envision DTP & The Nature Conservancy, Honolulu
Supervisor	Gareth Williams (Supervisor) & Craig Robertson (Supervisor)