Hempson, Tessa N. (2017) Coral reef mesopredator trophodynamics in response to reef condition. PhD thesis, James Cook University.

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Abstract

Habitat degradation in coral reef ecosystems is occurring at an unprecedented rate and scale around the world. This habitat decline is driven by both intensifying local stressors and the escalating effects of global climate change. Concurrently, the ubiquitous loss of large consumers from ecosystems, known as trophic downgrading, has important ramifications for the function and resilience of both terrestrial and marine ecosystems. Mesopredators represent an important component of coral reef ecosystems, both economically, supporting large reef fisheries, and ecologically, as potentially important drivers of reef trophodynamics. While there has been substantial focus on the effects of habitat degradation on the small-bodied reef fish community, which is closely associated with the reef benthos, relatively little is known about the implications for piscivorous reef mesopredators. These large-bodied, mobile species are less directly reliant on the reef benthos, and likely to experience the strongest effects of habitat degradation mediated via the fish community on which they prey. This thesis addresses this important research gap by focusing on four key implications of habitat degradation for mesopredators and their role in coral reef trophodynamics.

Dietary adaptability is likely to be an important factor in determining the vulnerability of piscivorous mesopredators to changing prey availability associated with habitat degradation. In chapter 2, I use stable isotope analyses of carbon (δ¹³C) and nitrogen (δ¹⁵N) to investigate whether coral trout (Plectropomus maculatus), in the Keppel Island group on the southern Great Barrier Reef (GBR), can switch their diet to exploit the altered prey base on degraded reefs. Coral bleaching and sediment-laden flood plumes have driven extensive live coral loss on these reefs. The resulting shift in dominant prey species from pelagic plankton-feeding damselfishes to benthic algal-feeding species, represents a shift in the principal carbon pathways in the food web. The δ¹³C signature in coral trout shifted from a more pelagic to a more benthic signal, reflecting the prey community shift, and demonstrating that trout appear to alter their diets as reefs degrade. Nitrogen signatures also indicated that trout with a more benthic carbon signature were feeding at a lower trophic level, indicating a shorter food chain on degraded reefs. Despite this apparent adaptability, mesopredator populations at this location are in steep decline, driven primarily by reduced total available prey biomass.

Thus, despite dietary flexibility conferring a degree of trophic resilience in the short term, mesopredators are nonetheless vulnerable to the effects of habitat degradation. Due the relative longevity of many mesopredator species, sublethal effects of changing prey resources may be difficult to detect. Chapter 3 investigates whether a common mesopredator species (Cephalopholis argus) in the Seychelles inner island group shows evidence of a loss of condition due to habitat degradation. Following extensive live coral loss during the 1998 mass bleaching event, some reefs have regained high coral cover, while others have experienced a regime shift to an algae-dominated state. Stable isotope analyses demonstrated that C. argus on regime-shifted reefs fed lower down the food chain, on a narrower range of carbon sources, than those on recovering reefs, suggesting a simplification of the food web. Histology of liver tissue showed reduced hepatocyte vacuolation in fish from regime-shifted reefs, and reduced lipid stores in spawning females. Reduced energy reserves can lead to decreased growth rates, fecundity and survivorship, ultimately resulting in long-term population declines.

Long-term effects of regime-shift in coral reef ecosystems can substantially alter the trophic structure of fish communities, yet understanding of how these changes manifest through time is limited. In chapter 4, I use a 20-year dataset documenting changes in the benthic and reef fish communities on the Seychelles inner island reefs, to examine how trophic structure has changed on recovering and regime-shifted reefs following the 1998 mass bleaching event. I demonstrate how reef fish communities become increasingly dissimilar, as the benthic states diverge with time since disturbance. Trophic pyramids of relative biomass on regime-shifted reefs developed a concave structure, with increased herbivore biomass supported by increased algal resources, a loss of mid trophic level specialist species, including corallivores, and biomass in the upper trophic levels maintained by large-bodied generalist species. In contrast, on recovering reefs, after an initial loss of mid trophic level biomass, pyramids developed a bottom-heavy structure, which is commonly predicted in stable ecosystems by the theory of energy transfer efficiency in food webs.

Benthic habitat and associated fish communities can also be altered via climate-driven shifts in coral assemblages. One of the predicted characteristics of novel future coral ecosystems is a loss of thermally sensitive coral taxa and an increasing dominance of taxa with higher thermal tolerance, many of which have low structural complexity. In chapter 5, I used a patch reef experiment at Lizard Island on the northern GBR to investigate the effects of thermally 'vulnerable' and 'tolerant' coral assemblages on the trophodynamics of reef mesopredators and their prey fish communities. Fish communities which established naturally on the low structure 'tolerant' patch reefs had lower diversity, abundance and biomass than 'vulnerable' reefs with higher structural complexity. The introduction of a mesopredator (Cephalopholis boenak) had a greater impact on the prey fish community composition of 'tolerant' reefs than 'vulnerable' reefs, and total lipid content of C. boenak indicated that those introduced to 'tolerant' reefs had lower energy reserves than those on 'vulnerable' reefs, indicating a sub-lethal cost to condition.

My research provides novel insight into the effects of habitat degradation on the trophodynamics of coral reef mesopredators, mediated via the fish community on which they prey. I demonstrate that while certain mesopredator species may be able to adapt their diets to changing prey availability, their trophic niche becomes altered as they feed further down the food chain, and they may experience sub-lethal costs due to reduced energy reserves. This work highlights the importance of improving our understanding of how mesopredators are affected by habitat degradation, particularly with respect to the long-term implications of sub-lethal effects for their populations. Sustainable management of these species into the future will require the explicit recognition of the potential for such costs to mesopredator condition as reefs degrade.

Item ID:	53079
Item Type:	Thesis (PhD)
Keywords:	coral reefs, coral trout, food chains, habitat degradation, mesopredator, Plectropomus maculatus, prey switching, stable isotope
Copyright Information:	© Copyright 2017 Tessa N. Hempson
Date Deposited:	10 Apr 2018 01:42
FoR Codes:	06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes:	97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50% 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 10% 96 ENVIRONMENT > 9603 Climate and Climate Change > 960305 Ecosystem Adaptation to Climate Change @ 40%
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