Trophic impact of large predators on coral reefs and management implications

Rizzari, Justin R. (2015) Trophic impact of large predators on coral reefs and management implications. PhD thesis, James Cook University.

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Abstract

Apex predators such as lions, wolves and orcas occupy the top of food chains and are known to exert strong top-down control of prey abundance, community structure and ecosystem productivity. Despite the importance of apex predators in both terrestrial and oceanic habitats, there is a paucity of information on this guild for coral reef ecosystems. Sharks are among the largest predators on coral reefs and may function as apex predators, but our understanding of the ecological role and importance of these organisms is poor. Recent declines in shark populations are also leading to concerns that these potentially important species may be effectively lost from reef systems before we realise their role in ecosystem dynamics. Thus, the overall focus of this thesis was to examine the status, trophic role and importance of large predators, namely reef sharks, on coral reefs, thereby providing management with improved tools for understanding how the removal of predators via fishing influences ecosystem properties.

The high mobility and generally low abundances of reef sharks has led to considerable debate regarding the optimal method/s for assessing their population status. Due to controversy surrounding previous stock assessments, the focus of Chapter 2 was to evaluate potential biases in five survey methods (timed-swim, towed-diver, baited-remote-underwater- video, stationary-point-count, and audible-stationary-count) for assessing reef shark populations, and whether these biases (if any) vary among reefs that are subject to different levels of human interaction due to conservation zoning. For timed-swim, towed-diver, and baited-remote-underwater-video (BRUV) surveys, shark encounter rates were constant over time, suggesting minimal bias caused by, for example, shark attraction to divers. For audible-stationary-count (ASC) surveys, encounter rates were elevated initially, then decreased rapidly, but the extent of upward bias did not differ between management zones. Timed-swim, BRUV, and ASC surveys produced comparable estimates of shark density, however, towed-diver-surveys produced significantly lower estimates of shark density. These findings provide no evidence for biases in diver-surveys: encounter rates with sharks were not elevated when divers first entered the water; behavioural responses of sharks were consistent across management zones; and diver-surveys yielded abundance estimates comparable to stationary non-diver surveys. These results indicate that estimates of abundance generated by diver-surveys such as underwater visual transects are reliable, minimally biased, and suitable for assessing shark populations in the field.

Quantifying the distribution and habitat use of sharks is critical for estimating their population sizes, understanding their ecological role, and for establishing appropriate conservation and management regimes. The goal of Chapter 3 was to survey shark populations across outer-shelf reefs of the Great Barrier Reef (GBR) in order to determine the diversity, abundance, and distribution of reef sharks across three major coral reef habitats: (1) the reef slope, (2) the back reef and (3) the reef flat. Results revealed that, of the variables that were examined, habitat was the best predictor of shark distribution and abundance. Specifically, overall shark abundance and diversity were significantly higher on the reef slope (and to a lesser degree, the back reef) than the reef flat. These results confirm that shark populations are not homogeneously distributed across coral reefs and also highlight the potential importance of the reef slope, with high levels of live coral cover, structural complexity, and diversity of potential prey species for sustaining reef shark populations.

The goal of Chapter 4 was to examine the top-down influence of reef sharks and mesopredators (predatory teleost fishes) on herbivores across a large spatial gradient. The abundance, size structure, and biomass of apex predators, mesopredators, and herbivores across fished, no-take, and no-entry management zones of the GBR were estimated. Shark abundance and mesopredator size and biomass were higher in no-entry zones than in fished and no-take zones, which indicates the viability of strictly enforced human exclusion areas as tools for the conservation of predator communities. However, changes in predator populations due to protection in no-entry zones did not have a discernible influence on the density, size, or biomass of different functional groups of herbivorous fishes. The lack of a relationship between predators and herbivores suggests that top-down forces may be much weaker than previously thought and that food web models developed for terrestrial systems may not be appropriate for describing reef communities across large spatial scales.

Using life-like models of a reef shark and another large predatory fish (coral-grouper), Chapter 5 examined the indirect effects of large predators on herbivore foraging behaviour and algal consumption. This study found that the presence of a model blacktip reef shark (1.7m length) or large coral-grouper (76 cm length) led to a substantial reduction in bite rate and species richness of herbivorous fishes feeding on a macroalgal thallus in the vicinity of the predator. The perceived risk of predation led to a near absolute localized cessation of macroagal removal. A smaller-sized model coral-grouper (48 cm length) also reduced herbivore diversity and activity but to a lesser degree than the larger model predators. These results highlight that the indirect effects of large predators on the foraging behaviour of herbivores may have flow-on effects on the biomass and distribution of macroalgae on a localized scale. Combined with the results of Chapter 4, this highlights that the ecological interactions and processes that contribute to ecosystem resilience may be more complex than previously assumed.

Due to their large size and conspicuous predatory behaviour, reef sharks are typically assumed to be apex predators on coral reefs, but this title is tenuous because little is known about their trophic ecology and functional role. In Chapter 6 stomach contents and stable isotope ratios (δ13Carbon and δ15Nitrogen) were used to estimate diet, trophic level and sources of primary production of three reef sharks (Triaenodon obesus, Carcharhinus melanopterus and Carcharhinus amblyrhynchos) from the GBR. Their assumed functional roles as apex predators were evaluated by qualitative and quantitative comparisons with other sharks and large predatory fishes. It was found that reef sharks do not occupy the apex of coral reef food chains, but instead appear to have functional roles similar to those of large predatory fishes, which are typically regarded as high-level mesopredators. These findings suggest that a high degree of functional redundancy may exist within this guild of large predators, potentially explaining why shark-induced trophic cascades are rare or subtle in coral reef ecosystems. Furthermore, these results indicate that reef sharks should be reassigned to an alternative trophic group such as mesopredators. This change will increase the accuracy of ecosystem models, providing resource managers with improved tools for understanding how reef communities function and how removal of predators might affect ecosystem properties.

The current body of work has advanced our understanding of reef sharks, particularly in terms of their response to protection from fishing, spatial distribution, behaviour towards divers and their functional roles on coral reefs. This thesis demonstrates that reef sharks respond positively to protection from fishing but that the flow-on effects to lower trophic levels are variable and inconsistent with theory surrounding trophic cascades. These advances will be important for improving ecosystem-based management strategies that aim to conserve not only individual species, but also their functions and interactions. Such an approach is paramount given that coral reefs around the globe face ever-intensifying human-induced threats such as overfishing and climate change.

Item ID: 43785
Item Type: Thesis (PhD)
Keywords: alpha predators; apex predators; conservation; coral reefs; ecological niche; ecological role; ecology; effect; impact; influence; large predators; protection; reef management; reef sharks; trophic levels; trophic status; trophic
Additional Information:

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

Chapter 2: Rizzari, Justin R., Frisch, Ashley J., and Connolly, Sean R. (2014) How robust are estimates of coral reef shark depletion? Biological Conservation, 176. pp. 39-47.

Chapter 3: Rizzari, Justin R., Frisch, Ashley J., and Magnenat, Katalin A. (2014) Diversity, abundance, and distribution of reef sharks on outer-shelf reefs of the Great Barrier Reef, Australia. Marine Biology, 161 (12). pp. 2847-2855.

Chapter 4: Rizzari, Justin R., Bergseth, Brock J., and Frisch, Ashley J. (2015) Impact of conservation areas on trophic interactions between apex predators and herbivores on coral reefs. Conservation Biology, 29 (2). pp. 418-429.

Chapter 5: Rizzari, Justin R., Frisch, Ashley J., Hoey, Andrew S., and McCormick, Mark I. (2014) Not worth the risk: apex predators suppress herbivory on coral reefs. Oikos, 123 (7). pp. 829-836.

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Date Deposited: 18 May 2016 02:12
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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