A functional analysis of herbivory on Ningaloo Reef, Australia

Johansson, Charlotte Lina (2012) A functional analysis of herbivory on Ningaloo Reef, Australia. PhD thesis, James Cook University.

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Roving herbivorous fishes play a key role in controlling the biomass of benthic algae on coral reefs. These fishes are examined here to provide insights into how the composition of herbivorous species and their ecological functions contribute to ecosystem resilience in a relatively un-impacted reef system, Ningaloo Reef in Western Australia. Atypical in the fact that it has very low levels of terrestrial run-off and or anthropogenic disturbance, this reef system provides a rare opportunity to study herbivory in its natural state. However, Ningaloo Reef has a very high natural presence of both macroalgae and sea urchins, both of which are often cited as being detrimental to some reef systems and key indicators of degraded coral reef systems. Between 2009-2012, visual censuses were used to quantify the distribution of these two indicators of reef decline, alongside those of four functional groups of herbivorous fish (browsers, excavators, grazers and scrapers), to provide a better understanding of the functional role that vertebrate and invertebrate herbivores play on coral reefs.

Extensive macroalgal beds present in the sandy lagoon and high densities of the sea urchin Echinometra mathaei recorded from the nearby reef slope were significant features of Ningaloo Reef. A marked cross-shelf variation in the composition of herbivorous fish functional groups was also found, with only the back reef and the reef slope supporting fish populations with high grazing rates (completely scraped every 43 and 59 days, respectively). To assess the relative importance and contribution of herbivorous fishes and urchins to grazing and bioerosion, feeding rates were quantified. Rates of bioerosion by fishes were low (1 and 2.3 kg m⁻² yr⁻¹ of CaCO₃) compared to those seen on the Great Barrier Reef, reflecting the lack of large excavating parrotfishes. However, the sea urchin E. mathaei, had an annual erosion capacity on the reef slope close to that of the most abundant excavating parrotfish, Chlorurus sordidus (0.4 and 0.54 kg m⁻² yr⁻¹ of CaCO₃, respectively), indicating that herbivory at Ningaloo Reef is divided between urchins and fishes. High, apparently natural, urchin and macroalgal abundances on this relatively intact reef system highlight the need for caution when using these metrics for evaluating the condition of a reef ecosystem.

To investigate why Ningaloo Reef has such high densities of sea urchins, diurnal predation events on tethered E. mathaei were recorded using stationary underwater video cameras. Despite the densities of E. mathaei varying from 0.1 individuals 100m⁻² in the lagoon to 219.7 individuals 100m-2 on the reef slope, the density and biomass of potential sea urchin predators did not differ among habitats. However, the predator assemblage on Ningaloo Reef differed markedly from those of other global regions suggesting that the composition of available urchin predators could be critical in determining the level of top-down control. Unlike many other studies on East African and Indo-Pacific reefs where balistids are dominant and active predators on sea urchins, labrids were the dominant sea urchin predators on Ningaloo Reef. The fact that labrids are the dominant predator in this system appears to reflect a lack of large balistid predators, suggesting that the predatory composition as well as bottom-up factors (e.g., water flow, food availability and habitat structure) may control densities and determine among-habitat distributions.

The effect of coral cover on herbivorous fishes was explored for each functional group over three size classes. Small excavating and scraping parrotfishes were found to have a strong dependency on live coral cover on the physically demanding exposed slope. Since coral tissue is not a part of the diets of small parrotfishes, this close association may instead allow these small and comparatively weaker swimmers to subsist and forage in strong water flows and avoid predation by utilizing the increased structural complexity of corals. In contrast, small schooling grazers on the slope were not constrained in the same manner, instead preferring areas with low levels of coral cover. While similar benefits to those provided by coral structure are gained by schooling, grazers can explore adjacent parts of the reef where the competition from coral promotes algal biomass. Although these contrasting relationships were evident on the slope, they were absent from protected back reefs. They were also restricted to the small size classes. Significant loss of coral cover may therefore be particularly important for small non-schooling fishes, especially in high energy habitats.

Species richness, generic richness, abundance and reserve capacity (the capacity of the remaining species to functionally compensate for the loss of the most abundant species) were used to examine the functional redundancy of herbivorous functional groups across habitats on Ningaloo Reef and a comparison was made with the Great Barrier Reef. Marked variations in the composition of herbivorous functions were found among habitats. Despite extensive management and limited fishing, coastal habitats in both reef systems had particularly low levels of herbivory in all the above metrics compared with offshore habitats, suggesting that coastal habitats are less likely to resist disturbances. By utilising these four metrics, which can easily be applied to any fish abundance dataset, this approach provides a simple yet robust estimate of the potential capacity of functional groups and habitats to cope with change, and further highlights the spatial variation in potential sensitivity to declining reef health.

High densities of sea urchins and extensive macroalgae beds are present on Ningaloo Reef. However, these two commonly cited indicators of reef degradation appear to represent a natural state at Ningaloo Reef, highlighting the unique configuration of this system. However, with fewer species and genera, as well as lower abundances and reserve capacity, the inshore coastal habitats of Ningaloo Reef and the Great Barrier Reef appear to be more vulnerable to disturbance than offshore reefs as far as herbivory is concerned. This highlights the sensitive nature and importance of herbivory in the macroalgal dominated lagoon at Ningaloo Reef. Furthermore, high densities of sea urchins on the reef slope and the strong association of smaller size-classes of excavators and scrapers to live coral cover identify a further level of vulnerability at Ningaloo Reef. Despite some signs of vulnerability, in terms of herbivorous processes, Ningaloo Reef appears to be a unique and relatively healthy coral reef ecosystem.

Item ID: 34423
Item Type: Thesis (PhD)
Keywords: browsers; coral reef fish; coral reefs; ecological functions; ecology; excavators; fish communities; functional groups; grazers; herbivores; herbivorous fishes; herbivory; macroalgae; Ningaloo Reef; scrapers; sea urchins; WA; Western Australia
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Johansson, C.L., Bellwood, D.R., and Depczynski, M. (2010) Sea urchins, macroalgae and coral reef decline: a functional evaluation of an intact reef system, Ningaloo, Western Australia. Marine Ecology Progress Series, 414. pp. 65-74.

Chapter 3: Johansson, C.L., Bellwood, D.R., Depczynski, M., and Hoey, A.S. (2013) The distribution of the sea urchin Echinometra mathaei (de Blainville) and its predators on Ningaloo Reef, Western Australia: the implications for top-down control in an intact reef system. Journal of Experimental Marine Biology and Ecology, 442. pp. 39-46.

Chapter 4: Johansson, C. L., Bellwood, D. R., and Depczynski, M. (2012) The importance of live coral for small-sized herbivorous reef fishes in physically challenging environments. Marine and Freshwater Research, 63 (8). pp. 672-679.

Chapter 5: Johansson, C.L., van de Leemput, I.A., Depczynski, M., Hoey, A.S., and Bellwood, D.R. (2013) Key herbivores reveal limited functional redundancy on inshore coral reefs. Coral Reefs, 32 (4). pp. 963-972.

Date Deposited: 19 Sep 2014 20:32
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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