Assessment of scale dependent function in reef fish, and application to the evaluation of coral reef resilience

Nash, Kirsty L. (2014) Assessment of scale dependent function in reef fish, and application to the evaluation of coral reef resilience. PhD thesis, James Cook University.

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In response to the anthropogenic pressures affecting ecosystems, and the resultant habitat and community changes these impacts cause, there has been increasing interest in using resilience approaches to study ecosystems. The resilience of a system its capacity to adapt to changing conditions whilst maintaining core processes, and resisting shifts to different regimes. Thus, the resilience concept allows the state of a system to be thought about within a dynamic, ecosystem-based framework. Despite this interest in resilience approaches, quantitative indicators of ecosystem resilience are rarely tested. Understanding and quantifying the functional roles played by species, and thus their importance in driving key ecosystem processes, has been suggested as one approach for quantifying resilience. In the context of coral reefs, herbivory by reef fishes has been identified as an important process controlling algae and supporting coral dominance. As a result, there is an extensive literature characterising how herbivorous species provide their function. However, there has been little evaluation of the spatial scales over which fish perform their functional roles. Knowledge of the scales over which fishes provide their function may be used to develop a broader indicator of resilience: cross-scale redundancy. The cross-scale resilience model, first proposed by Peterson et al. in 1998, suggests that the scale at which an individual provides its function will influence its response to scale-specific disturbances. Thus, the presence of species operating at different scales within a community (cross-scale redundancy), should be a useful indicator of resilience. Implementation and testing of cross-scale redundancy on coral reefs would go some way to addressing the need for empirical testing of resilience indicators.

To test these indicators, this thesis is split into two parts: in Part 1 (Chapter 2-5) I evaluate the spatial scales at which fish interact with the reef and provide their function; in Part 2 (Chapter 6-8) I investigate the application of the cross-scale resilience model in the context of coral reefs. The knowledge developed in Part 1 is essential for assessing the appropriateness of implementing the cross-scale resilience model for reef fish because these chapters test the underlying assumptions of the model used in Part 2.

The relationship between body size and home range provides a useful way of summarising the spatial scales at which communities of fish operate. In Chapter 2 I performed a quantitative review of studies examining home range in reef fishes, and assessed the interspecific relationship between body mass and home range area. Body mass and home range were positively related. Fishes appeared to occupy a smaller area per unit mass than terrestrial vertebrates. When the small home ranges of reef fish are considered in concert with their apparent reluctance to cross open areas, it suggests that reserves aimed at protecting fish biodiversity may be more effective if located across whole reefs as home ranges are less likely to cross reserve boundaries.

Home ranges may include areas that are used for activities such as sleeping, rather than focusing on those locations where the organism is providing the core functions of interest, such as grazing. Therefore, in Chapter 3 I assessed the allometric relationship between small-scale foraging movements and body size for herbivorous reef fishes within the functional groups: browsers, farmers, grazer/detritivores, and scraper/excavators. The relationship between vulnerability of species to fishing and their scale of foraging was also examined. I found evidence of a strong, positive, log linear relationship between scale of foraging movement and fish length. Some functional groups, such as scrapers/excavators, performed their role over a wide range of scales, whereas browsers were represented by few species and operated over a much narrower range of scales. Overfishing is likely to not only remove species operating at large scales, but also the browser group as a whole.

The spatial scales at which fish operate are not only affected by life history traits such as body size, they are also shaped by the habitats available to the individual. In Chapter 4, I assessed the influence of among-site variation in habitat condition on the short-term foraging range of two species of parrotfish. The primary predictor of these foraging movements was coral cover. The study suggests that future changes in coral cover are likely to alter the way reef herbivores forage. Habitat condition may also drive the underlying body size distribution of fish communities. In Chapter 5 I characterized patterns of cross-scale habitat complexity, and examined how this related to body-depth abundance distributions of associated fish assemblages over corresponding spatial scales. I found that reefs formed from different underlying substrata exhibit distinct patterns of cross-scale habitat complexity and this is reflected in the fish body depth distributions.

The second part of the thesis used knowledge generated in Part 1 to test the applicability of Peterson et al.'s cross-scale resilience model on coral reefs. This model was developed from the discontinuity hypothesis, which explores inherent scales of structure within ecosystems. In Chapter 6 I reviewed the conceptual framework underlying discontinuities. The chapter explored the utility of discontinuities for understanding cross-scale patterns by describing recent advances in examining non-linear responses to disturbance, and phenomena such as invasions, and resilience. I detailed outstanding knowledge gaps, in particular pertaining to the implementation of the cross-scale resilience model for taxa with indeterminate growth such as reef fishes.

To address the issue of applying the cross-scale resilience model to species with indeterminate growth, in Chapter 7 I performed a comparison of bird (determinate growth) and fish (indeterminate growth) body mass distributions, assessing the respective suitability of distinct analytical approaches for understanding habitat-size relationships in different ecosystems. I evaluated three size distribution indices: species size relationships, species sizedensity relationships and individual size-density relationships, and two types of analysis: looking for either discontinuities or abundance modes in the distributions. All the indices and analyses were useful for examining the relationship between habitat structure and size for species with determinate growth. In contrast, for species with indeterminate growth, such as fishes, individual size-density relationships were more useful.

Finally, in Chapter 8 I applied the cross-scale resilience model on coral reefs. I assessed the effectiveness of cross-scale redundancy in herbivores as an indicator of response diversity and benthic recovery on reefs monitored through a coral bleaching event. The distribution (redundancy) of herbivores operating across a broader range of spatial scales prior to the bleaching corresponded with increased reef recovery post-disturbance, as proposed by the cross-scale resilience model. Analysis of the change in biomass across size classes indicated that response diversity, with declines in small herbivores and increases in large herbivores, enhanced the overall herbivore biomass at recovering sites.

My research characterized the spatial ecology of reef fish communities, and herbivores in particular. This knowledge was used to underpin the testing of a potential resilience indicator: the cross-scale resilience model; a model that proved to be effective in the context of coral reefs. Critically, my research provides fundamental knowledge regarding the function provided by coral reef fishes, highlighting the spatial scales over which management is needed to support their critical functions. These results will help managers to predict the relative likelihood of different reefs declining or recovering following severe disturbances. Understanding the spatial ecology of fishes may hold the key to the management of coral reef recovery in the future.

Item ID: 41029
Item Type: Thesis (PhD)
Keywords: algal-dominated reef; allometry; Australia; body mass; climate change; climatic factors; competition; coral reef fishes; coral reefs; coral-dominated reef; discontinuity hypothesis; ecosystem function; ecosystem processes; extinction; fisheries; foraging range; function; functional group; functional role; Great Barrier Reef; habitat; herbivores; herbivory; hierarchy theory; invasion; marine ecology; mobility; multiple-scale analysis; nonlinear responses; redundancy; reef degradation; regime shift; resilience; rugosity; structural complexity; textural discontinuity hypothesis
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 3: Nash, Kirsty L., Graham, Nicholas A.J., and Bellwood, David R. (2013) Fish foraging patterns, vulnerability to fishing, and implications for the management of ecosystem function across scale. Ecological Applications, 23 (7). pp. 1632-1644.

Chapter 4: Nash, Kirsty L., Graham, Nicholas A.J., Januchowski-Hartley, Fraser A., and Bellwood, David R. (2012) Influence of habitat condition and competition on foraging behaviour of parrotfishes. Marine Ecology Progress Series, 457. pp. 113-124.

Chapter 5: Nash, Kirsty L., Graham, Nicholas A.J., Wilson, Shaun K., and Bellwood, David R. (2013) Cross-scale habitat structure drives fish body size distributions on coral reefs. Ecosystems, 16 (3). pp. 478-490.

Chapter 6: Nash, Kirsty L., Allen, Craig R., Angeler, David G., Barichievy, Chris, Eason, Tarsha, Garmestani, Ahjond S., Graham, Nicholas A.J., Granholm, Dean, Knutson, Melinda, Nelson, R. John, Nyström, Magnus, Stow, Craig A., and Sundstrom, Shana M. (2014) Discontinuities, cross-scale patterns, and the organization of ecosystems. Ecology, 95 (3). pp. 654-667.

Chapter 7: Nash, Kirsty L., Allen, Craig, Barichievy, Chris, Nyström, Magnus, Sundstrom, Shana, and Graham, Nicholas A.J. (2014) Habitat structure and body size distributions: cross-ecosystem comparison for taxa with determinate and indeterminate growth. Oikos, 123 (8). pp. 971-983.

Date Deposited: 29 Oct 2015 03:48
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050101 Ecological Impacts of Climate Change @ 33%
05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050102 Ecosystem Function @ 33%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 34%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 33%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 33%
96 ENVIRONMENT > 9603 Climate and Climate Change > 960305 Ecosystem Adaptation to Climate Change @ 34%
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