Functional niche partitioning in herbivorous coral reef fishes

Brandl, Simon Johannes (2016) Functional niche partitioning in herbivorous coral reef fishes. PhD thesis, James Cook University.

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Ecological niche theory predicts that the diversity of life hinges on differences in the ways in which species exploit available resources, i.e. their ecological niche. An organism's niche can be divided into the fundamental and the realized niche. The fundamental niche refers to a species' inherent potential capabilities (based on, for instance, its morphology), while the realized niche represents a species' behaviour when it can interact with the biotic and abiotic components of its environment. Tropical coral reefs stand out as one of the most diverse ecosystems on Earth, but patterns of niche partitioning among coral reef organisms are poorly understood. Herbivorous coral reef fishes are frequently considered to perform one of the most critical ecosystem processes on coral reefs, the removal of algal and detrital material from the reef substratum. However, while several classification schemes have previously been employed to characterize the functional role of different herbivores, detailed holistic investigations of niche differences among herbivore species are rare. Therefore, in this thesis, I use social, morphological, behavioural, and environmental factors to disentangle the functional niches of herbivorous fishes on coral reefs.

One of the most prominent social systems in animals is the association between two individuals, commonly termed a 'pair'. However, given that sexual reproduction in higher animals commonly requires the association between two partners, reproductive connotations frequently override the social-ecological benefits that a partner can provide and thus, the potential effects of pairing on an animal's functional niche. In teleost fishes, pairing is common but little is known about the potential drivers and consequences of this behaviour. My first objective, therefore, was to quantify the extent of pair-formation in coral reef fishes and to examine potential ecological/reproductive correlates of pair-formation. Of a total of 1,981 species of Indo-Pacific reef fishes, 341 (17.2%) are reported to form pairs, with pairforming species being particularly common (more than 50% of species) in five families. Pair forming species had few commonalities with regards to their reproductive strategies. Instead, two ecological traits appear to be prevalent in pairing species: 1) the consumption of small, benthic, and relatively immobile prey items, and 2) the maintenance of permanent burrows. Based on these results, I conclude that pairing may have important ecological benefits in coral reef fishes.

To further explore this hypothesis, I performed an ecomorphological assessment of pair-forming fishes, evaluating whether pairing fishes are also morphologically similar. Using a suite of six morphological traits, predominantly describing the cranial region and overall body shape of reef fishes, I demonstrate that morphology can accurately predict the prevalence of pairing behaviour in 47 species of benthos-feeding, reef fishes and that there is a strong relationship between morphology and pairing behaviour in three common families of reef fishes, the Acanthuridae, Chaetodontidae, and Siganidae. Basically, pair-forming fishes are characterized by concave foreheads, pointed snouts, deep bodies, and large eyes, attributes that relate to the feeding on small prey in topographically complex environments.

I then sought to examine specific benefits of pair-forming behaviour using four species of pairing rabbitfishes as model organisms. In doing so, I revealed evidence for a reciprocal cooperative system in rabbitfishes. In all four species, vigilance was strongly coordinated between pair members, with one individual assuming a head-up position elevated vertically above the substratum, while the partner was feeding, often in small cracks and crevices. Pair members altered their positions more often than one fish feeding continuously. Furthermore, compared to individual fishes, fishes in pairs exhibited longer vigilance bouts, more bites per foray, and deeper penetration of the substratum. By evaluating these findings against a set of six requirements for reciprocal cooperation, I show that the observed behaviour in rabbitfishes may be based on the reciprocal exchange of food and safety between pair members and that the presence of a partner permits the foraging in concealed microhabitats, unavailable for solitary fishes.

Based on these findings, I examined the fine-scale utilization of foraging microhabitats in herbivorous coral reef fishes. To do so, I introduced a novel individual-based analysis of functional niche overlap in animal communities, which uses convex hull volumes in order to assess niche breadth, overlap, and turnover vs. nestedness in multiple dimensions. Using this analysis on in situ behavioural data of the foraging behaviour of 21 species of herbivorous reef fishes, I show that there is limited functional redundancy among reef fish species, with an average niche overlap of only 15.2%. I further reveal a clear distinction between species that utilize predominantly flat, exposed surfaces of sand or bare rock, and species that feed on a wide range of different microhabitats, including cracks and crevices in dead corals and other complex substrata. Thus, the utilization of foraging microhabitats appears to represent an important axis of niche partitioning in herbivorous coral reef fishes.

Using the family Acanthuridae as a study subject, I then quantitatively explored ecological specialization in reef fishes with regards to both fundamental and realized niches. Specifically, I examined ecological specialization in ten surgeonfish species with regards to morphology and two realized niche axes associated with diet and foraging microhabitat utilization, and investigated the relationships between morphological and behavioural specialization. These relationships differed markedly from the traditional ecomorphological paradigm. While morphological specialization showed no relationship with dietary specialization, it exhibited a strong relationship with foraging microhabitat specialization. However, this relationship was inverted: species with specialized morphologies were microhabitat generalists while generalized morphotypes were microhabitat specialists. Thus, I highlight the potential importance of including niche axes beyond dietary specialization into ecomorphological frameworks and suggest that, on coral reefs, morphotypes commonly perceived as most generalized may, in fact, be specialized in exploiting flat and easily accessible microhabitats.

To investigate the role of concealed microhabitats for benthic organisms, I then performed an observational study on the distribution patterns of juvenile corals and early lifestage macroalgae. Specifically, I examined the microhabitat occupation of juvenile acroporid and pocilloporid corals and early life-stages of the macroalgae Turbinaria ornata, revealing that both corals and macroalgae were more prevalent in concealed microhabitats when compared to open or semi-concealed microhabitats. Corals were more common on the reef crest, while macroalgae were more abundant on the reef flat. The distribution patterns of herbivorous fishes also showed a distinct difference between the flat and crest, with species utilizing concealed microhabitats being more common on the crest. In particular, high abundance of pairing rabbitfishes and surgeonfishes of the genus Zebrasoma were positively related to high numbers of juvenile corals. Overall, this study suggested that coral reef crevices might be an important microhabitat for corals and algae alike and that the feeding activity of herbivorous fishes capable of feeding in crevices may influence this relationship.

Finally, I experimentally investigated the effects of micro-topographic refuges on grazing dynamics on coral reefs. Micro-topographic refuges decreased overall grazing pressure more than ten-fold and permitted access to only few species of cropping herbivores. In contrast, grazing pressure from detritivorous fishes was virtually absent, therefore precluding an entire ecosystem-process, the removal of particulates. As a consequence, benthic communities differed markedly between microhabitats, with micro-topographic refuges exhibiting longer algal filaments and a more diverse community, including scleractinian corals, which were completely absent from exposed microhabitats. Thus, although occurring on the scale of a few centimetres, micro-topographic refuges appear to generate fundamentally different dynamics between grazing fishes and the benthos.

Overall, the results of this thesis demonstrate that herbivory on coral reefs is not a uniform process, but a multifaceted interplay between morphological, behavioural, social, and environmental factors. This thesis has begun to disentangle the complex differences among herbivorous fish species, their drivers, and the consequences of their feeding behaviour for coral reefs. However, the results of this work suggest that micro-topographical complexity on the scale of a few centimetres represents an important ecological axis, along which fishes appear to partition their functional niches. Future research may focus on examining foraging micro-habitat utilization in reef fishes with an even higher resolution, the effects of reducing the grazing pressure exerted by crevice-feeding fishes, and the long-term consequences of the loss of functional diversity in herbivorous fishes on diversity patterns in the coral reef benthos.

Item ID: 45253
Item Type: Thesis (PhD)
Keywords: behavior; behaviour; coral reef fish; ecological niches; feeding; herbivorous fish; marine ecology; morphology; niche differentiation; niche partitioning; pair bonds; pair formation; pair forming
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Brandl, Simon J., and Bellwood, David R. (2014) Pair-formation in coral reef fishes: an ecological perspective. Oceanography and Marine Biology: an annual review, 52. pp. 1-80.

Chapter 3: Brandl, S.J., and Bellwood, D.R. (2013) Morphology, sociality, and ecology: can morphology predict pairing behavior in coral reef fishes? Coral Reefs, 32 (3). pp. 835-846.

Chapter 4: Brandl, Simon J., and Bellwood, David R. (2015) Coordinated vigilance provides evidence for direct reciprocity in coral reef fishes. Scientific Reports, 5.

Chapter 5: Brandl, Simon J., and Bellwood, David R. (2014) Individual-based analyses reveal limited functional overlap in a coral reef fish community. Journal of Animal Ecology, 83 (3). pp. 661-670.

Chapter 6: Brandl, Simon J., Robbins, William D., and Bellwood, David R. (2015) Exploring the nature of ecological specialization in a coral reef fish community: morphology, diet and foraging microhabitat use. Proceedings of the Royal Society of London Series B, Biological Sciences, 282 (1815). pp. 1-10.

Chapter 7: Brandl, S.J., Hoey, A.S., and Bellwood, D.R. (2014) Micro-topography mediates interactions between corals, algae, and herbivorous fishes on coral reefs. Coral Reefs, 33 (2). pp. 421-430.

Chapter 8: Brandl, Simon J., and Bellwood, David R. (2016) Microtopographic refuges shape consumer producer dynamics by mediating consumer functional diversity. Oecologia, 182 (1). pp. 203-217.

Date Deposited: 30 Aug 2016 05:37
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050102 Ecosystem Function @ 33%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 34%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 33%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960503 Ecosystem Assessment and Management of Coastal and Estuarine Environments @ 33%
96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 33%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 34%
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