Top-down control, trophic interactions, and the importance of predatory fishes on coral reefs

Hall, April Elizabeth (2015) Top-down control, trophic interactions, and the importance of predatory fishes on coral reefs. PhD thesis, James Cook University.

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Predators play critical roles in terrestrial and marine ecosystems, and can be responsible for maintaining balance and stability in ecological communities. Predators may exert top-down control on prey communities through predation on species at lower trophic levels, however the importance of top-down effects in structuring ecological communities has been widely debated by ecologists. Coral reefs contain a high diversity and abundance of piscivorous fishes, which primarily consume smaller fishes. The objective of this thesis was to examine the role of piscivorous fishes in structuring reef fish populations through predator-prey interactions. Focus was given on examining predator-prey interactions at a variety of spatial and biological scales. At large spatial scales, the importance of piscivores in structuring prey communities was examined by comparison of fish assemblages amongst management zones with varying predator densities (Chapter Two). At regional scales, the sub-lethal effects of predators on prey were examined by comparison of the population demographics of a prey species in areas of high and low predator biomass (Chapter Three). These studies were then followed up by a closer examination of predator-prey interactions using manipulative aquarium experiments (Chapters Four and Five).

Removal of apex predators has resulted in a variety of cascading effects in a range of systems, and studying the effects of such extirpations can give insight into the ecological role of predators. On the Great Barrier Reef (GBR), piscivorous fishes are the primary target of both recreational and commercial fisheries, and piscivore densities can be depleted on heavily fished reefs. Marine reserves however, have been effective in protecting and restoring piscivore populations, so there may be great variation in the density and biomass of piscivores between fished and unfished zones. These variations provide a robust experimental template to study to role of predatory fishes on coral reefs. In Chapter Two, the role of piscivores in influencing prey communities was examined by a comparison of fish assemblages in three management zones which represent a gradient of fishing intensity: marine reserves (no fishing), limited fishing zones (moderate fishing intensity) and open zones (highest fishing intensity). Fish counts and habitat surveys were conducted in each management zone at four locations on the GBR. There was great variation in predator biomass amongst zones, and strong evidence of associated prey release in heavily fished areas. The trophic composition of reef fish assemblages varied amongst zones; reefs open to fishing had much lower densities of piscivores, and higher densities of prey and herbivorous fishes compared to marine reserves.

In Chapter Three, the lethal and sub-lethal effects of predators were examined at a regional scale, by comparing the population demographics of the prey species Scolopsis bilineatus amongst management zones at the Palm Islands. The biomass of predators varied greatly between marine reserves and fishes zones at this location, as described in Chapter Two. For Scolopsis bilineatus, despite no variation in numerical abundance and mortality rates, there were strong differences in a variety of demographic traits for S. bilineatus between multiple areas of high and low predator biomass. These sub-lethal effects were sex dependant, and impacted females more strongly than males. Chapters One and Two, therefore, highlight the importance of predators in influencing prey communities on coral reefs, through lethal and sub-lethal top-down effects.

Coral reef fishes may be vulnerable to both predation and competition during the early life stages, and these processes may interact to influence mortality, growth and behaviour. Chapter Four compared the relative and interacting effects of competition and predation on two competing species of small damselfish; Pomacentrus amboinensis and P. moluccensis, which both exhibited patterns of prey release in heavily fished areas from Chapter Two. Using a multifactorial experiment, fish were subjected to the sight and smell of a known predator (Pseudochromis fuscus), the presence of the heterospecific competitor, or a combination of the two. Both predation and competition impacted the growth and behaviour of prey, and the presence of the predator tended to exacerbate competitive effects. There were strong differences between species according to dominance hierarchies, and subordinate fish suffered greater reductions in growth compared to dominant fish. These data highlight the importance of predator/competitor synergisms in influencing key behaviours and demographic parameters for juvenile coral reef fishes.

Chapter Five examined the physiological responses of predators to prey, and explored the mechanisms underpinning the behavioural response of prey to a predator. I used intermittent flow respirometry to demonstrate a strong metabolic response of P. amboinensis to visual predator cues. P. amboinensis had elevated metabolism for a 24 hour period when faced with a predator, but metabolism was not elevated when presented with an olfactory cue only, or when presented with non-predatory fish. These data highlight the energetic costs associated with predator-prey interactions, and demonstrate the capacity for prey to discriminate between predatory and nonpredatory fish, and respond accordingly. Outcomes from Chapters Four and Five demonstrated that behavioural and physiological mechanisms may underpin the response of prey to a predator, and this can ultimately scale up to the variations in abundance and demography observed in previous chapters.

In conclusion, this thesis provides a detailed examination of the importance of predator-prey interactions on coral reefs, and highlights the important role that predators play in regulating prey at multiple spatial and biological scales. These data are relevant to conservation and management of coral reefs, and reinforce the importance of preserving and restoring top-down trophic interactions in ecological systems.

Item ID: 44644
Item Type: Thesis (PhD)
Keywords: coral reefs; fish assemblages; fish communities; marine ecology; marine ecosystems; predation; predator-prey interactions; predator-prey relationships; predators; predatory fish; prey species; preying; reef fishes; trophic levels
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Boaden, A.E., and Kingsford, M.J. (2015) Predators drive community structure in coral reef fish assemblages. Ecosphere, 6 (4). pp. 1-33.

Date Deposited: 11 Aug 2016 04:36
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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