Controlling mesopredators: importance of behavioural interactions in trophic cascades

Palacios Otero, Maria del Mar (2017) Controlling mesopredators: importance of behavioural interactions in trophic cascades. PhD thesis, James Cook University.

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Trophic cascades illustrate the role of predation on ecological communities. In the absence of top-down control from apex predators, mid ranking species can drastically increase their abundance and foraging activity, creating stronger impacts on bottom prey populations. The consequences of such mesopredator release are difficult to predict, as intraguild predator-predator interactions can alter the impact of mesopredators on prey. Top-predators, through risk effects, can elicit antipredator responses that limit the behaviour and foraging activity of mesopredators. Moreover, positive (e.g., cooperative hunting) or negative behavioural interactions (e.g., interference) among mesopredators can modify each other's feeding rate, and hence any expected impact on prey. Although trophic cascades are of major concern for wildlife conservation and management purposes, there is limited understanding of the behavioural links in food webs and how they may influence ecosystem dynamics.

This thesis examined how behavioural interactions throughout the food web mediate the impacts (lethal and sub-lethal) of mesopredators on their prey. By using a model food chain of coral reef fishes [coral trout (Plectropomus leopardus) > dottybacks and groupers (Pseudochromis fuscus and Cephalopholis boenak, Epinephelus maculatus) > juvenile damselfish (Pomacentrus spp.)], this research provided key insights on the cascading consequences of top-predator loss on coral reefs. The thesis consists of several field and laboratory experiments that manipulated the lethal and sub-lethal effects of predation to examine the implications for the predators and their prey.

Fishes use visual and chemical stimuli to assess predation risk and develop antipredator responses. However, it remains unknown whether these cues in isolation are sufficient to induce behavioural changes in mesopredator-prey interactions. Hence, in Chapter 2, I examined how different combinations top-predator cues (visual, chemical, or both) alter mesopredator behaviour and modify their impact on juvenile damselfish prey. Laboratory results showed that even the independent sight or smell of a top-predator can strongly restrict the distance swum, area explored and foraging activity of mesopredators. In doing so, this mesopredator restriction indirectly triggered a behavioural release (e.g., increased their space use and activity) of their damselfish prey.

To further examine the impact of behavioural trophic cascades on juvenile fishes I tested the consequences of predation risk under natural field settings (Chapter 3). Using experimental patch reefs, I assessed the behaviour and survival of juvenile fishes placed on patches where cues from a top-predator fish (visual and chemical stimuli) or controls (PVC pipe or empty patch) had been deployed. After exposure to top-predator cues for 72 h, damselfishes from top-predator patches had significantly higher space use (46 %), feeding rate (95 %) and survival (67 %) than those from control patches. This study demonstrated that predation risk from large-sized predators can indirectly favour the persistence and behaviour of bottom level prey in the field.

In Chapter 4, I extended the results from Chapters 2 and 3 by examining whether cascading effects from top-predators could influence other phenotypic traits of bottom prey, such as physiology. Using intermittent-flow respirometry in a laboratory setting, I measured the oxygen (O2) uptake (a proxy for aerobic metabolism) of juvenile fish exposed to a combination of mesopredator and top-predator cues. Interestingly, results suggested that top-predators can benefit bottom prey physiologically by indirectly allowing them to maintain routine O2 uptake, minimising their metabolic stress. This study illustrated for the first time that top-predators can indirectly negate the effect of mesopredators on prey metabolic rates.

Each mesopredator species can impact prey differently. Moreover, their combined effects cannot typically be predicted by their independent contributions. Thus, in Chapter 5, I tested for multiple predator effects, exploring how intra- and inter-specific interactions among three mesopredator species influenced their combined lethal impact on bottom prey survival. By manipulating the density and diversity of mesopredators in a mesocosm experiment, I found that behavioral patterns within mesopredator pairs often matched those predicted from their hunting mode. However, the identity of the mesopredator species determined the strength of any positive or negative interactions (e.g., synergies, interference), and thus the nature and magnitude of MPEs on prey survival (i.e., risk-enhancing effects, risk-reducing effects, linear effects). Given the context-dependency of the predator-predator interactions, none of the mesopredators tested are expected to play identical functional roles on natural coral reefs.

The overarching aim of this thesis was to establish the importance of behavioural interactions in the control of mesopredators. This was achieved by conducting field and laboratory manipulations of a three-level food web of coral reef fishes. Results indicated that lethal and sub-lethal impacts of mesopredators on prey can be largely diminished by predation risk from top-predators (Chapters 2, 3, 4). However, intra- and inter-specific interactions with other mesopredators most often enhance prey mortality (Chapter 5). From a conservation perspective, these findings illustrate how a predatory-release of mid-ranking species could impair the replenishment, fitness, and development of newly-settled fish, and thus indirectly modify the structure and diversity of reef fish communities. From an ecological point of view, these findings highlight the role and importance of behavioural interactions in the balance of complex food webs.

Item ID: 49909
Item Type: Thesis (PhD)
Keywords: behavioural interactions, coral reef fish, mesopredators, metabolic rate, non-consumptive effects, predator-prey interactions, respirometry, sensory cues, trait-mediated indirect effects
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: del Mar Palacios , Maria, Warren, Donald T., and McCormick, Mark I. (2016) Sensory cues of a top-predator indirectly control a reef fish mesopredator. Oikos, 125 (2). pp. 201-209.

Chapter 4: Palacios, Maria M., Killen, Shaun S., Nadler, Lauren E., White, James R., and McCormick, Mark I. (2016) Top predators negate the effect of mesopredators on prey physiology. Journal of Animal Ecology, 85 (4). pp. 1078-1086.

Date Deposited: 21 Aug 2017 05:01
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 50%
06 BIOLOGICAL SCIENCES > 0608 Zoology > 060801 Animal Behaviour @ 25%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 25%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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