Habitat degradation and competition for resources in coral reef fishes

Boström Einarsson, Lisa (2016) Habitat degradation and competition for resources in coral reef fishes. PhD thesis, James Cook University.

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Habitat loss and degradation are among the most pressing threats to the persistence and diversity of species. They can directly lead to declining abundance through the loss of resources, or indirectly through disruption of important ecological interactions such as competition and predation. Coral reef ecosystems around the globe have experienced a decline in coral cover in the past few decades due to a suite of anthropogenic disturbances. Living corals not only provide the structural foundation in the reef ecosystem, but also critical resources such as food, shelter and suitable sites for reproduction for reef fishes. The loss of live coral often leads to a well-documented decline in reef fishes that associate with the coral reef matrix. However, our understanding of the causes of these declines is limited and the mechanisms are poorly understood. Habitat loss may directly impact on fitness parameters or population processes, or it may influence them indirectly by altering interactions such as competition for resources, successful sheltering from predators and habitat selection. The overall objective of this thesis is to explore how habitat degradation influences competition for resources in a common, habitat associated coral reef fish.

Competition over resources is recognised as a fundamental process in ecology, with important consequences for species coexistence, the distribution of species and the regulation of populations. The role of competition in the ecology of reef fishes has been the topic of debate over several decades. While early research in the 1980's focused on the partition of resources between species (i.e niche partitioning) or the chance colonisation of space (i.e 'lottery hypothesis'), others focused on alternate theories to explain patterns of density dependence (e.g. disturbance, predation, recruitment limitation). Since then, a large body of work has accumulated, with field experiments greatly increasing our understanding of the prevalence and importance of competition in coral reef fish communities. Chapter 2 compiles and synthesises the results of experimental tests of competition and shows that evidence for competition is pervasive, thus confirming its important role in structuring reef fish communities. Competition was found to be important both within and between species, with 72% of intraspecific tests and 56% of interspecific tests demonstrating a demographically significant consequence of competition. Competition within species (intraspecific competition) is likely to be particularly intense, given that individuals of the same species are likely to have a high degree of overlap in their resource requirements. A majority of studies of intraspecific competition explored numerical responses (i.e. survival or abundances) to competition. 59% found a negative effect of increasing conspecifics on their overall survival, while relatively few studies investigated sub-lethal effects of competition. Considering levels of competition depend on the availability of resources, the intensity of competition is likely to increase in response to habitat loss and degradation. However, the review emphasised the paucity of studies which have considered links between competition and resources, and the extent to which habitat loss and degradation alter the effects of competitive interactions are poorly understood.

A species' competitive response to habitat loss may affect multiple demographic parameters, and these effects may occur over different time scales. However, few studies have manipulated resource availability and documented the effects of habitat loss over time while measuring multiple demographic parameters. In Chapter 3 I evaluate the consequences of habitat loss on the abundance, body condition and behaviour of a common coral reef fish over four months following an experimental reduction in the availability of live coral habitat. I identified natural aggregations of Pomacentrus moluccensis sheltering in Acropora coral colonies, and experimentally reduced live coral tissue by exposing 60% of the coral colony to crown-of-thorns starfish. Throughout the four month post-disturbance period, P. moluccensis showed a strong association with the remnant live habitat on treatment colonies, and avoided the recently dead coral habitat. Densities within this live habitat increased following the disturbance, but gradually dropped until they matched those of control colonies, indicating density dependent mortality. Surprisingly, liver samples indicated that individuals on treatment colonies with 60% loss of live coral habitat had a higher body condition than those on control colonies with no habitat loss. Video analyses revealed P. moluccensis on treatment colonies opportunistically feeding on the algal matrix growing on the recently dead coral branches. These results indicate that successful competitors benefit by gaining access to a novel food source along the edge of prime shelter space within live coral. This edge effect allows species with a degree of flexibility in their resource requirements to benefit from living at a habitat boundary. Chapter 3 highlights a species complex response, both positive and negative, to habitat degradation.

While Chapter 3 demonstrated the importance of live coral in promoting the survival of habitat associated fishes, it is still unclear what causes the mortality of less successful individuals. It is commonly hypothesised that fish mortality is increased as a consequence of the loss of shelter space between branches as dead corals become overgrown by algae. In Chapter 4, I tested this hypothesis by quantifying changes in sheltering behaviour of a common damselfish, Pomacentrus moluccensis, following the death of its host coral colony. Recently dead colonies of Acropora were allowed to accumulate algae and invertebrates over a period of five weeks. Groups of P. moluccensis were then placed on either live or dead coral colonies, startled using a visual stimulus, and their sheltering responses compared. Pomacentrus moluccensis stopped sheltering amongst the coral branches immediately following the death of the coral, despite very little change in shelter space. Instead, most individuals swam away from the dead coral into the surrounding water where they were more exposed to predators. I argue that live coral is a necessary cue that elicits the appropriate behavioural sheltering response to potential predators. Findings in Chapter 4 suggest that the disruption of this cue poses a great threat to coral-associated fishes on degrading reefs.

Partial habitat loss clearly results in temporary crowding of reef fishes which may lead to density dependent habitat selection. Individuals are faced with the decision of either joining high density populations crowded into remnant high quality habitat or opting to move to low quality habitat. Chapter 5 investigates how habitat loss influences habitat selection, and ultimately the distribution, of P. moluccensis. In a survey of habitat use on 49 transects along the coral reef crest I found that P. moluccensis adults only chose dead coral colonies when the average density per live coral colony was higher than under natural conditions. These high densities on live coral colonies only occurred on reefs where >50% of colonies on were dead. This suggests that the loss of habitat causes crowding on remnant live coral until some fish start using less preferred dead colonies. I then conducted a choice experiment to investigate if density dependent habitat selection was the mechanism underlying this pattern. When presented with the choice of two colonies, fish were more likely to choose a near empty alternate colony when the other colony was severely crowded with conspecifics. The consequences of this behaviour are likely to be two-fold; first adult fish are forced to inhabit dead coral, and second their presence may encourage juvenile larvae to recruit to this unsuitable habitat if these recruits use conspecific presence as a cue to determine habitat quality. Chapter 5 provides the first example of how habitat loss induces density dependent habitat selection, adding to the growing body of work showing that habitat loss is impacting on critical ecological interactions on coral reefs.

In summary, this thesis has investigated effects of habitat degradation on key ecological processes determining the distribution of reef fishes, competition for resources and their interaction with the coral reef habitat. It showed complex demographic responses to coral loss that include both positive and negative effects. It established that live coral is critical, not just for the structure it provides, but also for eliciting adaptive behavioural responses to the threat of predation. Moreover, this thesis provides the first demonstration of the crowding hypothesis in the marine environment and is the first to investigate how density dependent habitat selection is affected by habitat degradation. The outcomes of this research highlight the importance of living corals in the ecology and behaviour of coral reef fishes, and their complex responses to coral reef habitat loss and degradation.

Item ID: 48560
Item Type: Thesis (PhD)
ISSN: 1616-1599
Keywords: climate change, coexistence, competition, coral declines, coral reef ecology, coral reef fishes, density dependence, habitat degradation, habitat modification, intraspecific competition, Pomacentrus, resource limitation
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Bonin, Mary C., Boström-Einarsson, Lisa, Munday, Philip L., and Jones, Geoffrey P. (2015) The prevalence and importance of competition among coral reef fishes. Annual Review of Ecology, Evolution, and Systematics, 46. pp. 169-190.

Date Deposited: 19 Apr 2017 02:19
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 100%
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