Year

2023

Degree Name

Doctor of Philosophy

Department

School of Earth, Atmospheric and Marine Sciences

Abstract

Climate change is rapidly altering ecosystems on a global scale, and coral reefs are particularly vulnerable to climate-induced disturbances. Coral reefs depend on mutualisms with their foundation species, i.e. corals, and yet most of the literature has focused on their mutualisms with only one type of symbiont (algae). Little is known about how coral-fish mutualisms respond to climatic disturbances, and yet cyclones and heatwaves are increasingly devastating coral reefs. We urgently need to assess how coral-fish mutualisms respond to disturbances as changes in mutualisms have the potential for causing ecosystem-level changes. Yet fish in coral-fish mutualisms have often been overlooked in studies regarding environmental disturbances. There are multiple aspects of the life history, behaviour, and movement of fish that may impact their mutualisms with corals. Here, I investigated (1) whether both symbionts in coral-fish mutualisms respond similarly to climatic disturbances, and (2) what mechanisms from the fish perspective are likely responsible for how coral-fish mutualisms respond to climatic disturbances. I used a model coral-fish mutualism between coral hosts from the genus Acropora and coral-dwelling gobies from the genus Gobiodon in which both organisms provide important benefits for the resilience of each partner. I implemented a comparative approach by investigating multiple goby and coral species encountered in study locations to provide genus-wide understandings of how their coral-goby mutualisms are impacted by climatic disturbances. Particularly important is that gobies can live in social groups and living in groups can improve coral maintenance. Accordingly, first I provided a comprehensive review on how climate change is impacting the sociality of coral reef fish as the sociality of these taxa have only recently been investigated. Studies have shown that climate change affected the habitat and physiology of fishes, and each of these effects impacted their sociality. The review highlighted key changes to the sociality of these fish depending on how corals respond to disturbances, like reduction in coral size, shifts in coral communities, and health of corals. Secondly, I set the scene by monitoring coral-goby mutualisms throughout four extreme disturbances in the northern Great Barrier Reef (GBR): two cyclones and two heatwaves that caused mass bleaching events. In the aftermath and after a few years of recovery, there were more coral species, but corals were almost three times smaller. For gobies though, there were two times fewer coral species, there were fewer gobies, and most corals became absent of gobies when previously most were occupied. Alarmingly, this study highlighted that gobies declined far more than corals and were far slower to recover than their hosts. Finally, I used a combination of observational and manipulative studies to investigate the potential for coral gobies to exhibit plasticity in their host use, sociality, and movement in relation to disturbances. Following the same four extreme disturbances, I found that gobies shifted hosts to the newly abundant coral species. Although exhibiting host plasticity may be an advantage in the short-term, using alternative coral hosts may reduce the fitness of gobies, i.e. their growth rates. I then investigated whether gobies shifted their social tendencies to live in groups or in pairs following these four extreme disturbances in the northern GBR and following a single extreme disturbance in the southern GBR. Gobies no longer lived in groups, rarely in pairs, and primarily lived as solitary individuals after the four disturbances, whereas there was relatively little change in their social tendencies after the single disturbance. This study suggests that if consecutive disturbances become the norm, gobies may continue to decline if they primarily stay solitary as they need to live in pairs to breed. I then completed another study to investigate how predation risk, coral size and health, and number of group members affected the movement of gobies. I translocated gobies in situ into corals with varying sizes, number of individuals, and health. I replicated the study in a relatively undisturbed environment in Papua New Guinea, and in the highly disturbed environment following the four extreme disturbances in northern GBR. Regardless of the disturbance state, gobies preferred to face high costs of predation and did alter their movement based on coral size, health, or number of group members, even when predation risk was higher in disturbed environments. This suggests that gobies do not alter their movement plasticity based on environmental disturbances even though predation risk is heightened. This means that gobies exhibited host and social plasticity, but they did not exhibit movement plasticity to disturbances. I found that each mechanism of plasticity was likely responsible for a reduced recovery potential of gobies compared to their coral hosts. By combining the findings from each chapter of the thesis, I suggest that coral-fish mutualisms are highly vulnerable to climate change as fish experience barriers to recovery via host, social, and movement plasticity. Future conservation strategies should address declines in fish in order to maintain coral-fish mutualisms important for coral health.

FoR codes (2008)

0602 ECOLOGY

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.