Effects of climate change-induced thermal stress and habitat degradation to the biodiversity and species composition of coral-associated invertebrates

Stella, Jessica (2015) Effects of climate change-induced thermal stress and habitat degradation to the biodiversity and species composition of coral-associated invertebrates. PhD thesis, James Cook University.

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Coral reefs have the highest animal diversity of any ecosystem on the planet, due to the vast number of invertebrate taxa that reside within its matrix. The majority of these taxa are small, cryptic and live in symbiotic relationships with other reef organisms, such as compact branching corals, and are ecologically important to their hosts. A portion of these are obligate users of a small range of coral species, while others are facultative, or generalist, users of a broad range of corals. Specialisation is known to be strongly linked to high rates of extinction during periods of environmental instability. Coral reefs are subject to many disturbances that can destabilise the coral reef community, with lasting effects on biodiversity and ecological functions. Thermal stress induces coral bleaching, which can lead to entire colony mortality and ultimately affect all species that recruit, feed, mate and shelter within the corals' branches. As bleaching events are increasing in frequency and intensity, it is critical to understand how thermal stress and consequent habitat degradation of coral reefs will affect the vast diversity of invertebrates that associate with and rely upon corals for their persistence. Combining field observations and experiments, this study aimed to document the diversity of invertebrates associated with common coral species of the Great Barrier Reef, examine their level of specialisation to particular host corals and to evaluate their responses to thermal stress and host coral degradation via coral bleaching and mortality.

Although the high biodiversity of coral reefs is attributed to the invertebrate groups found there, few studies have documented species diversity and community structure among coral-associated invertebrates and how they might vary among coral species. Chapter 2 examined species richness (diversity) and composition of animals associated with common species of branching corals. One hundred seventy eight nominal species from 12 different phyla were extracted from 4 host coral species. Twenty seven species (15% of all taxa collected) were found on only one of the four different coral species, which may potentially indicate specialisation among host corals. The distinct assemblages on different coral species, and the presence of potential specialists, suggests invertebrate communities will be sensitive to the differential loss of branching coral species resulting from coral reef degradation.

As climate change is driving habitat degradation among coral reefs via coral bleaching and mortality, numerous invertebrate taxa which are closely associated with corals are being threatened with loss of critical resources. Species specialised to a narrow range of host corals will experience greater extinction risks than generalist species, which may exploit a range of habitats. As coral declines, overall diversity may be impacted in one of two ways: either it will decline through the loss of specialised species or be promoted by the increase in habitat heterogeneity via partial coral mortality. Chapter 3 aimed to test these predictions test these predictions by sampling invertebrate assemblages from healthy, bleached and dead corals. Invertebrate diversity on healthy corals was nearly double that found on bleached colonies, but only half that found on predominantly dead colonies. This was explained by the marked decline of obligate species and the proliferation of facultative species. Different coral species were distinguished by their unique assemblages of obligate coral-dwellers, but dead corals supported communities of more random, numerous facultative species. Partial colony mortality (40-60 %) yielded the highest diversity and abundance of both obligate and facultative coral dwellers. However, as colony mortality increased, the community composition shifted from one dominated by a few obligate species to one marked by an abundance of facultative species. This study supports the hypothesis that moderate disturbances and spatial heterogeneity promote reef biodiversity, but phase shifts lead to large-scale coral loss are a major extinction risk for specialised coral-dwelling invertebrates.

Ecological specialization refers to how restricted certain animals are to a niche, as a result of evolutionary trade-offs. Corals represent a critical resource to many reef organisms, some of which have evolved to specialise on particular host corals. Episodes of coral bleaching are increasing both in frequency and intensity, yet the effects of bleaching on coral-reliant species remains poorly understood. Chapter 4 investigated the effects of host-colony bleaching on an obligate coral-dwelling crab during a natural bleaching event affecting 83 % of compact branching corals. Crabs monitored in situ over a 6 week period exhibited a significant decline in density on bleached corals and suffered a 40 % decline in fecundity. Host-colony bleaching also prompted crabs to emigrate and engage in aggressive interactions with crabs occupying healthy hosts, further threatening overall fitness. Decreased densities and clutch sizes, along with increased competitive interactions of symbionts ecologically important to their host corals could potentially result in a population decline of these symbionts with cascading effects on coral health.

Although mutualisms are ubiquitous in nature, our understanding of the potential impacts of climate change on these important ecological interactions is deficient. Chapter 5 examined a thermal-stress related shift from cooperation to antagonism between members of a mutualistic coral-dwelling community. Increased mortality of coral-defending crustacean symbionts was observed in response to experimentally elevated temperatures and host coral bleaching. Strong differential effects occurred among crustaceans as a function of species and sex, due to forceful eviction from the host coral by dominant individuals. Fecundity also suffered a dramatic decline (85 %), which could have deleterious consequences for population sustainability. Elevated temperature altered the fundamental nature of this interaction from cooperation to competition, leading to asymmetrical effects on species and/or sexes, illustrating the importance of evaluating not only individual responses to climate change, but also potentially fragile interactions within and among susceptible species.

In conclusion, this thesis demonstrates that many species of coral reef invertebrates rely on coral as a habitat, with some heavily dependent on a host coral for their fitness and persistence. It also identifies key species that are affected once the coral habitat becomes bleached and/or dies. It highlights effects climate change can have on species interactions, such as changing the nature of mutualisms. The findings of this thesis suggest that many specialist species are exposed to potential extinction should their host coral species decline, with generalist species benefitting from their demise. Overall reef biodiversity is maintained at its highest when disturbance regimes are intermediate both in frequency and severity. It also emphasises the importance not only of live coral, but also of the coral structure in supporting a vast diversity of invertebrates. Degradation of the physical reef structure has the greatest potential to threaten overall reef biodiversity, with implications for critical ecological functions and ecosystem productivity.

Item ID: 47441
Item Type: Thesis (PhD)
Keywords: biodiversity, climate change, community-level effects, competition, cooperation, coral bleaching, coral reefs, coral-associated invertebrates, coral-dwelling animals, global climate change, habitat degradation, habitat preference, habitat specialisation, mutualism, thermal stress
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Stella, J.S., Jones, G.P., and Pratchett, M.S. (2010) Variation in the structure of epifaunal invertebrate assemblages among coral hosts. Coral Reefs, 29 (4). pp. 957-973.

Chapter 4: Stella, J.S., Munday, P.L., and Jones, G.P. (2011) Effects of coral bleaching on the obligate coral-dwelling crab Trapezia cymodoce. Coral Reefs, 30 (3). pp. 719-727.

Chapter 5: Stella, J.S, Munday, P.L., Walker, S.P.W., Pratchett, M.S., and Jones, G.P. (2014) From cooperation to combat: adverse effect of thermal stress in a symbiotic coral-crustacean community. Oecologia, 174 (4). pp. 1187-1195.

Date Deposited: 23 Feb 2017 23:26
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 60%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 40%
SEO Codes: 96 ENVIRONMENT > 9603 Climate and Climate Change > 960305 Ecosystem Adaptation to Climate Change @ 50%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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