Competition and coexistence of reef-corals

Álvarez Noriega, Mariana (2019) Competition and coexistence of reef-corals. PhD thesis, James Cook University.

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Reef-building coral assemblages can be very species-rich, but the processes maintaining these levels of biodiversity are largely unknown. While understanding the processes that allow species coexistence has been a challenging task for ecologists for many decades, new methods allow the contributions of some coexistence-promoting mechanisms to be quantified. In this thesis, my overarching aim is to investigate how coral biodiversity is maintained: specifically, how environmentally-induced fluctuations in demographic rates influence coral species coexistence. To do this, I: i) quantify the relationship between size and fecundity for eight species of corals (to obtain parameter estimates for competition models) (Chapter 2); ii) test for trade-offs between competitive ability and demographic rates, and test the effect of competition on colony growth (to determine how to characterize competition in my models) (Chapter 3); and iii) evaluate the role of disturbance in promoting coral species coexistence (Chapter 4).

In Chapter 2, I estimated colony fecundity for eight coral species, using a five-year data set of coral demographic rates from Lizard Island, Great Barrier Reef, Australia. I found that size-dependent fecundity is much more similar between species with a similar morphology than among morphologies. This result suggests that colony morphology can be used as a proxy for coral fecundity, as previous work shows that morphology can also serve as a proxy for growth and survival. Using colony morphology as a proxy for coral demographic rates can help coral ecologists simplify the coexistence problem by considering the coexistence of typical species with different growth forms separately from coexistence of species with the same growth form.

In Chapter 3, I investigated the relationship between competitive ability and four important demographic rates: growth, mechanical stability, colony reproductive investment, and number of eggs per colony, by quantifying the proportion of the colony's periphery in competition in photographs of 30 colonies of each of 11 species followed yearly. Results show a trade-off between competitive ability and mechanical stability. However, there was a negligible effect of competition on colony growth, suggesting that any trade-off involving competitive ability in adult colonies is unlikely to influence species coexistence. This result suggests that if competition affects community dynamics, it does so at life-stages other than adults (for instance, via space-limited recruitment, which is well-documented).

In Chapter 4, I used a competition model to investigate the role of hydrodynamic disturbance in coral species coexistence. To describe coral population dynamics, I used integral projection models calibrated with the fecundity estimates obtained in Chapter 2, as well as with growth, survival, and susceptibility to mechanical disturbance estimates already available. I modelled competition via space-limited recruitment (larvae could settle only on space not occupied by resident (juvenile or adult) corals, regardless of the species of the larvae or resident. The model was calibrated for a population with a (mechanically unstable) tabular morphology and a population with a (mechanically stable) digitate morphology. I compared the results of invasibility analyses (to determine whether each competitor has a positive population growth rate when rare) between model scenarios with stochastic mortality induced by hydrodynamic disturbance, and model scenarios with a constant environment. I found that that coexistence was only possible in the presence of environmental fluctuations. There are two fluctuation-dependent mechanisms that could be responsible for this coexistence: the storage effect and relative nonlinearity of competition. The storage effect operates when competition and environmental conditions have subadditive effects on population growth, allowing species to 'store' benefits gained during favourable conditions, and thereby to persist during unfavourable ones. Relative nonlinearity of competition occurs when each competitor performs best at a different resource level, and the resource fluctuates between levels that favour one competitor, and levels that favour its opponent. A partitioning of the contribution of different coexistence-promoting mechanisms indicated that, of the two classes of fluctuation-dependent coexistence mechanisms, relative nonlinearity contributed positively to coexistence, but the storage effect did not.

Explaining biodiversity maintenance in coral assemblages has been a challenging task for coral ecologists. Few coexistence-promoting mechanisms have been tested for reef corals, and most of them have revealed low potential to contribute to coral biodiversity. Consequently, coral species coexistence has remained an enigmatic problem. Disturbance had been hypothesized to play an important role in coral species coexistence for many decades, but its role had been neither mechanistically characterized, nor the magnitude of its contribution quantified. This thesis reveals how coral biodiversity can be maintained by environmental fluctuations. More specifically, it shows that differences in susceptibility to disturbance among corals with different colony morphology can contribute to coexistence via a mechanism called relative nonlinearity of competition. Furthermore, it shows that environmental fluctuations affecting recruitment cannot promote coral species coexistence unless larvae directly compete against each other. Results shed light on how coral assemblages might respond if disturbance regimes are affected by climate change. Results suggest that predicted changes in the frequency and intensity of hydrodynamic disturbance regimes due to climate change will likely alter competitive dynamics in coral assemblages by increasing the competitive advantage of mechanically robust species.

Item ID: 60798
Item Type: Thesis (PhD)
Keywords: demographic strategies, fecundity, morphology, reef coral, coral reef, tradeoff, coral biodiversity, coexistence
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Copyright Information: Copyright © 2019 Mariana Álvarez Noriega.
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For this thesis, Mariana Alvarez Noriega received the Dean's Award for Excellence 2020, which recognises excellence in Higher Degree by Research and recipients of this award are commended by independent expert examiners as having made a substantial contribution to their field of research.

One publication arising from this thesis is stored in ResearchOnline@JCU, at the time of processing. Please see the Related URLs field. The publication is:

Chapter 2: Álvarez-Noriega, Mariana, Baird, Andrew H., Dornelas, Maria, Madin, Joshua S., Cumbo, Vivian R., and Connolly, Sean R. (2016) Fecundity and the demographic strategies of coral morphologies. Ecology, 97 (12). pp. 3485-3493.

Date Deposited: 03 Nov 2019 23:43
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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