The functional diversity and redundancy of corals

McWilliam, Mike (2018) The functional diversity and redundancy of corals. PhD thesis, James Cook University.

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Abstract

Corals are major contributors to a range of key ecosystem functions on tropical reefs, including calcification, photosynthesis, nutrient cycling and the provision of habitat structure. The abundance of corals is declining at local and regional scales, and the species composition of assemblages is responding to escalating human pressures, including anthropogenic global warming. An urgent challenge is to understand the functional consequences of these shifts in abundance and composition in different biogeographical contexts. To address this problem, I develop and analyse a series of coral traits to quantify the trait-based dissimilarity (functional diversity) and similarity (functional redundancy) of corals using multidimensional trait space. This thesis is focused on (i) biogeographical patterns in the functional diversity and redundancy of corals, (ii) how these patterns are changing in response to anthropogenic pressures, and (iii) the implications of these changes for the biodiversity and functioning of coral assemblages.

Biogeographical patterns of coral species richness are well known. However, the biogeography of coral functions in provinces and domains with high and low redundancy is poorly understood. My first objective was therefore to quantify the functional traits of all currently-recognized zooxanthellate coral species (n = 821) in both the Indo-Pacific and Atlantic domains, to examine the relationships between species richness and the diversity and redundancy of functional trait space. I found that trait diversity was conserved (> 75% of the global total) along latitudinal and longitudinal gradients in species richness, falling away only in species-poor provinces (richness < 200), such as the Persian Gulf (52% of the global total), Hawaii (37%), the Caribbean (26%) and the East-Pacific (20%), where redundancy is also diminished. In the more species-poor provinces, large and ecologically important areas of trait space are empty, or occupied by just a few, highly distinctive species. These striking biogeographical differences in redundancy could affect the resilience of critical reef functions, and highlight the vulnerability of relatively depauperate, peripheral locations which are often a low priority for targeted conservation efforts.

I next analyse temporal trends in the regional-scale trait diversity of corals before and after a severe episode of mass coral bleaching within the Great Barrier Reef (GBR). I show that in the aftermath of the record-breaking marine heatwave on the GBR in 2016, an exposure of 6°C-weeks or more drove an unprecedented, regional-scale shift in the trait composition of coral assemblages, reflecting markedly divergent responses to heat stress by different taxa. Fast-growing staghorn and tabular corals suffered a catastrophic die-off, transforming the three-dimensionality and ecological functioning of 29% of the 3,863 reefs comprising the world's largest coral reef system. The increasing prevalence of post-bleaching mass mortality of corals represents a radical shift in the disturbance regimes of tropical reefs, and poses a severe threat to the functional diversity of all regions. Nonetheless, long-term analysis is required to understand how trait composition is likely to be permanently affected by these recurrent disturbances.

A key challenge is to understand whether assemblages exposed to recurrent disturbances will lack important functional attributes, or whether a range of species with diverse ecological roles can respond differently to environmental change, and replace those in decline (response diversity). To explore these patterns, I next analysed case studies of long-term trends in coral composition from three biogeographical provinces (the Great Barrier Reef, French Polynesia, and Jamaica) to quantify shifts in multidimensional trait space throughout cycles of disturbance and recovery. The analysis shows that decades after disturbances, assemblages with diverse functional attributes have failed to recover at sites in all regions. Abundance-weighted trait diversity in recovering assemblages was diminished by 29% on the Great Barrier Reef, 18% in Polynesia, and 48% in Jamaica. Disturbance and recovery has favoured a subset of species with limited functional attributes, including smaller, shorter and morphologically simpler taxa with submassive, tabular or bushy morphologies. The degree to which depleted areas of trait space ('losers') were restored by taxa which have increased in abundance ('winners') reveals limited response diversity across locations.

To understand the ecological implications of these shifts in functional diversity through time, we must test the relationship between diversity and ecosystem function. To analyse this relationship at a local scale, experimental coral communities were assembled to quantify the performance of coral colonies with and without neighbours, and in the presence of conspecifics versus heterospecifics. I found a positive effect of coral species richness on primary productivity (gross and net photosynthesis), indicated by a 53% increase in productivity in multispecies assemblages (2 or 4 species) relative to monocultures. Productivity in monocultures was predicted by traits associated with different species morphologies. In contrast, multispecies assemblages maintained high levels of productivity even in the absence of the most productive species, reflecting non-additive effects of species richness on community functioning. Assemblage performances were regulated by positive and negative interactions between colonies, with many colonies performing better among functionally diverse heterospecific neighbours than in isolation (facilitation). Facilitation occurred primarily among flow-sensitive taxa with simple morphological traits, and did not occur under low flow, suggesting that modifications to flow microclimates by corals generated beneficial, interspecific interactions.

Overall, the results of this thesis suggest that future trajectories in the functioning of reefs will depend on how different biogeographical pools of distinctive and redundant species will reassemble in the wake of global warming. The thesis demonstrates that the diversity and identity of corals within colony aggregations can influence coral community productivity, and highlights the importance of traits or trait diversity in regulating ecosystem function. Nevertheless, the functional trait diversity of corals is changing rapidly at local and regional scales, driven by recurrent disturbances, including mass bleaching. The thesis also reveals a considerable amount of similarity, or redundancy, among corals, and the role of this redundancy in maintaining functional diversity through time. Nevertheless, redundancy varies in the major coral reef provinces of the world, revealing locations that are potentially more vulnerable to the collapse of ecological functions.

Item ID: 57108
Item Type: Thesis (PhD)
Keywords: species richness, functional diversity, functional redundancy, biogeography, resilience, neighbour diversity, coral assemblages
Copyright Information: Copyright © 2018 Mike McWilliam
Additional Information:

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: McWilliam, Mike, Hoogenboom, Mia O., Baird, Andrew H., Kuo, Chao-Yang, Madin, Joshua S., and Hughes, Terry P. (2018) Biogeographical disparity in the functional diversity and redundancy of corals. Proceedings of the National Academy of Sciences of the United States of America, 115 (12). pp. 3084-3089.

Chapter 3: Hughes, Terry P., Kerry, James T., Baird, Andrew H., Connolly, Sean R., Dietzel, Andreas, Eakin, C. Mark, Heron, Scott F., Hoey, Andrew S., Hoogenboom, Mia O., Liu, Gang, McWilliam, Michael J., Pears, Rachel J., Pratchett, Morgan S., Skirving, William J., Stella, Jessica S., and Torda, Gergely (2018) Global warming transforms coral reef assemblages. Nature, 556 (7702). pp. 492-496.

Chapter 5: McWilliam, Mike, Chase, Tory J., and Hoogenboom, Mia O. (2018) Neighbor diversity regulates the productivity of coral assemblages. Current Biology, 28 (22). pp. 3634-3639.

Date Deposited: 13 Feb 2019 04:07
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050101 Ecological Impacts of Climate Change @ 30%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 40%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 30%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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