The importance of structural complexity in coral reef ecosystems
TL;DR: Quantitative analyses of existing data indicate a strong negative relationship between structural complexity and algal cover, which may reflect the important role complexity plays in enhancing herbivory by reef fishes, and structural complexity should be incorporated into monitoring programs and management objectives.
Abstract: The importance of structural complexity in coral reefs has come to the fore with the global degradation of reef condition; however, the limited scale and replication of many studies have restricted our understanding of the role of complexity in the ecosystem. We qualitatively and quantitatively (where sufficient standardised data were available) assess the literature regarding the role of structural complexity in coral reef ecosystems. A rapidly increasing number of publications have studied the role of complexity in reef ecosystems over the past four decades, with a concomitant increase in the diversity of methods used to quantify structure. Quantitative analyses of existing data indicate a strong negative relationship between structural complexity and algal cover, which may reflect the important role complexity plays in enhancing herbivory by reef fishes. The cover of total live coral and branching coral was positively correlated with structural complexity. These habitat attributes may be creating much of the structure, resulting in a collinear relationship; however, there is also evidence of enhanced coral recovery from disturbances where structural complexity is high. Urchin densities were negatively correlated with structural complexity; a relationship that may be driven by urchins eroding reef structure or by their gregarious behaviour when in open space. There was a strong positive relationship between structural complexity and fish density and biomass, likely mediated through density-dependent competition and refuge from predation. More variable responses were found when assessing individual fish families, with all families examined displaying a positive relationship to structural complexity, but only half of these relationships were significant. Although only corroborated with qualitative data, structural complexity also seems to have a positive effect on two ecosystem services: tourism and shoreline protection. Clearly, structural complexity is an integral component of coral reef ecosystems, and it should be incorporated into monitoring programs and management objectives.
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TL;DR: Although conditions governing regime shift or recovery dynamics were diverse, pre-disturbance quantification of simple factors such as structural complexity and water depth accurately predicted ecosystem trajectories, foreshadow the likely divergent but predictable outcomes for reef ecosystems in response to climate change.
Abstract: Climate-induced coral bleaching is among the greatest current threats to coral reefs, causing widespread loss of live coral cover1. Conditions under which reefs bounce back from bleaching events or shift from coral to algal dominance are unknown, making it difficult to predict and plan for differing reef responses under climate change2. Here we document and predict long-term reef responses to a major climate-induced coral bleaching event that caused unprecedented region-wide mortality of Indo-Pacific corals. Following loss of >90% live coral cover, 12 of 21 reefs recovered towards pre-disturbance live coral states, while nine reefs underwent regime shifts to fleshy macroalgae. Functional diversity of associated reef fish communities shifted substantially following bleaching, returning towards pre-disturbance structure on recovering reefs, while becoming progressively altered on regime shifting reefs. We identified threshold values for a range of factors that accurately predicted ecosystem response to the bleaching event. Recovery was favoured when reefs were structurally complex and in deeper water, when density of juvenile corals and herbivorous fishes was relatively high and when nutrient loads were low. Whether reefs were inside no-take marine reserves had no bearing on ecosystem trajectory. Although conditions governing regime shift or recovery dynamics were diverse, pre-disturbance quantification of simple factors such as structural complexity and water depth accurately predicted ecosystem trajectories. These findings foreshadow the likely divergent but predictable outcomes for reef ecosystems in response to climate change, thus guiding improved management and adaptation.
589 citations
TL;DR: Pivotal to projecting the fate of coral reefs is the capacity of reef-building corals to acclimatize and adapt to climate change and the mechanisms that could enable adaptive plasticity in the coral holobiont, including the potential role of epigenetics and coral-associated microbes.
Abstract: Pivotal to projecting the fate of coral reefs is the capacity of reef-building corals to acclimatize and adapt to climate change. Transgenerational plasticity may enable some marine organisms to acclimatize over several generations and it has been hypothesized that epigenetic processes and microbial associations might facilitate adaptive responses. However, current evidence is equivocal and understanding of the underlying processes is limited. Here, we discuss prospects for observing transgenerational plasticity in corals and the mechanisms that could enable adaptive plasticity in the coral holobiont, including the potential role of epigenetics and coral-associated microbes. Well-designed and strictly controlled experiments are needed to distinguish transgenerational plasticity from other forms of plasticity, and to elucidate the underlying mechanisms and their relative importance compared with genetic adaptation.
290 citations
Cites background from "The importance of structural comple..."
...Nature Climate Change, Nature Publishing Group, 2017, 7 (9), pp....
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TL;DR: It is shown that juveniles of both corals and fishes are repelled by chemical cues from fished, seaweed-dominated reefs but attracted to cues from coral-dominated areas where fishing is prohibited, suggesting that species that appear passive in their choice of habitat may have stronger preferences than the authors thought.
Abstract: Coral reefs are in global decline, converting from dominance by coral to dominance by seaweed. Once seaweeds become abundant, coral recovery is suppressed unless herbivores return to remove seaweeds, and corals then recruit. Variance in the recovery of fishes and corals is not well understood. We show that juveniles of both corals and fishes are repelled by chemical cues from fished, seaweed-dominated reefs but attracted to cues from coral-dominated areas where fishing is prohibited. Chemical cues of specific seaweeds from degraded reefs repulsed recruits, and cues from specific corals that are typical of healthy reefs attracted recruits. Juveniles were present at but behaviorally avoided recruiting to degraded reefs dominated by seaweeds. For recovery, degraded reefs may need to be managed to produce cues that attract, rather than repel, recruiting corals and fishes.
227 citations
TL;DR: The 3D reconstruction of reef structure and complexity can be integrated with other physiological and ecological parameters in future research to develop reliable ecosystem models and improve capacity to monitor changes in the health and function of coral reef ecosystems.
Abstract: The structural complexity of coral reefs plays a major role in the biodiversity, productivity, and overall functionality of reef ecosystems. Conventional metrics with 2-dimensional properties are inadequate for characterization of reef structural complexity. A 3-dimensional (3D) approach can better quantify topography, rugosity and other structural characteristics that play an important role in the ecology of coral reef communities. Structure-from-Motion (SfM) is an emerging low-cost photogrammetric method for high-resolution 3D topographic reconstruction. This study utilized SfM 3D reconstruction software tools to create textured mesh models of a reef at French Frigate Shoals, an atoll in the Northwestern Hawaiian Islands. The reconstructed orthophoto and digital elevation model were then integrated with geospatial software in order to quantify metrics pertaining to 3D complexity. The resulting data provided high-resolution physical properties of coral colonies that were then combined with live cover to accurately characterize the reef as a living structure. The 3D reconstruction of reef structure and complexity can be integrated with other physiological and ecological parameters in future research to develop reliable ecosystem models and improve capacity to monitor changes in the health and function of coral reef ecosystems.
213 citations
TL;DR: It is found that structural complexity and reef zone are the strongest and most consistent predictors of reef fish abundance, biomass, species richness, and trophic structure, and that coral traits, diversity, and life histories provided additional predictive power for models of Reef fish assemblages, and were key drivers of structural complexity.
Abstract: With the ongoing loss of coral cover and the associated flattening of reef architecture, understanding the links between coral habitat and reef fishes is of critical importance. Here, we investigate whether considering coral traits and functional diversity provides new insights into the relationship between structural complexity and reef fish communities, and whether coral traits and community composition can predict structural complexity. Across 157 sites in Seychelles, Maldives, the Chagos Archipelago, and Australia's Great Barrier Reef, we find that structural complexity and reef zone are the strongest and most consistent predictors of reef fish abundance, biomass, species richness, and trophic structure. However, coral traits, diversity, and life histories provided additional predictive power for models of reef fish assemblages, and were key drivers of structural complexity. Our findings highlight that reef complexity relies on living corals-with different traits and life histories-continuing to build carbonate skeletons, and that these nuanced relationships between coral assemblages and habitat complexity can affect the structure of reef fish assemblages. Seascape-level estimates of structural complexity are rapid and cost effective with important implications for the structure and function of fish assemblages, and should be incorporated into monitoring programs.
201 citations
References
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TL;DR: The role that many organisms play in the creation, modification and maintenance of habitats does not involve direct trophic interactions between species, but they are nevertheless important and common.
Abstract: Interactions between organisms are a major determinant of the distribution and abundance of species. Ecology textbooks (e.g., Ricklefs 1984, Krebs 1985, Begon et al. 1990) summarise these important interactions as intra- and interspecific competition for abiotic and biotic resources, predation, parasitism and mutualism. Conspicuously lacking from the list of key processes in most text books is the role that many organisms play in the creation, modification and maintenance of habitats. These activities do not involve direct trophic interactions between species, but they are nevertheless important and common. The ecological literature is rich in examples of habitat modification by organisms, some of which have been extensively studied (e.g. Thayer 1979, Naiman et al. 1988).
5,407 citations
"The importance of structural comple..." refers background in this paper
...Much of this structure can be provided by the physical shape and complexity of living organisms, such as trees, grasses, kelp and corals, often termed ecosystem engineers or foundation species (Jones et al. 1994; Bruno and Bertness 2001)....
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...plexity of living organisms, such as trees, grasses, kelp and corals, often termed ecosystem engineers or foundation species (Jones et al. 1994; Bruno and Bertness 2001)....
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3,049 citations
TL;DR: The ecological roles of critical functional groups (for both corals and reef fishes) that are fundamental to understanding resilience and avoiding phase shifts from coral dominance to less desirable, degraded ecosystems are reviewed.
Abstract: The worldwide decline of coral reefs calls for an urgent reassessment of current management practices. Confronting large-scale crises requires a major scaling-up of management efforts based on an improved understanding of the ecological processes that underlie reef resilience. Managing for improved resilience, incorporating the role of human activity in shaping ecosystems, provides a basis for coping with uncertainty, future changes and ecological surprises. Here we review the ecological roles of critical functional groups (for both corals and reef fishes) that are fundamental to understanding resilience and avoiding phase shifts from coral dominance to less desirable, degraded ecosystems. We identify striking biogeographic differences in the species richness and composition of functional groups, which highlight the vulnerability of Caribbean reef ecosystems. These findings have profound implications for restoration of degraded reefs, management of fisheries, and the focus on marine protected areas and biodiversity hotspots as priorities for conservation.
3,032 citations
"The importance of structural comple..." refers background in this paper
...of fish known to provide a range of critical functional roles regarding the removal of turf and/or macroalgae from reefs (Bellwood et al. 2004; Burkepile and Hay 2008; Hoey and Bellwood 2008; Mumby 2009)....
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...…a significant positive relationship between Scaridae and complexity in our analysis, a family of fish known to provide a range of critical functional roles regarding the removal of turf and/or macroalgae from reefs (Bellwood et al. 2004; Burkepile and Hay 2008; Hoey and Bellwood 2008; Mumby 2009)....
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...Because Caribbean coral reefs can function differently to IndoPacific coral reefs (Bellwood et al. 2004; Roff and Mumby 2012), these two biogeographic regions were investigated separately where sufficient replication was available (fish density, total algal cover and coral cover), and the mean RI…...
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...Because Caribbean coral reefs can function differently to IndoPacific coral reefs (Bellwood et al. 2004; Roff and Mumby 2012), these two biogeographic regions were investigated separately where sufficient replication was available (fish density, total algal cover and coral cover), and the mean RI between the regions tested with a one-way ANOVA....
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TL;DR: Although the rate of coral loss has slowed in the past decade compared to the 1980s, significant declines are persisting and the ability of Caribbean coral reefs to cope with future local and global environmental change may be irretrievably compromised.
Abstract: We report a massive region-wide decline of corals across the entire Caribbean basin, with the average hard coral cover on reefs being reduced by 80%, from about 50% to 10% cover, in three decades. Our meta-analysis shows that patterns of change in coral cover are variable across time periods but largely consistent across subregions, suggesting that local causes have operated with some degree of synchrony on a region-wide scale. Although the rate of coral loss has slowed in the past decade compared to the 1980s, significant declines are persisting. The ability of Caribbean coral reefs to cope with future local and global environmental change may be irretrievably compromised.
2,034 citations
"The importance of structural comple..." refers background in this paper
...Critically, the widespread loss of live coral reported for regions such as the Caribbean (Gardner et al. 2003) is being accompanied by a reduction in overall reef structural complexity (Alvarez-Filip et al. 2009)....
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...Interestingly, despite differences in whether a relationship exited between complexity and live coral cover, measures of structural complexity (RI) were higher for the Caribbean than the Indo-Pacific on average....
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...It is possible that this reflects the substantial structure Montastraea reefs can afford (Harborne et al. 2012), which are often the focus of Caribbean studies since the loss of most Acropora dominated reef habitats....
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...Although the relationships were still consistently negative, they were no longer statistically significant when assessing the Caribbean or IndoPacific independently; however, the sample size was quite low, particularly for the Caribbean....
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...The significant relationships for fish density were consistent between the Caribbean and Indo-Pacific regions, suggesting reef fish density is strongly structured by benthic structural complexity in both regions....
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TL;DR: In this paper, the authors identify ecological goods and services of coral reef ecosystems, with special emphasis on how they are generated, and the consequences of human impacts on coral reefs are also discussed, including loss of resilience or buffer capacity.
1,721 citations
"The importance of structural comple..." refers background in this paper
...Dive tourism is an important economic benefit from coral reefs (Moberg and Folke 1999; Hicks 2011), and divers will preferentially dive on reefs of good condition....
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