Drivers of recruitment in sessile marine organisms are often poorly understood, due to the rapidly changing requirements experienced during early ontogeny. The complex suite of physical, biological, ...and ecological interactions beginning at larval settlement involves a series of trade-offs that influence recruitment success. For example, while cryptic settlement within complex microhabitats is a commonly observed phenomenon in sessile marine organisms, it is unclear whether trade-offs between competition in cryptic refuges and predation on exposed surfaces leads to higher recruitment. To explore the trade-offs during the early ontogeny of scleractinian corals, we combined field observations with laboratory and field experiments to develop a mechanistic understanding of coral recruitment success. Multiple experiments conducted over 15 months in Palau (Micronesia) allowed a mechanistic approach to study the individual factors involved in recruitment: settlement behavior, growth, competition, and predation, as functions of microhabitat and ontogeny. We finally developed and tested a predictive recruitment model with the broader aim of testing whether our empirical insights explained patterns of coral recruitment and quantifying the relative importance of each trade-off. Coral settlement was higher in crevices than exposed microhabitats, but post-settlement bottlenecks differed markedly in the presence (uncaged) and absence (caged) of predators. Incidental predation by herbivores on exposed surfaces at early post-settlement (<3 mm) stages and targeted predation by corallivores at late post-settlement (3–10 mm) stages exceeded competition in crevices as major drivers of mortality. In contrast, when fish were excluded, competition with macroalgae and heterotrophic invertebrates intensified mortality, particularly in crevices. As a result, post-settlement trade-offs were reversed, and recruitment was more than twofold higher on exposed surfaces than crevices. Once post-settlement bottlenecks were overcome, survival was higher on exposed surfaces regardless of fish exclusion. However, maximum recruitment occurred in crevices of uncaged treatments, being ninefold higher than caged treatments. Overall, we characterize recruitment success throughout the earliest life-history stages of corals and uncover some intriguing trade-offs between growth, competition and predation, highlighting how these change and even reverse during ontogeny and under alternate disturbance regimes.
The Ecological Role of Sharks on Coral Reefs Roff, George; Doropoulos, Christopher; Rogers, Alice ...
Trends in ecology & evolution (Amsterdam),
05/2016, Letnik:
31, Številka:
5
Journal Article
Recenzirano
Sharks are considered the apex predator of coral reefs, but the consequences of their global depletion are uncertain. Here we explore the ecological roles of sharks on coral reefs and, conversely, ...the importance of reefs for sharks. We find that most reef-associated shark species do not act as apex predators but instead function as mesopredators along with a diverse group of reef fish. While sharks perform important direct and indirect ecological roles, the evidence to support hypothesised shark-driven trophic cascades that benefit corals is weak and equivocal. Coral reefs provide some functional benefits to sharks, but sharks do not appear to favour healthier reef environments. Restoring populations of sharks is important and can yet deliver ecological surprise.
Stable isotopes reveal that sharks span an extended range of trophic levels with true apex species being higher than most common reef sharks.
Dietary analysis reveals that most common reef sharks are mesopredators occupying a similar trophic level to large piscivorous fishes.
Evidence for shark-induced trophic cascades that benefit herbivorous fishes is weak or equivocal on coral reefs.
Sharks can exert non-consumptive or ‘fear’ effects that disrupt the foraging of potential prey.
While coral reefs provide a range of ecological benefits for sharks, the link between healthy reefs and shark abundance is unclear.
Projections of coral reefs under climate change have important policy implications, but most analyses have focused on the intensification of climate‐related physical stress rather than explicitly ...modelling how coral populations respond to stressors. Here, we analyse the future of the Great Barrier Reef (GBR) under multiple, spatially realistic drivers which allows less impacted sites to facilitate recovery. Under a Representative Concentration Pathway (RCP) 2.6 CMIP5 climate ensemble, where warming is capped at ~2°C, GBR mean coral cover declined mid‐century but approached present‐day levels towards 2100. This is considerably more optimistic than most analyses. However, under RCP4.5, mean coral cover declined by >80% by late‐century, and reached near zero under RCP ≥6.0. While these models do not allow for adaptation, they significantly extend past studies by revealing demographic resilience of coral populations to low levels of additional warming, though more pessimistic outcomes might be expected under CMIP6. Substantive coral populations under RCP2.6 would facilitate long‐term genetic adaptation, adding value to ambitious greenhouse emissions mitigation.
We integrated sea warming projections from six climate models with an individual‐based ecological model to project future coral cover on the Great Barrier Reef (GBR). Under a Representative Concentration Pathway (RCP) 2.6 CMIP5 climate ensemble, where warming is capped at ~2°C, GBR mean coral cover declined mid‐century but approached present‐day levels towards 2100. This implies that demographic resilience may assist in sustaining coral cover under mild additional warming; however, such resilience was ineffective under RCP4.5, RCP6.0 and RCP8.5 warming scenarios.
Global environmental change is happening at unprecedented rates. Coral reefs are among the ecosystems most threatened by global change. For wild populations to persist, they must adapt. Knowledge ...shortfalls about corals' complex ecological and evolutionary dynamics, however, stymie predictions about potential adaptation to future conditions. Here, we review adaptation through the lens of quantitative genetics. We argue that coral adaptation studies can benefit greatly from “wild” quantitative genetic methods, where traits are studied in wild populations undergoing natural selection, genomic relationship matrices can replace breeding experiments, and analyses can be extended to examine genetic constraints among traits. In addition, individuals with advantageous genotypes for anticipated future conditions can be identified. Finally, genomic genotyping supports simultaneous consideration of how genetic diversity is arrayed across geographic and environmental distances, providing greater context for predictions of phenotypic evolution at a metapopulation scale.
Coral reefs are among the ecosystems most threatened by global change: whether corals can adapt to a warmer world is an urgent research question. Here, we argue that coral adaptation studies can benefit greatly from “wild” quantitative genetic methods—studying traits in populations experiencing natural selection, extending analyses to multivariate phenotypes, and integrating studies with population genomics to provide greater empirical context for phenotypic evolution at a metapopulation scale.
Some of the most profound effects of climate change on ecological communities are due to alterations in species interactions rather than direct physiological effects of changing environmental ...conditions. Empirical evidence of historical changes in species interactions within climate-impacted communities is, however, rare and difficult to obtain. Here, we demonstrate the recent disappearance of key habitat-forming kelp forests from a warming tropical–temperate transition zone in eastern Australia. Using a 10-y video dataset encompassing a 0.6 °C warming period, we show how herbivory increased as kelp gradually declined and then disappeared. Concurrently, fish communities from sites where kelp was originally abundant but subsequently disappeared became increasingly dominated by tropical herbivores. Feeding assays identified two key tropical/subtropical herbivores that consumed transplanted kelp within hours at these sites. There was also a distinct increase in the abundance of fishes that consume epilithic algae, and much higher bite rates by this group at sites without kelp, suggesting a key role for these fishes in maintaining reefs in kelp-free states by removing kelp recruits. Changes in kelp abundance showed no direct relationship to seawater temperatures over the decade and were also unrelated to other measured abiotic factors (nutrients and storms). Our results show that warming-mediated increases in fish herbivory pose a significant threat to kelp-dominated ecosystems in Australia and, potentially, globally.
Extreme disturbances often lead to community reorganisations, yet sometimes ecosystems unexpectedly fail to recover. Such surprising outcomes may pinpoint important yet overlooked mechanisms that ...drive ecosystems into undesirable states. Using long-term field observations, experimental manipulations and mechanistic modelling, we document the drivers of an unexpected phase shift from coral to macroalgal dominance following typhoon disturbance on reefs in Palau (Micronesia). After extensive coral mortality, an ephemeral bloom of a canopy-forming macroalga (Liagora sp.) provided physical refuge from herbivore grazing, resulting in the establishment of a secondary, understory macroalga (Lobophora spp.). After disappearance of Liagora canopies and resulting loss of grazing refuge, the Lobophora patches continued to expand and led to a macroalgal (Lobophora-) dominated state that has persisted for more than 2 years. We developed a mechanistic model of Lobophora patch dynamics parameterised with rates of growth measured in situ to simulate the observed proliferation of Lobophora under variable grazing refuges in space and time. Model simulations showed that short-term escapes from grazing were pivotal in allowing establishment of patches of Lobophora. Ephemeral grazing refuges created an opportunity to reach a cover above which Lobophora growth exceeds grazing, so that Lobophora could expand after disappearance of Liagora canopies. Critically, in the absence of grazing refuge, herbivore biomass was sufficient to prevent the establishment of Lobophora patches. Our model demonstrates that with rapid algal growth and low grazing, a relatively minor grazing refuge (6 month) is sufficient to escape herbivore control after extensive coral mortality, leading to unexpected recovery failure. Transient fluctuations in the intensity of control mechanisms, such as herbivore grazing, can have disproportionate and longlasting effects on community structure. Overall, this study stresses that our perception of reef dynamics must integrate the time scales at which reefs can be sensitive to transient changes in mechanisms promoting coral dominance.
Under projections of global climate change and other stressors, significant changes in the ecology, structure and function of coral reefs are predicted. Current management strategies tend to look to ...the past to set goals, focusing on halting declines and restoring baseline conditions. Here, we explore a complementary approach to decision making that is based on the anticipation of future changes in ecosystem state, function and services. Reviewing the existing literature and utilizing a scenario planning approach, we explore how the structure of coral reef communities might change in the future in response to global climate change and overfishing. We incorporate uncertainties in our predictions by considering heterogeneity in reef types in relation to structural complexity and primary productivity. We examine 14 ecosystem services provided by reefs, and rate their sensitivity to a range of future scenarios and management options. Our predictions suggest that the efficacy of management is highly dependent on biophysical characteristics and reef state. Reserves are currently widely used and are predicted to remain effective for reefs with high structural complexity. However, when complexity is lost, maximizing service provision requires a broader portfolio of management approaches, including the provision of artificial complexity, coral restoration, fish aggregation devices and herbivore management. Increased use of such management tools will require capacity building and technique refinement and we therefore conclude that diversification of our management toolbox should be considered urgently to prepare for the challenges of managing reefs into the 21st century.
One striking feature of coral reef ecosystems is the complex benthic architecture which supports diverse and abundant fauna, particularly of reef fish. Reef‐building corals are in decline worldwide, ...with a corresponding loss of live coral cover resulting in a loss of architectural complexity. Understanding the dynamics of the reef architecture is therefore important to envision the ability of corals to maintain functional habitats in an era of climate change. Here, we develop a mechanistic model of reef topographical complexity for contemporary Caribbean reefs. The model describes the dynamics of corals and other benthic taxa under climate‐driven disturbances (hurricanes and coral bleaching). Corals have a simplified shape with explicit diameter and height, allowing species‐specific calculation of their colony surface and volume. Growth and the mechanical (hurricanes) and biological erosion (parrotfish) of carbonate skeletons are important in driving the pace of extension/reduction in the upper reef surface, the net outcome being quantified by a simple surface roughness index (reef rugosity). The model accurately simulated the decadal changes of coral cover observed in Cozumel (Mexico) between 1984 and 2008, and provided a realistic hindcast of coral colony‐scale (1–10 m) changing rugosity over the same period. We then projected future changes of Caribbean reef rugosity in response to global warming. Under severe and frequent thermal stress, the model predicted a dramatic loss of rugosity over the next two or three decades. Critically, reefs with managed parrotfish populations were able to delay the general loss of architectural complexity, as the benefits of grazing in maintaining living coral outweighed the bioerosion of dead coral skeletons. Overall, this model provides the first explicit projections of reef rugosity in a warming climate, and highlights the need of combining local (protecting and restoring high grazing) to global (mitigation of greenhouse gas emissions) interventions for the persistence of functional reef habitats.
PDF
