Coral Reef Community Composition in the Context of Disturbance History on the Great Barrier Reef,Australia

Much research on coral reefs has documented differential declines in coral and associated organisms. In order to contextualise this general degradation, research on community composition is necessary in the context of varied disturbance histories and the biological processes and physical features thought to retard or promote recovery. We conducted a spatial assessment of coral reef communities across five reefs of the central Great Barrier Reef, Australia, with known disturbance histories, and assessed patterns of coral cover and community composition related to a range of other variables thought to be important for reef dynamics. Two of the reefs had not been extensively disturbed for at least 15 years prior to the surveys. Three of the reefs had been severely impacted by crown-of-thorns starfish outbreaks and coral bleaching approximately a decade before the surveys, from which only one of them was showing signs of recovery based on independent surveys. We incorporated wave exposure (sheltered and exposed) and reef zone (slope, crest and flat) into our design, providing a comprehensive assessment of the spatial patterns in community composition on these reefs. Categorising corals into life history groupings, we document major coral community differences in the unrecovered reefs, compared to the composition and covers found on the undisturbed reefs. The recovered reef, despite having similar coral cover, had a different community composition from the undisturbed reefs, which may indicate slow successional processes, or a different natural community dominance pattern due to hydrology and other oceanographic factors. The variables that best correlated with patterns in the coral community among sites included the density of juvenile corals, herbivore fish biomass, fish species richness and the cover of macroalgae. Given increasing impacts to the Great Barrier Reef, efforts to mitigate local stressors will be imperative to encouraging coral communities to persist into the future.     

Thresholds and Multiple Stable States in Coral Reef Community Dynamics

Multiple stable states occur when more than one type of communitycan stably persist in a single environmental regime. Simpletheoretical analyses predict multiple stable states for (1)single species dynamics via the Allee effect, (2) two-speciescompetitive interactions characterized by unstable coexistence,(3) some predator-prey interactions, and (4) some systems combiningpredation and competition. Potential examples of transitionsbetween stable states on reefs include the failure of Diademaantillarum and Acropora cervicornis to recover following catastrophicmortality, and the replacement of microalgal turf by unpalatablemacroalgae after rapid increase in the amount of substratumavailable for colonization by algae. Subtidal marine ecosystemsin general, and reefs in particular, have several attributeswhich favor the existence of multiple stable states. Studiesof transitions between states often need to rely upon poorlycontrolled, unreplicated natural "experiments," as transitionstypically require pulses of disturbance over very large spatialscales. The stability of a state must often be inferred fromanalyses of the dynamics of participants at that state, as generationtimes and the potential for further extrinsic disturbance precludethe use of persistence as an indicator of stability. The potentialfor multiple stable states strongly influences our interpretationof variability in space and time and our ability to predictreef responses to natural and man-made environmental change.     

Anthropogenic Stressors,Inter-Specific Competition and ENSO Effects on a Mauritian Coral Reef

Much of the western Indian Ocean suffered widespread loss of live coral in 1998 and interest is now focussed on the indirect effects of this coral loss on other components of the ecosystem, in particular fishes. However, it is just as important to identify changes in fish assemblages at locations that did not suffer coral mortality to understand local versus regional drivers. We surveyed benthic and fish communities on a reef flat in Mauritius five times between 1994 and 2005. The design allowed for comparison through time, along the coast and between inshore and offshore reef locations. The benthic community demonstrates a clear trend along the coast, likely in response to a dredged water ski lane, but little change through time. Branching Acropora colonies dominate much of the live coral and best explain patterns in the fish assemblage (P < 0.01). Few changes in overall fish species richness through time were identified, and observed changes were within fishery target families rather than species reliant on live coral. Departure from expected levels of taxonomic distinctness suggests degradation in the community associated with the dredged ski lane. Non-metric multi-dimensional scaling of the fish assemblage demonstrates a similar pattern to that seen in the benthos; greater differences along the coast (Global R = 0.34) than through time (Global R = 0.17) and no trend between reef positions. SIMPER analysis identified two species of Stegastes as the main drivers of trends in the MDS plot and the most dominant of these, S. lividus, appears to be reducing species richness of the remaining fish community. The study highlights Mauritius as a regional refugia of thermally-sensitive corals and specialised fish, suggesting a need for careful management.     

Length-Based Assessment of Coral Reef Fish Populations in the Main and Northwestern Hawaiian Islands

The coral reef fish community of Hawaii is composed of hundreds of species, supports a multimillion dollar fishing and tourism industry, and is of great cultural importance to the local population. However, a major stock assessment of Hawaiian coral reef fish populations has not yet been conducted. Here we used the robust indicator variable “average length in the exploited phase of the population (L¯)”, estimated from size composition data from commercial fisheries trip reports and fishery-independent diver surveys, to evaluate exploitation rates for 19 Hawaiian reef fishes. By and large, the average lengths obtained from diver surveys agreed well with those from commercial data. We used the estimated exploitation rates coupled with life history parameters synthesized from the literature to parameterize a numerical population model and generate stock sustainability metrics such as spawning potential ratios (SPR). We found good agreement between predicted average lengths in an unfished population (from our population model) and those observed from diver surveys in the largely unexploited Northwestern Hawaiian Islands. Of 19 exploited reef fish species assessed in the main Hawaiian Islands, 9 had SPRs close to or below the 30% overfishing threshold. In general, longer-lived species such as surgeonfishes, the redlip parrotfish (Scarus rubroviolaceus), and the gray snapper (Aprion virescens) had the lowest SPRs, while short-lived species such as goatfishes and jacks, as well as two invasive species (Lutjanus kasmira and Cephalopholis argus), had SPRs above the 30% threshold.     

Cyanobacteria-shrimp colonies in the Mariana Islands

Aquatic Ecology - Cyanobacteria have multifaceted ecological roles on coral reefs. Moorena bouillonii, a chemically rich filamentous cyanobacterium, has been characterized as a pathogenic organism...     

Ecological Physiology of a Coral Pathogen and the Coral Reef Environment

Laboratory studies on the ecological physiology of a coral pathogen were carried out to investigate growth potential in terms of environmental factors that may control coral diseases on reefs. The disease chosen for this study, white plague type II, is considered to be one of the major diseases of Caribbean scleractinian corals, affecting a wide range of coral hosts and causing rapid and widespread tissue loss. It is caused by a single pathogen, the bacterium Aurantimonas coralicida. A series of laboratory experiments using a pure culture of the pathogen was carried out to examine the roles of temperature, pH, and O₂ concentration on growth rate. Results revealed optimal growth between 30 and 35°C, and between pH values of 6 and 8. There was a distinctive synergistic relationship between pH and temperature. Increasing temperature from 25 to 35°C expanded the range of pH tolerance from a minimum of 6.0 down to 5.0. O₂ concentration directly affected growth rate, which increased with increasing O₂. The combined effects of increasing O₂ and increasing temperature resulted in a synergistic effect of more rapid growth. These laboratory results are discussed in terms of the coral host and the range of the environmental factors that occur on coral reefs. We conclude that changing environmental conditions in the reef environment, in particular observed increases in water temperature, may be promoting coral diseases by allowing coral pathogens to expand their ecological niches. In the case of the white plague type II pathogen, elevated temperature would allow A. coralicida to colonize the low pH environment of the coral surface mucopolysaccharide layer as an initial stage of infection. The synergistic effect between temperature and oxygen concentration appeared to be less environmentally relevant for this coral pathogen.     

Additive Partitioning of Coral Reef Fish Diversity across Hierarchical Spatial Scales throughout the Caribbean

There is an increasing need to examine regional patterns of diversity in coral-reef systems since their biodiversity is declining globally. In this sense, additive partitioning might be useful since it quantifies the contribution of alpha and beta to total diversity across different scales. We applied this approach using an unbalanced design across four hierarchical scales (80 sites, 22 subregions, six ecoregions, and the Caribbean basin). Reef-fish species were compiled from the Reef Environmental Education Foundation (REEF) database and distributions were confirmed with published data. Permutation tests were used to compare observed values to those expected by chance. The primary objective was to identify patterns of reef-fish diversity across multiple spatial scales under different scenarios, examining factors such as fisheries and demographic connectivity. Total diversity at the Caribbean scale was attributed to β-diversity (nearly 62% of the species), with the highest β-diversity at the site scale. α¯-diversity was higher than expected by chance in all scenarios and at all studied scales. This suggests that fish assemblages are more homogenous than expected, particularly at the ecoregion scale. Within each ecoregion, diversity was mainly attributed to alpha, except for the Southern ecoregion where there was a greater difference in species among sites. β-components were lower than expected in all ecoregions, indicating that fishes within each ecoregion are a subsample of the same species pool. The scenario involving the effects of fisheries showed a shift in dominance for β-diversity from regions to subregions, with no major changes to the diversity patterns. In contrast, demographic connectivity partially explained the diversity pattern. β-components were low within connectivity regions and higher than expected by chance when comparing between them. Our results highlight the importance of ecoregions as a spatial scale to conserve local and regional coral reef-fish diversity.     

Sporadic Disturbances in Fluctuating Coral Reef Environments: El Nino and Coral Reef Development in the Eastern Pacific

The disastrous effects of the intense 1982–83 El Niño-SouthernOscillation (ENSO) bring new insight into the long-term developmentof eastern Pacific coral reefs. The 1988–83 ENSO sea surfacewarming event caused extensive reef coral bleaching (loss ofsymbiotic zooxanthellae), resulting in up to 70–95% coralmortality on reefs in Costa Rica, Panama, Colombia and Ecuador.In the Galapagos Islands (Ecuador), most coral reefs experienced>95% coral mortality. Also, several coral species experiencedextreme reductions in population size, and local and regionalextinctions. The El Niño event spawned secondary disturbances,such as increased predation and bioerosion, that continue toimpact reef-building corals. The death of Pocillopora colonieswith their crustacean guards eliminated coral barriers now allowingthe corallivore Acanthaster planci access to formerly protectedcoral prey. Sea urchins and other organisms eroded disturbedcorals at rates that exceed carbonate production, potentiallyresulting in the elimination of existing reef buildups. In otherreefbuilding regions following extensive, catastrophic coralmortality, rapid recovery often occurs through the growth ofsurviving corals, recruitment of new corals from nearby sourcepopulations, and survival of consolidated reef surfaces. Inthe eastern Pacific, however, the return of upwelling conditionsand the survival of coral predators and bioeroders hamper coralreef recovery by reducing recruitment success and eroding coralreef substrates. Thus, coral reef growth that occurs betweendisturbance events is not conserved. Repeated El Niñodisturbances, which have occurred throughout the recent geologichistory of the eastern Pacific, prevent coral communities fromincreasing in diversity and limit the development and persistenceof significant reef features. The poor development of easternPacific coral reefs throughout Holocene and perhaps much ofPleistocene time may result from recurrent thermal disturbancesof the intensity of the 1982–83 El Niño event.     

Population Dynamics and Spatial Distribution of Coral Reef Fishes: Comparison Between Continuous and Isolated Habitats

Recent studies have shown that there are high degrees of spatial and temporal stability in coral reef fish assemblage structures in a continuous habitat, in contrast to results of observations in isolated habitats. In order to determine the reason for the difference in temporal stability of fish assemblage structures in a continuous habitat site and an isolated habitat site, population dynamics and spatial distributions of coral reef fishes (six species of pomacentrids and two species of apogonids) in the two habitat site were investigated over a 2-year period in an Okinawan coral reef. The population densities of pomacentrid and apogonid species increased in juvenile settlement periods at both sites, but the magnitude of seasonal fluctuation in population density was significantly greater at the isolated habitat site, indicating that the rate of juvenile settlement and mortality rate in the isolated habitat were greater than those in the continuous habitat. The magnitude of aggregation of fishes, which affects density-dependent biological interactions that modify population density such as competition and predation, was also significantly greater at the isolated habitat site, especially in the juvenile settlement season. Most of the fishes at the isolated habitat site exhibited more generalized patterns of microhabitat selection because of less coral coverage and diversity. The seasonal stability in the species composition of fishes was greater at the continuous habitat site than that at the isolated habitat. Our findings suggest that the relative importance of various ecological factors responsible for regulation of the population density of coral reef fishes (e.g., competition, predation, microhabitat selection and post-settlement movement) in a continuous habitat site and the isolated habitat site are different.     

Eco-Evolutionary Community Dynamics: Covariation between Diversity and Invasibility across Temperature Gradients *

Abstract Understanding biodiversity gradients is a long-standing challenge, and progress requires theory unifying ecology and evolution. Here, we unify concepts related to the speed of evolution, the influence of species richness on diversification, and niche-based coexistence. We focus on the dynamics, through evolutionary time, of community invasibility and species richness across a broad thermal gradient. In our framework, the evolution of body size influences the ecological structure and dynamics of a trophic network, and organismal metabolism ties temperature to eco-evolutionary processes. The framework distinguishes ecological invasibility (governed by ecological interactions) from evolutionary invasibility (governed by local ecology and constraints imposed by small phenotypic effects of mutation). The model yields four primary predictions: (1) ecological invasibility declines through time and with increasing temperature; (2) average evolutionary invasibility across communities increases and then decreases through time as the richness-temperature gradient flattens; (3) in the early stages of diversification, richness and evolutionary invasibility both increase with increasing temperature; and (4) at equilibrium, richness does not vary with temperature, yet evolutionary invasibility decreases with increasing temperature. These predictions emerge from the "evolutionary-speed" hypothesis, which attempts to account for latitudinal species richness gradients by invoking faster biological rates in warmer, tropical regions. The model contrasts with predictions from other richness-gradient hypotheses, such as "niche conservatism" and "species energy." Empirically testing our model's predictions should help distinguish among these hypotheses.     

Mollusk collecting and environmental change during the Prehistoric Period in the Mariana Islands

Archaeological research in the Mariana Islands has revealed changes in mollusk collecting during the Prehistoric Period (approximately 1500 BC to AD 1521). The earliest people at Tumon Bay, Guam and Chalan Piao, Saipan collected mostly bivalves, especially the arc clam Anadara antiquata. After several hundred years, they no longer collected A. antiquata, but collected smaller bivalves instead. By AD 1000, they collected mostly gastropods, primarily the coral reef species Strombus gibberulus gibbosus. One possible explanation is that the people preferred the large arc clam but overharvested it until they were forced to eat the smaller bivalves and then the snails. However, recent evidence in the form of mangrove wood and mangrove pollen supports another explanation, one of non-anthropogenic environmental change. In this case, the relative sea-level decline, which took place in the Marianas within the last 4,000 years, caused the demise of mangrove habitats and of the arc clam at Tumon Bay, Guam and Chalan Piao, Saipan. As mangrove habitats were diminished by sea-level decline, collecting effort shifted to coral reefs, and S. gibberulus gibbosus was harvested throughout the remainder of the Prehistoric Period and into the Historic Period. Southern Guam is the only area in the Marianas in which A. antiquata increased in abundance during the Prehistoric Period. The same types of evidence, mangrove wood and mangrove pollen, indicate that, in contrast to the situation at Tumon Bay and Chalan Piao, mangroves increased in abundance in southern Guam.     

Coral communities and reef growth in the southern Great Barrier Reef

 Fringing reef development is limited around 22° S along the inner Great Barrier Reef, although there is substantial development north and south of this latitude. This study examined the relationships among coral communities and the extent of reef development. Reefs were examined to determine coral composition, colony abundance, colony size and growth form between the latitudes 20°S and 23°S. Major reef framework builders (scler- actinian genus Acropora and families Faviidae and Poritidae) dominated reefs north and south of 22°S, but declined significantly at 22°S where foliose and encrusting corals (Turbinaria and Montipora spp.) were most common. Porites spp. were present at 22° S but had encrusting morphologies. Consistently high turbidity at this latitude, caused by a 10 m tidal range and strong tidal flows, resuspends silts from the shallow shelf, and appears to have precluded reef development throughout the Holocene, by limiting the abundance, stunting the growth, and shortening the life expectancies of reef framework corals. The distinctions between ‘natural’ and ‘human-induced’ degradation may be interpreted on the basis of the relationship between Holocene development and current benthic community longevity. A mismatch between substantial past reef building capacity (a broad and/or thick reef) and non-existent or limited present reef-building capacity could signify anything from a long-period, natural cycle to an unprecedented deterioration in ecosystem function caused by human influence. Accepted: 29 July 1996     

Quantification of Loss and Change in Floridian Reef Coral Populations

Six coral reef locations between Miami and Key West were markedwith stainless steel stakes and rephotographed periodicallybetween 1984 and 1991. The monitored areas included two photostationsin the Looe Key National Marine Sanctuary, two photostationsin the Key Largo National Marine Sanctuary, and two photostationsin the Biscayne National Park. Stations were monitored for speciesnumber, percent cover, and species diversity of the scleractinianand hydrozoan stony corals. Monitoring began in 1984 for photostationsin the National Marine Sanctuaries and in 1989 for stationsin the National Park. All six areas lost coral species between the initial surveyyear and 1991. Survey areas lost between one and four species;these losses constituted between 13% and 29% of their speciesrichness. Five of the six areas lost live coral cover. Basedupon photographs taken repeatedly at these locations, net lossesranged between 7.3% and 43.9%. In the one station showing anincrease in coral cover, the increase was only for the canopybranches of Acropora palmata; understory branches of this samespecies lost surface area at the same rate as canopy branchesgained area. For most of the common species, there was a reductionin the total number of living colonies in the community, anda diminution in the number of large, mature colonies. Throughoutthe study period, there was no recruitment to any of the photostationsby any of the massive frame building coral species. Mortality of this magnitude is often associated with hurricanedamage, but in this survey the losses occurred during a periodwithout catastrophic storms. Sources of mortality identifiablein the photographs include (1) black band disease and (2) "bleaching"other potential sources of mortality are also considered. Weconclude, for our survey areas, that loss rates of this magnitudecannot be sustained for protracted periods if the coral communityis to persist in a configuration resembling historical coralreef community structure in the Florida Keys.     

Full-Sibs in Cohorts of Newly Settled Coral Reef Fishes

Reef fishes exhibit a bipartite life cycle where a benthic adult stage is preceded by a pelagic dispersal phase during which larvae are presumed to be mixed and transported by oceanic currents. Genetic analyses based on twelve microsatellite loci of 181 three-spot dascyllus (Dascyllus trimaculatus) that settled concurrently on a small reef in French Polynesia revealed 11 groups of siblings (1 full sibs and 10 half-sibs). This is the first evidence that fish siblings can journey together throughout their entire planktonic dispersal phase (nearly a month long for three-spot dascyllus). Our findings have critical implications for the dynamics and genetic structure of fish populations, as well as for the design of marine protected areas and management of fisheries.     

Fluorescence-Based Classification of Caribbean Coral Reef Organisms and Substrates

A diverse group of coral reef organisms, representing several phyla, possess fluorescent pigments. We investigated the potential of using the characteristic fluorescence emission spectra of these pigments to enable unsupervised, optical classification of coral reef habitats. We compiled a library of characteristic fluorescence spectra through in situ and laboratory measurements from a variety of specimens throughout the Caribbean. Because fluorescent pigments are not species-specific, the spectral library is organized in terms of 15 functional groups. We investigated the spectral separability of the functional groups in terms of the number of wavebands required to distinguish between them, using the similarity measures Spectral Angle Mapper (SAM), Spectral Information Divergence (SID), SID-SAM mixed measure, and Mahalanobis distance. This set of measures represents geometric, stochastic, joint geometric-stochastic, and statistical approaches to classifying spectra. Our hyperspectral fluorescence data were used to generate sets of 4-, 6-, and 8-waveband spectra, including random variations in relative signal amplitude, spectral peak shifts, and water-column attenuation. Each set consisted of 2 different band definitions: ‘optimally-picked’ and ‘evenly-spaced.’ The optimally-picked wavebands were chosen to coincide with as many peaks as possible in the functional group spectra. Reference libraries were formed from half of the spectra in each set and used for training purposes. Average classification accuracies ranged from 76.3% for SAM with 4 evenly-spaced wavebands to 93.8% for Mahalanobis distance with 8 evenly-spaced wavebands. The Mahalanobis distance consistently outperformed the other measures. In a second test, empirically-measured spectra were classified using the same reference libraries and the Mahalanobis distance for just the 8 evenly-spaced waveband case. Average classification accuracies were 84% and 87%, corresponding to the extremes in modeled water-column attenuation. The classification results from both tests indicate that a high degree of separability among the 15 fluorescent-spectra functional groups is possible using only a modest number of spectral bands.