Spatial Resilience of Coral Reefs

There have been several earlier studies that addressed the influence of natural disturbance regimes on coral reefs. Humans alter natural disturbance regimes, introduce new stressors, and modify background conditions of reefs. We focus on how coral reef ecosystems relate to disturbance in an increasingly human-dominated environment. The concept of ecosystem resilience—that is, the capacity of complex systems with multiple stable states to absorb disturbance, reorganize, and adapt to change—is central in this context. Instead of focusing on the recovery of certain species and populations within disturbed sites of individual reefs, we address spatial resilience—that is, the dynamic capacity of a reef matrix to reorganize and maintain ecosystem function following disturbance. The interplay between disturbance and ecosystem resilience is highlighted. We begin the identification of spatial sources of resilience in dynamic seascapes and exemplify and discuss the relation between “ecological memory” (biological legacies, mobile link species, and support areas) and functional diversity for seascape resilience. Managing for resilience in dynamic seascapes not only enhances the likelihood of conserving coral reefs, it also provides insurance to society by sustaining essential ecosystem services. Received 25 February 2000; accepted 31 January 2001.     

The Changing Health of Coral Reefs

Throughout their entire global range coral reefs are in decline. Coral bleaching, macroalgal overgrowth and coral diseases — responses signaling the declining health of coral reefs — have occurred with increasing frequency and intensity in recent decades. Decreased calcification may also be affecting coral reefs over longer time scales. Declines in coral reef health have been attributed to various natural and anthropogenic processes, but assignment of causality has proved problematic. Coral bleaching has been observed during extreme climate events such as El Niño; furthermore, there are indications that exposure to UV radiation, air, infectious microbes, and elevated temperature plays a role in the dramatic increase of coral bleaching since the mid-1970s. Macroalgal overgrowth is usually ascribed to eutrophi-cation and coral diseases to weakening of the coral host resistance by anthropogenic pollution. An issue precluding a strict anthropogenic cause of coral reef decline is that both overgrowth and coral diseases are known to occur, although less frequently, on reefs remote from human development. While its causes are still being unraveled, the overall decline in coral reef health sends an unambiguous signal that the coral reef system is losing its ability to withstand sudden or persistent environmental changes.     

Communities of Coral Reefs of Central Vietnam

Communities of macrobenthos on fringing reefs of central Vietnam were studied by the hydrobiological method using scuba diving. The species composition (more than 600 species), population densities of common species of the macrobenthos, and the degree of coverage of the substrate by corals and macrophytes were examined. Bottom communities classified on the basis of these data are indicative of the similarity of the investigated reefs with those of southern and northern Vietnam.     

Diel Variability in Seawater pH Relates to Calcification and Benthic Community Structure on Coral Reefs

Community structure and assembly are determined in part by environmental heterogeneity. While reef-building corals respond negatively to warming (i.e. bleaching events) and ocean acidification (OA), the extent of present-day natural variability in pH on shallow reefs and ecological consequences for benthic assemblages is unknown. We documented high resolution temporal patterns in temperature and pH from three reefs in the central Pacific and examined how these data relate to community development and net accretion rates of early successional benthic organisms. These reefs experienced substantial diel fluctuations in temperature (0.78°C) and pH (>0.2) similar to the magnitude of 'warming' and 'acidification' expected over the next century. Where daily pH within the benthic boundary layer failed to exceed pelagic climatological seasonal lows, net accretion was slower and fleshy, non-calcifying benthic organisms dominated space. Thus, key aspects of coral reef ecosystem structure and function are presently related to natural diurnal variability in pH.     

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.     

How Microbial Community Composition Regulates Coral Disease Development

Reef coral cover is in rapid decline worldwide, in part due to bleaching (expulsion of photosynthetic symbionts) and outbreaks of infectious disease. One important factor associated with bleaching and in disease transmission is a shift in the composition of the microbial community in the mucus layer surrounding the coral: the resident microbial community—which is critical to the healthy functioning of the coral holobiont—is replaced by pathogenic microbes, often species of Vibrio. In this paper we develop computational models for microbial community dynamics in the mucus layer in order to understand how the surface microbial community responds to changes in environmental conditions, and under what circumstances it becomes vulnerable to overgrowth by pathogens. Some of our model''s assumptions and parameter values are based on Vibrio spp. as a model system for other established and emerging coral pathogens. We find that the pattern of interactions in the surface microbial community facilitates the existence of alternate stable states, one dominated by antibiotic-producing beneficial microbes and the other pathogen-dominated. A shift to pathogen dominance under transient stressful conditions, such as a brief warming spell, may persist long after environmental conditions have returned to normal. This prediction is consistent with experimental findings that antibiotic properties of Acropora palmata mucus did not return to normal long after temperatures had fallen. Long-term loss of antibiotic activity eliminates a critical component in coral defense against disease, giving pathogens an extended opportunity to infect and spread within the host, elevating the risk of coral bleaching, disease, and mortality.     

A Theoretical Model of Pattern Formation in Coral Reefs

We present a mathematical model of the growth of coral subject to unidirectional ocean currents using concepts of porous-media flow and nonlinear dynamics in chemical systems. Linear stability analysis of the system of equations predicts that the growth of solid (coral) structures will be aligned perpendicular to flow, propagating against flow direction. Length scales of spacing between structures are selected based on chemical reaction and flow rates. In the fully nonlinear system, autocatalysis in the chemical reaction accelerates growth. Numerical analysis reveals that the nonlinear growth creates sharp fronts of high solid fraction that, as predicted by the linear stability, advance against the predominating flow direction. The findings of regularly spaced growth areas oriented perpendicular to flow are qualitatively supported on both a colonial and a regional reef scale. Received 1 October 2001; accepted 22 May 2002.     

Coral Community Adaptability to Environmental Change at the Scales of Regions, Reefs and Reef Zones

SYNOPSIS. Projected global increases in temperature, sea level,storminess and atmospheric carbon dioxide (CO₂) are likely tocause changes in reef coral communities which the present humangeneration will view as deleterious. It is likely coral communitytrajectories will be influenced as much by the reduction inintervals between extreme events as the projected increasesin means of environmental parameters such as temperature, atmosphericCO₂ and sea-level. Depressed calcification rates in corals causedby reduced aragonite saturation state of water may increasevulnerability of corals to storms. Moreover, reduction in intervalsbetween storms and other extreme events causing mass mortalityin corals (coral predators, diseases, bleaching) are likelyto more frequently "set back" reef coral communities to earlysuccessional stages or alternate states characterized by non-calcifyingbenthos (plants, soft corals, sponges). The greater the areaand the longer the duration of dominance of putative "coral/corallinealgae" zones of coral reefs by non-calcifying stages, the lesswill be the reefs capacity to accrete limestone bulk lockedup in the big skeletal units of late successional stages (i.e.,very large old corals). Averaged over decades to centuries,the effects of such changes on the coral community's carryingcapacity for other biota such as fish are unpredictable. A "shiftingsteady-state mosaic" null model may provide a useful conceptualtool for defining a baseline and tracking changes from thatbaseline through time.     

Ecological Consequences of Sediment on High-Energy Coral Reefs

Sediments are widely accepted as a threat to coral reefs but our understanding of their ecological impacts is limited. Evidence has suggested that benthic sediments bound within the epilithic algal matrix (EAM) suppress reef fish herbivory, a key ecological process maintaining reef resilience. An experimental combination of caging and sediment addition treatments were used to investigate the effects of sediment pulses on herbivory and EAMs and to determine whether sediment addition could trigger a positive-feedback loop, leading to deep, sediment-rich turfs. A 1-week pulsed sediment addition resulted in rapid increases in algal turf length with effects comparable to those seen in herbivore exclusion cages. Contrary to the hypothesised positive-feedback mechanism, benthic sediment loads returned to natural levels within 3 weeks, however, the EAM turfs remained almost 60% longer for at least 3 months. While reduced herbivore density is widely understood to be a major threat to reefs, we show that acute disturbances to reef sediments elicit similar ecological responses in the EAM. With reefs increasingly threatened by both reductions in herbivore biomass and altered sediment fluxes, the development of longer turfs may become more common on coral reefs.     

Context-Dependent Diversity-Effects of Seaweed Consumption on Coral Reefs in Kenya

Consumers and prey diversity, their interactions, and subsequent effects on ecosystem function are important for ecological processes but not well understood in high diversity ecosystems such as coral reefs. Consequently, we tested the potential for diversity-effects with a series of surveys and experiments evaluating the influence of browsing herbivores on macroalgae in Kenya’s fringing reef ecosystem. We surveyed sites and undertook experiments in reefs subject to three levels of human fishing influence: open access fished reefs, small and recently established community-managed marine reserves, and larger, older government-managed marine reserves. Older marine reserves had a greater overall diversity of herbivores and browsers but this was not clearly associated with reduced macroalgal diversity or abundance. Experiments studying succession on hard substrata also found no effects of consumer diversity. Instead, overall browser abundance of either sea urchins or fishes was correlated with declines in macroalgal cover. An exception was that the absence of a key fish browser genus, Naso, which was correlated with the persistence of Sargassum in a marine reserve. Algal selectivity assays showed that macroalgae were consumed at variable rates, a product of strong species-specific feeding and low overlap in the selectivity of browsing fishes. We conclude that the effects of browser and herbivore diversity are less than the influences of key species, whose impacts emerge in different contexts that are influenced by fisheries management. Consequently, identifying key herbivore species and managing to protect them may assist protecting reef functions.     

Movements of Fishes Within and Among Fringing Coral Reefs in Barbados

Movement of coral reef fishes across marine reserve boundaries subsequent to their initial settlement from the plankton will affect the ability of no-take reserves to conserve stocks and to benefit adjacent fisheries. However, the mobility of most exploited reef species is poorly known. We tagged 1443 individuals of 35 reef fish species captured in Antillean fish traps in the Barbados Marine Reserve and adjacent non-reserve over a two-month period. Trapping and visual surveys were used to monitor the movements of these fish during the trapping period and the subsequent two months. Estimates of distances moved were corrected for the spatial distribution of sampling effort and for the number of recaptures of individual fish. Recapture rates for individual species ranged from 0–100% (median=38%). Species mobility estimated by recapture and resighting were highly correlated. Most species were strongly site attached, with the majority of recaptures and resightings occurring at the site of tagging. However, only one of 59 tagged jacks (Caranx latus, C. ruber) was ever resighted, suggesting emigration from the study area. All species were occasionally recorded away from the sites where they had been tagged (20–500m), and several species, including surgeonfish, Acanthurus bahianus, A. coeruleus, filefish, Cantherhines pullus, butterflyfish, Chaetodon striatus, angelfish Holocanthus tricolor and parrotfish, Sparisoma viride, ranged widely within reefs. In contrast, few movements were observed between reefs separated by more than 20m of sand and rubble, and no emigration from the Reserve was recorded. Most reef fishes vulnerable to Antillean traps appear sufficiently site-attached to benefit from reserves. However, many species move over a wide enough area to take them out of small reserves on continuous reef. Use of natural home range boundaries could minimize exposure of fishes in reserves to mortality from adjacent fisheries.     

Dynamic Stability of Coral Reefs on the West Australian Coast

Monitoring changes in coral cover and composition through space and time can provide insights to reef health and assist the focus of management and conservation efforts. We used a meta-analytical approach to assess coral cover data across latitudes 10–35°S along the west Australian coast, including 25 years of data from the Ningaloo region. Current estimates of coral cover ranged between 3 and 44% in coral habitats. Coral communities in the northern regions were dominated by corals from the families Acroporidae and Poritidae, which became less common at higher latitudes. At Ningaloo Reef coral cover has remained relatively stable through time (∼28%), although north-eastern and southern areas have experienced significant declines in overall cover. These declines are likely related to periodic disturbances such as cyclones and thermal anomalies, which were particularly noticeable around 1998/1999 and 2010/2011. Linear mixed effects models (LME) suggest latitude explains 10% of the deviance in coral cover through time at Ningaloo. Acroporidae has decreased in abundance relative to other common families at Ningaloo in the south, which might be related to persistence of more thermally and mechanically tolerant families. We identify regions where quantitative time-series data on coral cover and composition are lacking, particularly in north-western Australia. Standardising routine monitoring methods used by management and research agencies at these, and other locations, would allow a more robust assessment of coral condition and a better basis for conservation of coral reefs.     

Patterns of Coral Recruitment and Post-settlement Mortality on Bermuda's Reefs: Comparisons to Caribbean and Pacific Reefs

The recruitment of juvenile corals and post-settlement mortalityare important processes for coral population dynamics and reefcommunity ecology. I monitored juvenile coral recruitment andsurvival on a severely disturbed reef in Bermuda from 1981 to1989 and on adjacent healthy reefs from 1986 to 1990. Poritesastreoides was the dominant recruiting species at all sites,due to the release of brooded planulae that may settle rapidly.The dominant corals on Bermuda's reef, Diploria spp., were poorrecruiters, perhaps due to the broadcast mode of reproductionof these species. However, Diploria spp. have lower juvenilemortality rates compared to P. astreoides, which may explaintheir abundance on Bermuda's reefs. Brooding corals, primarily agariciids, were the dominant recruitson Atlantic reefs compared to high recruitment rates by spawningacroporids in the Pacific, which may be the result of differentenvironmental conditions and/or evolutionary trends in the twooceans. The latter group also suffered high post-settlementmortality compared to brooding coralsin both the Atlantic andthe Pacific. Massive corals in both oceans had generally lowrecruitment rates, related to their spawning mode of reproduction,and low rates of post-settlement mortality. The dominant roleof long-lived massive corals on the Atlantic and Pacific reefscan be understood in terms of their life-history strategy incomparison to the relatively short-lived Pacific acroporidsand Atlantic agariciids that rely on different strategies tomaintain their populations.     

Boring by Macro-organisms in the Coral Montastrea annularis on Barbados Reefs

The fauna boring into Montastrea annularis includes sponges, bivalves, sipunculid and polychaete worms and barnacles. Sponges are most important in hard tissue destruction and account for more than 90% of the total boring in most heads. Bivalves and barnacles are locally important. Sipunculids and sabellids account for less than 4% of the total boring. The volume removed from coral samples by boring ranged from 3–60% and samples from a deeper bank reef were more highly bored than fringing reef samples. An average of 20% of the volume of bank reef corals, and 5% of the volume of fringing reef corals, was removed by boring. The distribution of individual borers is not a function of depth. The density and variety of borers and the extent of boring in coral heads is greater in older heads. The ratio of living coral surface to dead encrusted areas on colonies also influences borer density and the extent of boring.     

Environmental and Biotic Correlates to Lionfish Invasion Success in Bahamian Coral Reefs

Lionfish (Pterois volitans), venomous predators from the Indo-Pacific, are recent invaders of the Caribbean Basin and southeastern coast of North America. Quantification of invasive lionfish abundances, along with potentially important physical and biological environmental characteristics, permitted inferences about the invasion process of reefs on the island of San Salvador in the Bahamas. Environmental wave-exposure had a large influence on lionfish abundance, which was more than 20 and 120 times greater for density and biomass respectively at sheltered sites as compared with wave-exposed environments. Our measurements of topographic complexity of the reefs revealed that lionfish abundance was not driven by habitat rugosity. Lionfish abundance was not negatively affected by the abundance of large native predators (or large native groupers) and was also unrelated to the abundance of medium prey fishes (total length of 5–10 cm). These relationships suggest that (1) higher-energy environments may impose intrinsic resistance against lionfish invasion, (2) habitat complexity may not facilitate the lionfish invasion process, (3) predation or competition by native fishes may not provide biotic resistance against lionfish invasion, and (4) abundant prey fish might not facilitate lionfish invasion success. The relatively low biomass of large grouper on this island could explain our failure to detect suppression of lionfish abundance and we encourage continuing the preservation and restoration of potential lionfish predators in the Caribbean. In addition, energetic environments might exert direct or indirect resistance to the lionfish proliferation, providing native fish populations with essential refuges.     

Cyanobacteria Associated with Coral Black Band Disease in Caribbean and Indo-Pacific Reefs

For 30 years it has been assumed that a single species of cyanobacteria, Phormidium corallyticum, is the volumetrically dominant component of all cases of black band disease (BBD) in coral. Cyanobacterium-specific 16S rRNA gene primers and terminal restriction fragment length polymorphism analyses were used to determine the phylogenetic diversity of these BBD cyanobacteria on coral reefs in the Caribbean and Indo-Pacific Seas. These analyses indicate that the cyanobacteria that inhabit BBD bacterial mats collected from the Caribbean and Indo-Pacific Seas belong to at least three different taxa, despite the fact that the corals in each case exhibit similar signs and patterns of BBD mat development.