Peter J. Mumby

Coral Reefs Under Rapid Climate Change and Ocean Acidification

Atmospheric carbon dioxide concentration is expected to exceed 500 parts per million and global temperatures to rise by at least 2°C by 2050 to 2100, values that significantly exceed those of at least the past 420,000 years during which most extant marine organisms evolved. Under conditions expected in the 21st century, global warming and ocean acidification will compromise carbonate accretion, with corals becoming increasingly rare on reef systems. The result will be less diverse reef communities and carbonate reef structures that fail to be maintained. Climate change also exacerbates local stresses from declining water quality and overexploitation of key species, driving reefs increasingly toward the tipping point for functional collapse. This review presents future scenarios for coral reefs that predict increasingly serious consequences for reef-associated fisheries, tourism, coastal protection, and people. As the International Year of the Reef 2008 begins, scaled-up management intervention and decisive action on global emissions are required if the loss of coral-dominated ecosystems is to be avoided.

Mangroves enhance the biomass of coral reef fish communities in the Caribbean

Mangrove forests are one of the worlds most threatened tropical ecosystems with global loss exceeding 35% (ref. 1). Juvenile coral reef fish often inhabit mangroves, but the importance of these nurseries to reef fish population dynamics has not been quantified. Indeed, mangroves might be expected to have negligible influence on reef fish communities: juvenile fish can inhabit alternative habitats and fish populations may be regulated by other limiting factors such as larval supply or fishing. Here we show that mangroves are unexpectedly important, serving as an intermediate nursery habitat that may increase the survivorship of young fish. Mangroves in the Caribbean strongly influence the community structure of fish on neighbouring coral reefs. In addition, the biomass of several commercially important species is more than doubled when adult habitat is connected to mangroves. The largest herbivorous fish in the Atlantic, Scarus guacamaia, has a functional dependency on mangroves and has suffered local extinction after mangrove removal. Current rates of mangrove deforestation are likely to have severe deleterious consequences for the ecosystem function, fisheries productivity and resilience of reefs. Conservation efforts should protect connected corridors of mangroves, seagrass beds and coral reefs.

Rising to the challenge of sustaining coral reef resilience

Phase-shifts from one persistent assemblage of species to another have become increasingly commonplace on coral reefs and in many other ecosystems due to escalating human impacts. Coral reef science, monitoring and global assessments have focused mainly on producing detailed descriptions of reef decline, and continue to pay insufficient attention to the underlying processes causing degradation. A more productive way forward is to harness new theoretical insights and empirical information on why some reefs degrade and others do not. Learning how to avoid undesirable phase-shifts, and how to reverse them when they occur, requires an urgent reform of scientific approaches, policies, governance structures and coral reef management.

Thresholds and the resilience of Caribbean coral reefs

The deteriorating health of the world’s coral reefs threatens global biodiversity, ecosystem function, and the livelihoods of millions of people living in tropical coastal regions. Reefs in the Caribbean are among the most heavily affected, having experienced mass disease-induced mortality of the herbivorous urchin Diadema antillarum in 1983 and two framework-building species of coral. Declining reef health is characterized by increases in macroalgae. A critical question is whether the observed macroalgal bloom on Caribbean reefs is easily reversible. To answer this question, we must resolve whether algal-dominated reefs are an alternative stable state of the ecosystem or simply the readily reversible result of a phase change along a gradient of some environmental or ecological parameter. Here, using a fully parameterized simulation model in combination with a simple analytical model, we show that Caribbean reefs became susceptible to alternative stable states once the urchin mortality event of 1983 confined the majority of grazing to parrotfishes. We reveal dramatic hysteresis in a natural system and define critical thresholds of grazing and coral cover beyond which resilience is lost. Most grazing thresholds lie near the upper level observed for parrotfishes in nature, suggesting that reefs are highly sensitive to parrotfish exploitation. Ecosystem thresholds can be combined with stochastic models of disturbance to identify targets for the restoration of ecosystem processes. We illustrate this principle by estimating the relationship between current reef state (coral cover and grazing) and the probability that the reef will withstand moderate hurricane intensity for two decades without becoming entrained in a shift towards a stable macroalgal-dominated state. Such targets may help reef managers face the challenge of addressing global disturbance at local scales.

A mid-term analysis of progress toward international biodiversity targets

In 2010, the international community, under the auspices of the Convention on Biological Diversity, agreed on 20 biodiversity-related “Aichi Targets” to be achieved within a decade. We provide a comprehensive mid-term assessment of progress toward these global targets using 55 indicator data sets. We projected indicator trends to 2020 using an adaptive statistical framework that incorporated the specific properties of individual time series. On current trajectories, results suggest that despite accelerating policy and management responses to the biodiversity crisis, the impacts of these efforts are unlikely to be reflected in improved trends in the state of biodiversity by 2020. We highlight areas of societal endeavor requiring additional efforts to achieve the Aichi Targets, and provide a baseline against which to assess future progress. Although conservation efforts are accelerating, their impact is unlikely to improve the global state of biodiversity by 2020. Indicators of progress and decline The targets set by the Convention on Biological Diversity in 2010 focused international efforts to alleviate global biodiversity decline. However, many of the consequences of these efforts will not be evident by the 2020 deadline agreed to by governments of 150 countries. Tittensor et al. analyzed data on 55 different biodiversity indicators to predict progress toward the 2020 targets—indicators such as protected area coverage, land-use trends, and endangered species status. The analysis pinpoints the problems and areas that will need the most attention in the next few years. Science, this issue p. 241

Coral reef management and conservation in light of rapidly evolving ecological paradigms

The decline of many coral reef ecosystems in recent decades surprised experienced managers and researchers. It shattered old paradigms that these diverse ecosystems are spatially uniform and temporally stable on the scale of millennia. We now see reefs as heterogeneous, fragile, globally stressed ecosystems structured by strong positive or negative feedback processes. We review the causes and consequences of reef decline and ask whether management practices are addressing the problem at appropriate scales. We conclude that both science and management are currently failing to address the comanagement of extractive activities and ecological processes that drive ecosystems (e.g. productivity and herbivory). Most reef conservation efforts are directed toward reserve implementation, but new approaches are needed to sustain ecosystem function in exploited areas.

Trophic cascade facilitates coral recruitment in a marine reserve

Reduced fishing pressure and weak predator–prey interactions within marine reserves can create trophic cascades that increase the number of grazing fishes and reduce the coverage of macroalgae on coral reefs. Here, we show that the impacts of reserves extend beyond trophic cascades and enhance the process of coral recruitment. Increased fish grazing, primarily driven by reduced fishing, was strongly negatively correlated with macroalgal cover and resulted in a 2-fold increase in the density of coral recruits within a Bahamian reef system. Our conclusions are robust because four alternative hypotheses that may generate a spurious correlation between grazing and coral recruitment were tested and rejected. Grazing appears to influence the density and community structure of coral recruits, but no detectable influence was found on the overall size-frequency distribution, community structure, or cover of corals. We interpret this absence of pattern in the adult coral community as symptomatic of the impact of a recent disturbance event that masks the recovery trajectories of individual reefs. Marine reserves are not a panacea for conservation but can facilitate the recovery of corals from disturbance and may help sustain the biodiversity of organisms that depend on a complex three-dimensional coral habitat.

Mapping marine environments with IKONOS imagery: enhanced spatial resolution can deliver greater thematic accuracy

The IKONOS 2 satellite was launched in late 1999 and carries the first commercial multispectral instrument to achieve 4 m spatial resolution. The cost and accuracy of using IKONOS imagery to map shallow-water marine environments is evaluated and compared directly to that using a suite of satellite and airborne instruments including Landsat Multispectral Scanner (MSS) and Thematic Mapper (TM), Satellite Pour l’Observation de la Terre (SPOT) High-Resolution Visible (HRV) multispectral and panchromatic, and Compact Airborne Spectrographic Imager (CASI). Evaluations were conducted in the Turks and Caicos Islands which are ideally suited to aquatic remote sensing because of the large areas of clear, shallow water containing a range of different habitats. Over 600 field sites were surveyed and used to define habitat categories, supervise image classification, and make an independent assessment of thematic map accuracy. For the highresolution IKONOS imagery, pixel sizes were small enough to allow within-habitat textural information to be added to the classification. Making full use of this textural information in supervised classifications significantly improved (P<.01) thematic map accuracy for finelevel habitat discrimination (13 classes). Although significantly (P<.001) and almost 20% more accurate than Landsat TM, like other satellite-borne sensors, IKONOS data were unable adequately to discriminate 13 categories of coral, algal, and seagrass habitats (overall accuracy 50%). Comparison with CASI in a restricted area where only nine of these habitats were represented indicated that CASI (81% user accuracy) was significantly (P<.01) more accurate than IKONOS (64% user accuracy). Comparisons of IKONOS, Landsat TM, and CASI along a transect in the maximally penetrating blue part of the spectrum suggest that the poor descriptive resolution of satellite sensors is partly due to loss of radiance contrast, presumably as a result of atmospheric Rayleigh scattering. Problems of scattering, coupled with the relatively poor spectral resolution of satellite sensors such as IKONOS, currently constrain their use for fine-level marine habitat mapping. IKONOS data enabled significantly more accurate mapping at a geomorphological scale (e.g., coral vs. seagrass) than other satellite sensors except Landsat TM. While reasonably high accuracies (75%) were obtained for such coarse-level habitat mapping, Landsat TM was more costeffective than IKONOS, even for small (50 km 2 ) areas. IKONOS may only be a cost-effective option if (i) independent field data are available to identify habitat patches, (ii) the area to be mapped is fairly small (<500 km 2 ), and (iii) small-scale (<10 m) habitat dynamics are to be monitored. IKONOS data are unlikely to identify deterioration in coral reefs directly because changes in community structure cannot be resolved spectrally from the satellite.

The Impact Of Exploiting Grazers (Scaridae) On The Dynamics Of Caribbean Coral Reefs

Coral reefs provide a number of ecosystem services including coastal defense from storms, the generation of building materials, and fisheries. It is increasingly clear that the management of reef resources requires an ecosystem approach in which extractive activities are weighed against the needs of the ecosystem and its functions rather than solely those of the fishery. Here, I use a spatially explicit simulation model of a Caribbean coral reef to examine the ecosystem requirements for grazing which is primarily conducted by parrotfishes (Scaridae). The model allows the impact of fishing grazers to be assessed in the wider context of other ecosystem processes including coral-algal competition, hurricanes, and mass extinction of the herbivorous urchin Diadema antillarum. Using a new analytical model of scarid grazing, it is estimated that parrotfishes can only maintain between 10% and 30% of a structurally complex forereef in a grazed state. Predictions from this grazing model were then incorporated into a broader simulation model of the ecosystem. Simulations predict that scarid grazing is unable to maintain high levels of coral cover (> or = 30%) when severe hurricanes occur on a decadal basis, such as occurs in parts of the northern Caribbean. However, reefs can withstand such intense disturbance when grazing is undertaken by both scarids and the urchin Diadema. Scarid grazing is predicted to allow recovery from hurricanes when their incidence falls to 20 years or less (e.g., most of Central and South America). Sensitivity analyses revealed that scarid grazing had the most acute impact on model behavior, and depletion led to the emergence of a stable, algal-dominated community state. Under conditions of heavy grazer depletion, coral cover was predicted to decline rapidly from an initial level of 30% to less than 1% within 40 years, even when hurricane frequency was low at 60 years. Depleted grazers caused a population bottleneck in juvenile corals in which algal overgrowth caused elevated levels of postsettlement mortality and resulted in a bimodal distribution of coral sizes. Several new hypotheses were generated including a region-wide change in the spatial heterogeneity of coral reefs following extinction of Diadema. The management of parrotfishes on Caribbean reefs is usually approached implicitly through no-take marine reserves. The model predicts that depletion of grazers in nonreserve areas can severely limit coral accretion. Other studies have shown that low coral accretion can reduce the structural complexity and therefore quality of the reef habitat for many organisms. A speculative yet rational inference from the model is that failure to manage scarid populations outside reserves will have a profoundly negative impact on the functioning of the reserve system and status of non-reserve reefs.

The tropicalization of temperate marine ecosystems: climate-mediated changes in herbivory and community phase shifts

Climate-driven changes in biotic interactions can profoundly alter ecological communities, particularly when they impact foundation species. In marine systems, changes in herbivory and the consequent loss of dominant habitat forming species can result in dramatic community phase shifts, such as from coral to macroalgal dominance when tropical fish herbivory decreases, and from algal forests to ‘barrens’ when temperate urchin grazing increases. Here, we propose a novel phase-shift away from macroalgal dominance caused by tropical herbivores extending their range into temperate regions. We argue that this phase shift is facilitated by poleward-flowing boundary currents that are creating ocean warming hotspots around the globe, enabling the range expansion of tropical species and increasing their grazing rates in temperate areas. Overgrazing of temperate macroalgae by tropical herbivorous fishes has already occurred in Japan and the Mediterranean. Emerging evidence suggests similar phenomena are occurring in other temperate regions, with increasing occurrence of tropical fishes on temperate reefs.