Corals reefs worldwide have suffered signficant impact as a result of rising sea temperatures. The effect on reef ecosystems located in the Indian Ocean was particularly severe in 1998 when 45% of living coral was killed. The development of management interventions well adapted to changing environmental conditions requires a comprehensive understanding of factors affecting reef thermal stress and resilience. In this study McClanahan et al. (2011) seek to identify coral reef areas of low climate stress that are resilient to climate change. The authors mapped climatic-oceanographic stress and coral reef diversity in the western Indian Ocean (WIO) in order to establish a relationship between high diversity coral reefs and areas of low climate stress. The results from an environmental stress model and susceptibility index of the WIO confirmed moderate correlation, showing that the southern and eastern parts are areas with low environmental stress. Meanwhile, regions in the north and west were identified with high fish diversity, and regions from Tanzania to northwestern Madagascar with high coral diversity. McClanahan et al. suggest that these areas are ideal locations for management efforts aimed at protecting coral reef from climate change disturbances.
McClanahan, T. R., Maina, J. M. and Muthiga, N. A. 2011, Associations between climate stress and coral reef diversity in the western Indian Ocean. Global Change Biology, 17: 2023–2032. doi: 10.1111/j.1365-2486.2011.02395.x
Tim McClanahan and colleagues at the Wildlife Conservation Society conducted their research in the western Indian Ocean (WIO), an area stretching from the coast of East Africa to the banks of the Mascarene Plateau. The authors measured the environmental stress of the WIO by using a multivariate stress model (SMI) that measured environmental exposure, sea surface temperature (SST) rate of rise, and chlorophyll concentrations, among a number of various oceanographic factors. The map generated from these variables demonstrated their relationship to coral bleaching, also measured in the model. In order to quantify biodiversity, visual observations were used to measure the richness of coral communities and belt-transect surveys projected numbers of fish species. The degree of interrelatedness of the abovementioned data was then evaluated using pairwise correlation analysis, referred to as Moran’s Index. With Moran’s Indices of 0.40, 0.13, and 0.28 for coral community susceptibility and the numbers of fish and coral species, there was less an a 1% likelihood that the clustered patterns are due to chance. These variables were then synthesized onto a final map using an algebraic sum equation.
The SMI pairwise comparisons demonstrated that several of the variables measured were statistically significant, albeit not strongly. While coral community susceptability and number of coral species were correlated with modelled stress, numbers of fish species were positively correlated with it. Although coral species exhibited the highest numbers at intermediate latitudes between 5 and 101S, the number of fish species was greatest at northern latitudes. Additionally, a map of the data identified the southern WIO region as low stress with some moderate stress regions along the Tanzanian-Mozambique border and a few very high stress areas in the northern regions. These results demonstrate the importance of the Tanzania-Mozambique border as an area of low-stress and high-diversity well suited for coral ecosystems.
This study identifies Madagascar, the Mascarene Islands, and the region from southern Kenya to northern Mozambique as areas of low environmental stress and high biodiversity. McClanahan et al. maintain that conservation strategies should focus on the protection of these coral reef ecosystems. However, the relationship between measures of environmental stress and biodiversity are often conflicting for these regions within the WIO. Although the ability to clearly identify locations possessing desired environmental conditions and species diversity is difficult, a sustainable strategy for climate change would support appropriate restrictions within these habitats.
The first hypothesis posited by Mumby et al. proved to hold true; the spatial distribution of thermal stress and larval connectivity are similar enough that networks may be stratified according to the response of corals to bleaching. Furthermore, satellite measurements of SST reveal that there is enough larval supply to generate a reserve network. Results from the second hypothesis indicate that the key difference in response scenarios is not corals ability to adapt to global warming, but the differences between the alternate scenarios. Although the authors provided potential selection sites for reef reserves based on coral adaptation to stress, they maintain that further research is needed in this area. The framework Mumby et al. develop may be adapted to future improvements in research regarding larval connection and coral response to climate change.
Atmospheric levels of carbon dioxide are currently at 380ppm, and are expected to increase within the next few decades. Such increases in atmospheric concentrations result in increased levels of dissolved CO2 in the oceans. Elevated levels of dissolved CO2 increase ocean acidification and reduce carbonate-ion saturation. Scientists predict that these changes will have varied effects on marine species. A number of studies have demonstrated the variability in the responses of different aquatic species to increased levels of CO2. However, most studies have only compared species belonging to different orders. The study conducted by Ferrari et al. seeks to provide a comparison of physiological responses to elevated CO2 among closely related species. By doing so, they can test whether species sharing related life histories and ecology have similar tolerance levels to CO2. The ability to group species according to their CO2 tolerance could play a key role in predicting changes to marine communities in the future. Recent studies have demonstrated that CO2 exposure causes olfactory impairment in a number of reef fish. Given that reef fish rely upon their chemosensory ability to detect predators, any olfactory impairment is expected to increase mortality rates. Ferrari et al. tested the effects of CO2 exposure on the antipredator responses of four damselfish species; their research revealed that contrary to expectations, the four congeneric species varied drastically in CO2 tolerance. Additionally, using Pomacentrus chrysurus as a model species, their results suggest that reef fish larvae exposed to high levels of CO2 showed decreased antipredator responses to risk.—Cecilia Ledesma
Ferrari, M. C. O., Dixson, D.L., Munday, P. L., McCormick, M. I., Meekan, M. G., SIH, A. and Chivers, D. P. (2011), Intrageneric variation in antipredator responses of coral reef fishes affected by ocean acidification: implications for climate change projections on marine communities. Global Change Biology, 17: 2980–2986. doi: 10.1111/j.1365-2486.2011.02439.x
Maud C. O. Ferrari and scientists from various institutes collaborated on research submitted to Global Change Biology. The experiments for this study were conducted in November and December 2009 at the Lizard Island Research Station located on the Great Barrier Reef, Australia. Four damselfish species, P. chrysurus, P. moluccensis, P. amboinensis and P. nagasakiensis, were collected in light traps at the end of the planktonic larval stage. Once transferred to rearing aquariums, the fish were exposed to CO2-enriched air for 4 consecutive days. The tested levels of were approximately 700 and 850 ppm, levels of dissolved CO2 predicted under greenhouse gas emission scenarios. Immediately after the treatment period was over the fish were used in the experiment. Ferrari et al. measured the antipredator behavioral responses of the fish by quantifying their foraging, swimming activity, and microhabit use after detection of predator risk cues. These activities represent common antipredator responses in prey fishes. In order to compare behavior and mortality rates of fish exposed to CO2, P. chrysurus were placed on small patch reefs for about 11 hours and their behavior and survival were monitored.
The authors found that all four species showed varied antipredator responses when exposed to levels of CO2 higher than 390 ppm. While P. chrysurus, P. moluccensis, P. amboinensis and P. nagasakiensis all displayed significant losses of antipredator behavior, P. amboinensis remained less affected at CO2 levels of 700 ppm. The activities of CO2-treated P. chrysuru in the field were compared to those of control fish. The comparision indicates that P. chrysuru suffered much higher mortality rates, were more active, moved further and higher away from the reef, displayed higher feeding rates and were bolder than control fish. Thus, the CO2-exposed fish clearly lacked the adaptive antipredator responses to risk cues exhibited by control fish.
P. amboinensis appear as the more sensitive species; at CO2 levels of 700 ppm, it showed a 95% reduction in antipredator response. Meanwhile, at those same levels P. nagasakiensis only showed a 30% reduction in antipredator responses. Given similar history traits between the species, the authors cannot explain the variation in responses. They note that in order to better predict changes in marine communities under increasing CO2 levels future work should focus on the role of physiology in explaining these variations. Additionally, the reduced antipredator behavior observed in the lab translated into decreased survival rates under natural conditions; these fish showed as much as a fivefold to sevenfold incresae in mortality when placed in the reefs. These results indicate that interspecific variation in response to rising CO2 may result in changes to the composition of prey species; this would in turn effect the biodiversity of coral reef communities. However, the impact of increased levels of dissolved CO2 on marine ecosystems will depend on both the magnitude of variation between species and the ability of these species to adapt to changing environmental conditions.