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.