The degradation of coral reef ecosystems results from the combined impacts of global climate change and local activities. Although improvements to global warming trends are unlikely to occur in the immediate future, several conservation tools exist for coral reef managers. In this study, Edwards et al. (2010) examine the role that fisheries conservation tools can play in promoting the recovery of Caribbean coral reefs from disturbance events. Using an individual-based model of ecological factors and accounting for spatial differences, they test the impact of disturbances on coral populations. Simulations for contrasting regions throughout the Caribbean demonstrate that regional differences in hurricane frequency cause different spatial patterns of reef health. While greater “patchiness” of coral cover occurred in Belize, more frequent disturbances in the Bahamas increased coral cover. Although substantial variation exists among regions, coral bleaching is shown to contribute to the decline in coral reef health over time. Additionally, the protection of herbivores fails to prevent reef degradation, but it does delay rates of coral loss over the tested time period. These results demonstrate that the impact of local conservation measures on reef ecosystem will vary spatially and temporally. Cecilia Ledesma
Edwards, H.J., Elliott, I.A., Eakin, C.M., Irikawa, A., Madin, J.S., McField, M., Morgan, J.A., van Woesik, R., Mumby, P.J. (2010) How much time can herbivore protection buy for coral reefs under realistic regimes of hurricanes and coral bleaching? Global Change Biology 17, 2033-2048.
Helen Edwards collaborated with a number of scientists to investigate potential management interventions for preventing future degradation of coral reef ecosystems. They demonstrate that by altering the management of local factors, the vulnerability of coral reefs to changes in temperature, storms, and ocean chemistry can be reduced. By establishing a spatially realistic model of climate-change impacts on Caribbean reefs, they were able to study the extent to which local changes in grazing can influence their dynamics. An individual-based model of the Montastraea annularis zone of a Caribbean coral reef was used to model the ecological dynamics after coral populations expereienced spatially-realistic disturbances. The simulations were carried out for three contrasting regions of the Caribbean, including Belize, Benaire, and the Bahamas. Climatologies were acquired from the US National Oceanographic and Atmospheric Administration (NOAA) and future sea surface temperatures (SSTs) were derived from the SERS A1 scenario. Although scenario A1 describes a “worst case” scenario, the authors argue that it is an appropriate prediction since current growth rates of CO2 emission exceed this trajectory. In order to model bleaching caused by elevated temperatures, the projected temperatures and SST were then used to calculate the number of degree heating months (DHMs). Since the response of coral to bleaching events has not yet been monitored for areas in the Caribbean, Edwards et al. instead examined the response of nonacroporid Pacific species that best compared with corals in the Caribbean. Data on the frequency of past storms were used to simulate predicted environmental conditions at reef locations across the Caribbean. Changes in coral cover within reef communities were measured for a 90-year period in which bleaching and hurricanes occur.
The results from this study show that the frequency of predicted thermal stress events increased with time. Although no severe bleaching events were predicted for the Bahamas between 2012 and 2039, sites in Belize experienced severe bleaching events between 2060 and 2099. Meanwhile, sites in Bonaire experienced few severe events before 2070. In the areas with high rates of hurricane incidence the coral cover was generally lower and more variable, increasing overall net-reef recovery. When grazing was carried out by an unexploited community of parrotfishes and reefs were only impacted by coral bleaching, coral cover maintained its initial state for approximately the first 40 years, after which it steadily declined. When hurricanes were modelled in combination with bleaching events, reefs in each region of the Caribbean exhibited net decline. Contrary to the authors’ expectations, the presence of a community of urchins did not prevent reef decline under global warming. Overall, regions across the Caribbean characterized by very different physical disturbance regimes showed a similar difference in the impacts from local disturbances, thus resulting in spatial variation in predicted coral cover.
Of significant consequence, this study found that although the rates of decline vary substantially with geographic location, climate change impacts are detrimental throughout the Caribbean. Spatial variation in hurricane frequency and strength led to significant differences in disturbance regimes in the Bahamas and Belize. Meanwhile, analysis showed that Bonaire has the least damaging disturbance environment of the three regions. Furthermore, this study does not take into account that the frequency or severity of storms may change in the future, potentially underestimating the negative impacts of hurricanes on reef health. Even for the highest grazing scenarios with stable levels of coral cover, coral cover declined once coral bleaching was added to the disturbance simulation. This underscores the critical need to reduce global greenhouse gas and carbon dioxide emissions.