by Dawn Barlow
Kennedy et al. (2013) examine the structural and ecological human-caused damage on coral reefs in the Caribbean since the 1960s and then generate modeled predictions for what coral reefs in the Caribbean might look like between now and the year 2080 if practices of local conservation and global action were to be implemented. They examine driving ecological factors of Caribbean reefs, reconstructing past disturbances to address the important roles that all of the many pieces play in contributing to overall health of the reefs. Some of these disturbances between the 1960s and the 2000s include depletion of the reef due to overfishing, loss of branching corals because of disease, bleaching, and bioerosion, hyperabundance of urchins because their predators were lost to overfishing, loss of urchins because of disease, poor watershed management that has led to changes in nutrient abundance, and ongoing climate change since the 1960s. In order for a coral reef to maintain its structure, the rate at which carbonate is produced must be greater than the rate of erosion—the carbonate budget must be positive. In order to maintain a carbonate budget that is either positive or at equilibrium, Kennedy et al. suggest that local management for the protection of grazers such as parrotfishes is important and can create a positive carbonate budget for reefs starting out with higher coral cover and keep reefs with lower coral cover to begin with near equilibrium, at least in the short term. In the long term however, aggressive mitigations will need to take effect on a global scale for there to be hope of maintaining a carbonate budget near equilibrium.
This study is a report based on past studies of Caribbean reef systems and projections for the future under different scenarios using climate models. Kennedy et al. choose the Caribbean as their model system for multiple reasons—there has been ongoing research in the region on carbonate budgets throughout the time period that they are examining, carbonate budgets and ecological dynamics are simpler to model because of low diversity relative to other reef systems, and there have been severe disturbances of many sorts over the past decades that have been studied and can be used to predict and understand future trajectories of ecosystem functioning. Simulation models of climate change, ecosystem dynamics, and carbonate processes were run based on parameters drawn from published literature as well as some unpublished data on Caribbean reefs and climate data from IPCC AR5 earth system models. Interventions to reduce local stressors were separated from global efforts to mitigate greenhouse gas emissions to assess the action needed to sustain net carbonate production. The authors run models on reefs with a relatively healthy coral cover (~20%) and more degraded coral cover (~10%), and look at the effect of local protection of grazing parrotfishes. This analysis is considered under “business as usual” CO2 conditions, as well as under a scenario where there is a move toward a lower carbon economy.
Major historical disturbances are outlined over the past several decades, beginning in the 1960s and 1970s, when the reefs had already experienced fisheries exploitation. This led to a boom in urchin populations because their natural predators were removed from the system, and this caused a net loss of reef structure due to an interaction between the urchins and crustose coralline algae. In the 1980s there was a region-wide die-off of branching coral because of disease, causing the most negative carbonate budget documented in the past decades. This event was followed by regional mass mortality in urchins, and the loss of these herbivores caused an abundance of macroalgae. In the 1990s and 2000s, bleaching events contributed to the overall lowered rates of carbonate production since the 1960s.
The authors found that under “business as usual” conditions, there was strong net erosion in the more degraded reefs regardless of local conservation efforts, and for reefs with relatively healthy coral cover, protection of parrotfishes caused a delay in the onset of erosion for approximately a decade. However, under the low emissions scenario, local conservation efforts were able to keep the carbonate budget at equilibrium for degraded reefs. When local parrotfish conservation measures were added to the low emissions scenario, there was a substantial increase in the carbonate budget. Thus they were able to conclude that it is necessary to begin with a relatively healthy reef for both local and global interventions to have a significant effect when the simulations were ended in 2080.
Grazers such as parrotfishes, and to a certain extent, urchins, are important to the ecosystem because they keep the macroalgae from dominating and provide space for coral recruits to settle. This implies that will be necessary to prevent overfishing of these grazers because coral cover declines in the functional absence of parrotfish, lowering the overall carbonate budget. Kennedy et al. suggest that carbonate budgets can be used to set target levels for coral, water quality, and herbivory to be better able to deliver biodiversity goals for reef management. This study suggests that local management and protection do have some preventative effect on the degradation of reefs, but that if we wish to see a lasting positive change in the carbonate budget or even carbonate budget equilibrium, dramatic global CO2 emissions reduction measures will be necessary.
Kennedy, E. V., Perry, C. T., Halloran, P. R., Iglesias-Prieto, R., Schonberg, C. H.L., Wisshak, M., Form, A. U. Carricart-Gavinet, J. P., Fine, M., Eakin, C. M., Mumby, P. J. Avoiding Coral Reef Functional Collapse Requires Local and Global Action. Current Biology 23, 912–918.