Coral reef ecosystems are affected by various anthropogenic and natural disturbances, potentially crippling the resilience of the ecosystem. For this reason, identifying appropriate reef management policies is largely dependent on the interaction between multiple stressors. Notably, in Caribbean coral reefs, fishing efforts and hurricane impacts on local areas threaten the future sustainability of reefs. In this study, Blackwood et al. (2011) seek to measure the combined impacts of multiple stressors, including coral-algal interactions and grazing by herbivorous reef fish. The authors developed an analytic model that considers changes in structural complexity, the direct impacts of hurricanes, and various levels of fishing efforts. The model simulations focus on the effects of these interacting elements for specific geographic locations. However, in order to simplify the investigation, this study focuses solely on relatively short times scales, and does not account for changes in ocean temperature, chemistry, or sea level rise. The results enable the authors to determine whether the stressors behave synergistically, and to predict potential coral reef recovery patterns from interacting stressors. Additionally, they propose appropriate management policies, recommending either local reef restoration or fisheries management based on their results.
Blackwood, J. C., Hastings, A., Mumby, P. J. 2011. A model-based approach to determine the long-term effects of multiple interacting stressors on coral reefs. Ecological Applications 21, 2722-2733.
Julie Blackwood from the University of California, Davis, collaborated with various scientists to produce this study. The research conducted is an extension of previous work quantitatively measuring coral-algae interactions using an analytic model. Two distinct states of coral recovery were investigated: the complexity of reef structure after having suffered an acute disturbance such as a hurricane, and coral recovery from events such as bleaching or disease in which the reef structure remained intact. Rather than solely focusing on states of high coral cover and coral depletion, the authors included parrotfish dynamics and their mortality from fishing rates. The numerical responses of parrotfish to changes in habitat conditions are measured simultaneously with food limitation on parrotfish abundance. This allows the authors to determine the long-term effects of multiple coral stressors.
Here, rugosity is defined as the structural complexity of a reef ecosystem. This analysis accounts for slow changes in rugosity that occur between hurricane impacts, direcly affected by the resilience of reefbuilding corals and by bioerosion. Hurricane impacts are measured as “jump processes,” meaning that they occur randomly and the impacts are instantaneous. Furthermore, it is assumed that recovery trajectories from hurricanes immediately resume to prehurricane rates. Three different levels of damage to coral reefs are then simulated, consideringgeographic differences. The authors assume that hurricanes reduce coral rugosity by 50% by impacting reef structure, 5% by pruning out coral colonies, and an intermediate case where a hurricane has the same proportional effect on rugosity as coral cover. Varying hurricane frequencies are considered in order to identify diverse geographic conditions, and are simulated for a 100 year time period.
The results from this study suggest that different management guidelines are necessary for different regions in the Caribbean. Given differences in local environmental conditions and hurricane frequencies, the resilience of reef ecosystems to disturbances is highly variable. Generally, there appears to be greater resilience to fishing efforts for higher frequency hurricanes only when hurricane damage levels and the growth rate of macroalgae are sufficiently low. Otherwise, higher impact levels cause substantial damage to the strucutural complexity, reducing refugia for parrotfish and limiting the ecosystem services provided by the corals. However, when macroalgal growth rates are high, even regions with low frequency hurricanes have strong resilience to fishing effort. These results indicate that in regions with frequently occurring hurricanes, management should put greater emphasis on determining whether reef restoration methods are cost effective. Additionally, the authors maintain that managers should adapt their policy implementations according to shifts in local hurricane frequencies. For example, reefs experiencing low frequency hurricanes demonstrate less resilience to fishing effort, therefore there should be greater emphasis on fishing regulations to promote resilience. Since all levels of hurricane frequencies threaten the structural complexity of reef ecosystems, future studies need to focus on the impacts of global warming. Previous studies suggest that global warming will likely increase hurricane intensity,resulting in increased relevance of policy recommendations of high hurricane frequencies through time.