The resilience of coral reefs: The role of shelter limitation, connectivity, functional redundancy and response diversity
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Knowledge of factors that enhance or undermine the resilience of coral reefs helps us understand and predict the response of reefs to environmental disturbances in the past, present and future. If the cumulative stress of disturbances like overfishing, sediment-runoff, eutrophication, hurricanes and bleaching events exceeds a certain threshold, coral reefs may exhibit non-linear behaviour and undergo discontinuous phase shifts from a state of coral to a state of macroalgal dominance. Overgrown reefs are characterized by a low biodiversity and a heavily compromised capacity to provide ecosystem services to man. The macroalgae state has proven to be highly stable and resistant to restoration efforts like reductions in fishing pressure. The chances of preventing and escaping this deadlock of macroalgal dominance differ widely between reefs and are known to depend on resilience factors like reef connectivity and the composition and diversity of key functional groups like herbivores. Here we use a dynamic model to examine the role of connectivity, shelter limitation in herbivorous fish and the ambiguous role of sea urchins in phase shift dynamics of a multiple-stable coral reef. Sea urchins provide a mixed blessing to reef resilience as they may compensate for a loss in fish herbivory but also, if unchecked, inflict serious damage to coral colonies through bioerosion processes at high urchin densities. Our results suggest that connectivity significantly enhances the reversibility of a phase shift from coral to macroalgal dominance but does little to prevent it. In contrast, a strong shelter limitation in herbivorous fish compromises the reversibility of a shift and may preclude a reversal even in highly connected reefs. Our findings also substantiate the ambiguous role of sea urchins in coral reef resilience. Sea urchins were found to be capable of preventing shifts by compensating for a loss in fish herbivory at high fishing pressures. However, if bioerosion effects at high urchin densities were accounted for, an overfishing-induced shift in the composition of the herbivore guild towards high urchin densities may also trigger phase shifts. These bioerosion-induced shifts may be either continuous or discontinuous depending on the overall resilience of the reef. Despite their demonstrated importance for reef resilience, explicit herbivore dynamics are often neglected when modelling phase shift dynamics in coral reefs. We advocate for a stronger consideration of these dynamics and particularly of the role of sea urchins in future studies on coral reef resilience.