The selectivity of both natural disturbances and human impacts is a key issue for predicting their longer-term consequences. If cyclones, overfishing or coral bleaching affected all species equally, they would have no direct impact on species composition. Overall abundance would decline, but there would be no direct change in the relative abundance of each species. Massive destruction (e.g. more than a 90% decline in coral cover) is exceptional, even at the relatively small spatial scale that ecologists normally study. A large number of studies demonstrate selective mortality among corals and other organisms—from cyclones, predators, diseases, and human impacts. As well as the immediate changes that selective mortality induces, natural disturbances and human impacts also promote longer-term changes. For example, delayed mortality from outbreaks of disease among injured survivors (Fig. 9.8), bioerosion of damaged coral skeleton, and altered predator-prey relationships (if one survives better than the other) may occur for years after a cyclone has struck. Some species also rebound faster than others after an acute disturbance, further changing the composition of coral reefs.
Increasingly, many coral reefs today have a diminished ability to recover from recurrent natural disasters, due to other chronic human impacts. For example, the recovery of Red Sea corals after catastrophic mortality from low tides were much lower on a reef flat that was chronically impacted by oil-spills, compared to a nearby location that was free of oil. This erosion of resilience (Box 9.3) is often characterised by major shifts in species composition, called phase-shifts. Overfishing typically leads to dominance by fleshy macroalgae (Fig. 9.9), due to the depletion of herbivorous fishes. Nutrient addition can also contribute to algal blooms, and promote suspension feeders such as molluscs and sponges at the expense of corals. Managing the resilience of reefs (Chapter 12), so that they can continue to absorb natural and human impacts without fundamentally changing and degrading, is the major challenge for the future.
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