How Caribbean coral reefs can suddenly disappear
Everyone who has had the opportunity to snorkel in the sea around a coral reef (or anyone who watches nature shows on television) knows how divers this ecosystem is. Parrotfish nibble on corrals, a little “Nemo” hides away in anemones and larger fish swim in large schools. Although the system seems stable, it can suddenly shift to another stable state, a state with hardly any fish, dead corrals and blooming algae. This shift has several underlying causes, which make the system vulnerable to the actual triggers of the shift. Intensive land-use causes nutrient-loading which allows algae to grow in higher numbers. This nevertheless does not directly trigger an algae bloom, since intensive fishing also leads to an increased number of sea urchins. These urchins control the algae population, by scraping them of the corrals. However, once a pathogen reduces the number of sea urchins, and herbivorous fish have also been diminished, the corrals can quickly be overgrown with fleshy brown algae. While young algae are easily eaten by urchins and fish, adult algae are far less edible. This makes the switch difficult to reverse, also because the algae prevent the settlement of coral larvae. This scenario occurred in 1983 in the Florida Keys, and the coral reefs are still recovering.
Three lessons can be learned from this example:
1. A shift in dominance between two different life forms can result in a shift of a contrasting ecosystem state.
2. The trigger that causes the abrupt shift is usually a random stochastic event, such as the pathogen in the Coral reefs example.
3. Biological, physical and chemical mechanisms are involved in stabilizing feedbacks, which can prevent a sudden shift.
Source: Scheffer et al. (2001)