Source: American Scientist 
February 9, 2010
At the conclusion of his Darwin Medal Lecture at the 11th International Coral Reef Symposium in 2008, Terry Hughes, who is director of the Australian Research Council's Centre of Excellence for Coral Reef Studies at James Cook University, projected two side-by-side images onto massive screens in the darkened hall. On the left was an image of a canoe in which two passengers sat, comfortably dry and smiling. On the right was the same canoe, only upside down, with the passengers in the water. Hughes explained that these are the two equilibrium states for a canoe: upright and capsized. At equilibrium, the canoe resists shifting from one state to the other. But with enough forcing, a tipping point is reached at which the canoe can shift rapidly into the opposite state of equilibrium, sometimes to the dismay of the passengers.
Hughes's apt metaphor underscored a key message of his lecture: that coral reefs have tipping points as well. And although they may resist change at first, showing few outward signs of stress, when shifts do take place, they can occur more rapidly than anyone had previously predicted and are tremendously difficult to reverse.
To read the full text of the article, click here .