The extent to which alternate states are stable or reversible is presently not well understood. Such a process during which an otherwise stable ecosystem changes into another stable state is called a phase shift.

Stability & phase shifts
Many complex systems—ecosystems, economies—exhibits a dynamic behaviour by which they revert back to a stable equilibrium after minor or lesser pertubations from the outside. This is thanks to various buffer and corrective mechanisms, which kick in with growing effect the further away from the steady state equilibrium the system are brought. A very simple model of this principle is a ball in bowl. The ball will always come to rest in the middle. Unless, that is, you shake the bowl so violently that the ball falls out. And in that case, question is, where does the ball end up then?

Where do coral reefs end up?
Regarding coral reef ecosystems, on the other hand, there has been little success in predicting such regime or phase shifts. Mainly because the increased instability of coral reef ecosystems and their state prior to their collapses has often gone unrecognized,
even on reefs which have been well studied.

This cryptic loss of coral reef resilience can manifest itself in numerous ways. For example, in the Caribbean, the collapse of many coral reefs was long preceded
by dwindling stocks of fishes and increased nutrient and sediment runoff from land. In the 1950s, what prevented blooms of macroalgae from smothering the reefs was the increasing presence of a single species of sea urchin, Diadema antillarum, which grazed upon the algae. This is an example of a buffering mechanism that kicks in acting to return the system to its steady state—the ball seeks to roll back. In the 1970s, the densities of Diadema reached extraordinarily high levels on overfished reefs, averaging more than ten individuals per square metre in shallow waters. In 1983, the magnitude and crowded conditions of Diadema populations eventually led to an epidemic, which spread throughout the Caribbean, leading to a near total collapse of the populations and precipitating macro-algal blooms that still persist today. In addition, remnant coral populations
are further affected by increasingly prevalent coral disease and bleaching induced by global warming. The ball fell out of the bowl all together.

In retrospect, it is now clear that long before the present widespread loss of coral cover, many Caribbean reefs were on an unrecognized trajectory to collapse.
The symptoms included loss of macro-fauna and reduced fish stocks. And as the fishes role as the dominant herbivores were gradually replaced by a single species of sea urchins, this led to destructive overgrazing and bioerosion by food-limited sea urchins, and reduced recruitment of corals. Let us look further into the dynamics of these phase shifts.