The status of coral reef ecosystems is routinely measured and
monitored using a small number of metrics, usually abundances of
important taxonomic groups, especially corals. However, coral
cover is not a reliable metric of resilience, because a healthy reef
that is recovering towards a coral-dominated equilibrium can have
substantially less coral than one that is locked into a downward
trajectory to dominance by macroalgae. Coral cover only becomes a
definitive indicator of phase-shifts if the same site is monitored for
many years, and if the mechanisms and feedbacks have been
identified. The spectacular decline of corals in many parts the
Caribbean in the 1980s came as an ecological surprise because
people then and now commonly mistake high coral abundance as
an indicator of resilience.