In the 1980's S. planifrons was a common sight among the fore-reef terraces throughout the Caribbean, an abundant habitat at that time. It was thought that these fish were not at carrying capacity and recruitment-limited. However, with the decline of Acropora species, a decline caused or exacerbated by climate change, diseases, hurricanes, pollution and overfishing, this view is changing. Acropora are fast growing species and are thought to have held their own against damselfish, a trait that other coral species haven't exhibited, and with the decline in their populations other corals are feeling the strain.
Basically, the damselfish are not finding their optimal or preferred habitat - a habitat that can withstand herbivory by the fish. So they are looking for acceptable alternatives, slow-growing coral heads (especially the Montastraea annularis species complex) , and are eating them to death. This is a change that is killing many coral heads across the Caribbean, a change that could take hundreds of years to recover from. And even the fish themselves are limited by habitat and are also showing a decline. A likely course of action to help fix the problem would be to conserve staghorn corals, conserving fish stocks, and restoring coral (mainly staghorns) populations.
Here's the article:
Precht WF, Aronson RB, Moody RM, Kaufman L. (2010) Changing Patterns of Microhabitat Utilization by the Threespot Damselfish, Stegastes planifrons, on Caribbean Reefs. PLoS ONE: 5(5), e10835. (DOI: 10.1371/journal.pone.0010835)
(image from reefbuilders.com)