PARROTFISH eat algae and seaweed. These brightly colored fish with beaklike mouths inhabit coral reefs, the wellsprings of ocean life. Without them and other herbivores, algae and seaweed would overgrow the reefs, suppress coral growth and threaten the incredible array of life that depends on these reefs for shelter and food.

This was happening in Bermuda, until the government in 1990 banned fish traps that were decimating the parrotfish population. Today, Bermuda’s coral reefs are relatively healthy, a bright spot in the wider Caribbean, where total coral cover has declined by half since 1970.

Last month, in a reminder of just how dire the situation facing the world’s coral reefs is, the National Oceanic and Atmospheric Administration said it was listing 20 species of coral as threatened under the Endangered Species Act, including all of what were once the most abundant Caribbean corals. The action focused primarily on the projected impacts of global warming and ocean acidification. Carbon dioxide emissions are increasing ocean temperatures and making them more acidic — and less hospitable for corals.

But climate change is only half the story. Up to now, the impacts of climate change on reefs have been much less destructive than the localized effects of overfishing, runoff pollution from the land and the destruction of habitats from coastal development. Those problems have exploded in intensity over the past century and will continue to increase sharply with population growth.