It’s because of the prodigious ability of coral to aggregate nutrients that coral reefs are some of the most biodiverse ecosystems on Earth. But while algae and grasses grow faster than corals, they’re normally kept in check by sea urchins, which slowly graze over a reef, and grazing fish like parrotfish. However, Caribbean sea urchins experienced a massive die-off in the 70s, and overfishing has caused grazing fish populations to plummet. The problem is that while coral provides the base of the nutrient cycle in a reef, it can’t compete against algae and grasses in the absence of predators, which will eventually drive out all of the coral and leave the reef barren.
Compounding the problem is the fact that zooxanthellae, which live inside living coral and are the main drivers of that whole nutrient-gathering thing, are very temperature sensitive. When water temperatures increase rapidly, the zooxanthellae ditch the coral en masse (an event known as bleaching), which leaves the coral pretty much dead. Because climate change has caused Caribbean waters to increase in temperature rather rapidly, the region has experienced more bleaching events. With that double whammy, the Caribbean has become the poster child for reef system collapse.