Complex sentence: Overfishing has reduced the herbivorous fish that keep algae populations in check and caused a phase shift where algae covers 24 percent of the reefs where corals once stood.
Split into simpler sentences: Overfishing has reduced the herbivorous fish that keep algae populations in check and it has caused a phase shift from coral reefs to algae reefs. Today, algae covers 24 percent of the reefs where corals once stood.