In the 1960s, a guy named Bob Paine picked a small stretch of rocky beach in Washington state and evicted its sea stars, crowbarring them off the rocks and throwing them back into the ocean. Within a year, the beach’s demographics had changed dramatically: barnacles and then mussels replaced algae and limpets. Species richness, or the number of different species present, hadn’t gone down by the number of sea stars–it had halved.
Paine realized the reason behind the shift was that without the sea stars around to eat barnacles and mussels, their populations skyrocketed, and their demand for algae and limpets increased, causing those populations to crash. He called the three-step chain a “trophic cascade”; the sea stars at the top that shaped the chain he called an “apex predator.”