In 1966, an ecologist at the University of Washington named Robert Paine removed all the ochre starfish from a short stretch of Pacific shoreline on Washington’s Olympic Peninsula. The absence of the predator had a dramatic effect on its ecosystem. In less than a year, a diverse tidal environment collapsed into a monoculture of mussels because the starfish was no longer around to eat them.

By keeping mussel numbers down, the starfish had allowed many other species to thrive, from seaweed to sponges. Paine’s research led to the well-known concept of keystone species: The idea that some species in an ecosystem have prevailin