Are Keystone species always predators?

How do predators, prey, and keystone species act as agents of natural selection in populations?

• can you explain to me. i'm little confused.

• Answer:

  A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance... Such species play a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and numbers of various other species in the community... As was described by Dr. Robert Paine in his classic 1966 paper, some sea stars may prey on sea urchins, mussels, and other shellfish that have no other natural predators.. If the sea star is removed from the ecosystem, the mussel population explodes uncontrollably, driving out most other species, while the urchin population annihilates coral reefs...