keystone and sea star !? please help?

Keystone species are species in an ecological community that plays an important role. Sea stars are keystone species because they help regulate/sustain the other organisms in the community.

Sea stars are top of the food chain in the intertidal zones. If the sea stars are all killed off, then the whole food chain will shift. Since sea stars consume mussels which are their main source of food in the intertidal zone, the mussel population would increase and now would affect the other species like barnacles and other organisms because the mussels are now competing for more space and pushing other organisms away.