Keystone species are species without which our eco-system would change. For example, snow geese. They feed in salt marshes and keep other harmful species from taking over there. They also allow many different plants to grow there making for a more diverse habitat.
Sea otters are another example. Their favorite food is sea urchins. Sea urchins eat kelp. The sea urchins eat the kelp at the bottom of the ocean; therefore the kelp will drift away, its roots being eaten. If the otters didn’t eat sea urchins, the kelp forests would diminish if not disappear. This would affect the oceans as a whole. Also, what might happen if the sea urchins exhausted the supply of kelp? Not only would we not have it for our own use, but the sea urchins would probably die off because their food source would be gone. Then what would the otters do? It’s like dominoes—it’s a chain reaction.
That’s why certain species are called keystone species. They are the key stones to our eco-system. Without them the eco-system would change drastically.