Keystone species

A keystone species is an animal or plant that is important to an ecosystem. They are called keystone species because they hold an ecosystem together like a keystone holds together an arch. An example of a keystone species is the sea otter, who live in the ocean. They eat sea urchins, which would otherwise eat too many of the seaweed and kelp plants. Without these plants, the whole ocean ecosystem would be changed and many other animals would not survive.