In an ecosystem, keystone species are those

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 Keystone species is a species that helps to define the whole ecosystem and plays a critical role in maintaining its structure. The Keystone species have an important regulatory effect on other species in the ecosystem. Without the keystone species, the ecosystem will be very much different or may not exist. If the keystone species disappear from the ecosystem, no other species will fill the ecological niche. Any organism can be a keystone species if they have a huge influence on food webs. 

Sea otters protect the kelp forests from sea urchins by regulating the sea urchin population. In the absence of sea otters, sea urchins overgraze kelp and destroy the kelp forest. Therefore, in the kelp forest ecosystem, sea otters are the keystone species.

A keystone species may be carnivores (gray wolves in the Greater Yellowstone Ecosystem, USA), herbivores (elephants in African savannas or the Serengeti plains, Tanzania), and mutualists (hummingbird and indigenous plants in woody grasslands of Patagonia, South America).