Keystone is the main center stone in the arch that keeps the arch from falling down.

The center stone, referred to as the keystone, distributes the weight above it to the rest of the bottom stones. This keystone controls the rest of the stones below it. Remove the keystone from the arch and the arch breaks down. this was the idea behind the Romans building an aqueduct to transport water.

This idea of the controlling keystone was applied to explaining the importance of living species in an ecosystem. Hence keystone species. They can be carnivores or herbivores, plant or animal, marine or terrestrial.

Keystone 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. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether. Some keystone species, such as the wolf, are also apex predators. The role that a keystone species plays in its ecosystem is analogous to the role of a keystone in an arch. While the keystone is under the least pressure of any of the stones in an arch, the arch still collapses without it. Similarly, an ecosystem may experience a dramatic shift if a keystone species is removed, even though that species was a small part of the ecosystem by measures of biomass or productivity.

A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance, a concept introduced in 1969 by the zoologist Robert T.Paine.   In image below, starfish is main predator or ketstone. All the rest of the ecosystem species are kept under control. Below is the food chain for starfish:

Below is a video of keystone species balancing out their ecosystems:

Keystone: nature restoring biological balance in food chain; length = 19:29 minutes

Wikipedia, "Keystone."  Keystone

Wikipedia,"Primary production."  Wiki:Primary production