Many ecosystems contain species that are called keystone because without them the system would collapse into barrenness. I don’t know if every ecosystem contains a keystone, but there are many examples. I would have guessed the keystones would be the creatures at the bottom of an ecosystem’s food chain, but it turns out there are examples with keystones at the top, namely the predators.
In the 1960s, marine biologist, Bob Paine conjectured that there were predator keystone species, but testing required controlled environments. He chose tide pools as one of the easiest. Starfish prey on mussels using their succored legs to open them. Paine removed starfish by hand from a chosen pool, tossing them into an adjacent one over a long period. After months there were more mussels but less of everything else. Years later only mussels remained. Well, since mussels eat by filtering plankton from the sea water, plankton must have been there, but not easy to detect as they appear as mere specks.
After this breakthrough discovery, more predators were discovered to be keystone, but not all keystone are predators. Keystones include:
African elephants tramp down trees and provide corridors that contain fire. They excrete seeds that maintains grassland. Grizzly bears toss their salmon bones, which fertilize the soil. Sharks eat dead diseased fish, curbing pandemics Sea otters control the sea urchin opulation, which destroy kelp. Krill feed whales, seals, penguins, squid, and fish Beavers build dams that provide habitats for salmon and other creatures. Bees, Hummingbirds and Honey possums are absolutely critical pollinators to our food supply. Parrotfish are the janitors of coral reefs. Prairie dogs are gardeners of the prairies, positively impacting water supply. African termites fertilize the land and provide small homes. Woodpeckers also provide space for nests. Saguaro cactus are condos for birds and small animals, They provide fruit in season.
Given the importance of keystone species, efforts are being made to restore areas of our planet that have suffered. Wolves have been reintroduced into Yosemite. By keeping the deer population down, the variety and quantity of vegetation has increased followed by streams and other animals who now thrive. Dwindling wild dog populations in Africa have been boosted by World Wild Life funds with a similar benefit.
Wildebeests have restored the Serengeti. They eat grass that burns easily, and without fires trees recovered supporting elephants, giraffes, birds, and other species including predators like lions.
Bass are keystone in our rivers.
Wild dogs photo: https://www.maxpixel.net/ph