Keystone species: like a stone bridging the two sides of an arch, once removed the structure crumbles. One of ecologists favorite example in the last twenty years has been the grey wolf.
Humans have a long history of destroying ecosystems and obliterating biodiversity. More than likely we’ve seen the loss of more keystone species than we’re aware of because we didn’t take careful note of how the ecosystem was before we changed it.
Modern humans, however, have on occasion reintroduced species. An infamous example are the grey wolves at Yellowstone National Park. As expected, they kept the deer populations down and there was a trickle effect to other plants and animals–less vegetarian consumption allowed for better habitat, and those animals in turn fed other predators.
Geography changed as well. Without deer spending most of their time at rivers, the plants had a chance to grow along the banks and stabilize erosion. With less erosion the water quality improved and life in and down stream are better.
Scientists expected the trickle effect within the biological communities. The physical ecosystem being integrated with that trickle wasn’t.
From Youtube’s Sustainable Human channel.