What effect does a keystone predator have on its habitat?
A keystone predator keeps the population of a certain species (usually their prey) in check.
If we remove a keystone predator from it's habitat, their prey population would increase, leading to an abnormal change in the ecosystem of an area. On the other hand, if a keystone predator is introduced to an already existing ecosystem it will affect the ecosystem at large and might allow other species to enter the area as well. Here is a a good example using wolves as a keystone species in Yellowstone: https://www.livingwithwolves.org/wolves-a-keystone-species/
Wolves are carnivores and prey on usually larger animals by hunting in packs, the leftovers allow the entry of birds of prey in the area.