Answer:
another species that controls another species? I'm guessing to control their population. an example of that is wolves being unleashed into yellow stone Park to control the over population of the deer there.
it helped the park quite a bit in the long run. there's articles about it if you want to read up on it if you want. but trees grew 5x in height in the areas the deer started to avoid. alot of animals started moving back in like beavers, songbirds, owls, weasels, etc.