In an ecosystem, one group consumes another. In the forest or in the woods moose eat plants but wolves eat the moose but if there is too much wolves the moose population decreases so the moose population grows since there isn't a lot of wolves eating them but too many moose cause scarce food so the moose population decreases and that goes on and on. That is a 3 link chain since moose eats plants and wolves eat the moose.