European fern is a poisonous weed that has spread and destroyed many pastures in the northern hemisphere in recent years. A cheap and self-sustaining way to deal with this weed is to introduce the plant's natural enemies. As a result, some scientists have suggested that fern-eating moths from the southern hemisphere could be re-stocked to the northern hemisphere affected by the weed as a way to control fern growth.
The stocked moths were able to survive in the northern hemisphere and form a group large enough to reduce the fern population and prevent its growth is necessary for the success of the scientist's proposal to control the European fern.