Answer:
The keystone species is the wolf.
Step-by-step explanation:
Every species of organism is important to the ecosystem, so if just one species gets extinct for whatever reason there will be consequences for the other species. How big the consequences will be, depends on the species and on how big of an effect it has on the ecosystem. Some species have relatively small importance, while some have huge importance, and the ones that have huge importance are called keystone species.
The keystone species have such big importance to an ecosystem that with their presence everything is under control, but if they go extinct then the ecosystem suffers badly and changes dramatically. In this case, a keystone species is the wolf. The wolves control the elk population, which in turn means that there is no shortage of food for the elk and they don't damage the young trees. By not damaging the young trees, they grow, so there is a much larger area covered by forest. More forest means less erosion, the water of the streams is better controlled, and there is less meandering. If the wolves are out of the picture, as they were for some time, the picture is totally opposite.