why is it present under the apex list?
They often kept animals from grazing in riparian areas, which protected beavers from having their food sources encroached upon.
Keystone species
 In addition, wolves often kept animals from grazing in riparian areas, which protected beavers from having their food sources encroached upon. 
why are certain traits adaptations in one ecosystem but not in another apex UNANSWERABLE which species are present in an apex? Gray wolf
wiki:11751797
Keystone species [SEP] The wolf, Yellowstone's apex predator
In addition, wolves often kept animals from grazing in riparian areas, which protected beavers from having their food sources encroached upon. The removal of wolves had a direct effect on beaver populations, as their habitat became territory for grazing. Increased browsing on willows and conifers along Blacktail Creek due to a lack of predation caused channel incision because the beavers helped slow the water down, allowing soil to stay in place. Furthermore, predation keeps hydrological features such as creeks and streams in normal working order. When wolves were reintroduced, the beaver population and the whole riparian ecosystem recovered dramatically within a few years.