▁ex termin ation ▁of ▁the ▁w olf ▁population .< ref ▁name =" d ow "></ ref </ ref > ▁ ▁A ▁century ▁later , ▁in ▁the ▁mid - tw enti eth ▁century , ▁it ▁began ▁to ▁be ▁believed ▁that ▁the ▁earlier ▁policy ▁of ▁" prote ction ▁from ▁disturb ance ▁was ▁in ade qu ate ▁to ▁preserve ▁park ▁values ", ▁and ▁that ▁is ▁that ▁direct ▁human ▁inter vention ▁was ▁necessary ▁to ▁restore ▁the ▁landscape ▁of ▁National ▁Park s ▁to ▁its ▁‘ ’ natural ’ ’ ▁condition . ▁In ▁ 1 9 6 3 ▁the ▁Leopold ▁Report ▁argued ▁that ▁" A ▁national ▁park ▁should ▁represent ▁a ▁v ign ette ▁of ▁primitive ▁America ". ▁This ▁policy ▁change ▁eventually ▁led ▁to ▁the ▁rest oration ▁of ▁wol ves ▁in ▁Y ellow stone ▁Park ▁in ▁the ▁ 1 9 9 0 s . ▁ ▁However , ▁recent ▁research ▁in ▁various ▁discipl ines ▁indicates ▁that ▁a ▁pr ist ine ▁natural ▁or ▁" prim itive " ▁landscape ▁is ▁a ▁myth , ▁and ▁it ▁now ▁real ised ▁that ▁people ▁have ▁been ▁changing ▁the ▁natural ▁into ▁a ▁cultural ▁landscape ▁for ▁a ▁long ▁while , ▁and ▁that ▁there ▁are ▁few ▁places ▁un touch ed ▁in ▁some ▁way ▁from ▁human ▁influence . ▁The ▁earlier ▁conservation ▁policies ▁were ▁now ▁seen ▁as ▁cultural ▁inter vent ions . ▁The ▁idea ▁of ▁what ▁is ▁natural ▁and ▁what ▁artificial ▁or ▁cultural , ▁and ▁how ▁to ▁maintain ▁the ▁natural ▁elements ▁in ▁a ▁landscape , ▁has ▁been ▁further ▁complicated ▁by ▁the ▁discovery ▁of ▁global ▁war ming ▁and ▁how ▁it ▁is ▁changing ▁natural