 The gray wolf, after being extirpated in the 1920s and absent for 70 years, was reintroduced to the park in 1995 and 1996.Since then a three-tiered trophic cascade has been reestablished involving wolves, elk ("Cervus elaphus"), and woody browse species such as aspen ("Populus tremuloides"), cottonwoods ("Populus" spp.