Northern-range elk numbers have changed through four successive population phases over time: low numbers in prehistory up to ~1883; censuses of >6,000 up to 20,000-35,000 from 1884-1958; <6,000 from 1959-1970 during park reductions; and >6,000 from 1971 to the low 2,000s after reductions stopped. Varying sources of evidence indicate alternating responses to each of these phases by all measured components of the northern-range biota. The pre-1872 northern-range ecosystem can be characterized by a hierarchical model constrained at the top by human and large-carnivore predators that limited elk numbers, prevented heavy herbivory, and allowed development of a vegetation that provided ample food for other herbivores, habitat for other faunal components, and constraints on hydrology. Removing the predatory constraints allowed elk increase that altered the structure of the system and progressively reduced biodiversity. These changes have disproved the ecological-effect tenets of the natural-regulation hypothesis. Expansion of the northern range since 1988 and the reestablishment of wolves could return the system structure back to its 1872 state, although it is too early to ascertain whether this will occur. Meanwhile, climate warming could alter system structure to conditions not previously experienced.
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