Wolves were reintroduced to Yellowstone National Park in 1995,
and their return had a major impact on other species in the park,
especially elk and aspens. Elk began avoiding areas where
wolves could easily prey upon them, such as near riverbanks,
especially riverbanks with downed logs, where escape would be
difficult. With decreasing elk presence, aspens flourished in
these areas. The effects depicted here are a powerful example of
the influence of a keystone species on an ecological community.Cause and Effect
Assessment available inWolf population
Percentage
of aspen trees
grazed upon
Height of
aspen saplings
(in cm)
LegendTrees in
uplands with
downed logsTrees in uplands
without logsTrees on
riverbanks with
downed logsTrees on
riverbanks
without logsElk population
(in thousands)