Yellowstone National Park
Wolf Case Study
Wyoming’s Yellowstone
Species Interactions
cause trophic cascades
• Trophic cascade: changes in one level will create
changes in other trophic levels.
• Trophic Levels:
tertiary
consumers
5
• Tertiary Consumer
secondary
consumers
5000
• Secondary Consumer
primary
consumers
500,000
• Primary Consumer
producers
5,000,000
• Primary Producer/ plants
Food Web in Yellowstone
• The wolves eat
moose, pronghorn,
mule deer, elk, bison,
and bighorn sheep.
• They share these
prey (food) with other
predators like:
coyotes, mountain
lions, and grizzly and
black bears.
The wolves are a keystone species
• Keystone species: they have an unusually
large effect on the ecosystem.
• In 1926 unrestricted hunting had
eliminated the wolves from the park
community.
• In 1920 An annual census of elk was
initiated by park managers.
Intervening in nature
• From 1920-1968 the park service
selectively killed elk herds so they wouldn’t
go over caring capacity.
• In 1968 they stopped because of public
pressure.
• Once the killing stopped the elk population
rapidly increased.
Effects of the increase in Elk
• The elk eat aspen trees and once their
population took off there were no more
young trees.
• The elk also severely browsed (ate)
streamside willows and this didn’t allow
beavers to eat or make dams.
– The absence of beavers dams made less
biodiversity because of loss of this habitat.
–Everywhere else there were no elk,
there were tree populations which
told scientists it was due to the elk
that trees were becoming depleted.
Bring Back the Wolves
• In 1995 after the wolves had been gone
for 70 years, park managers decided to
bring some to the park again.
• They had noticed that in the Lamar Valley,
where they brought back the wolves that
the tree problem began to go away and
other things also became more
established.
Lamar Valley in Yellowstone
– Once the wolves were back the elk there
avoided the aspen groves where wolves could
easily attack them.
– This allowed the tree population to increase
and also allowed the beaver colonies to
increase from one in 1996 to seven in 2003.
– These beavers where then able to make
dams and mini ecosystems for all other
creatures that rely on them.
–The presence of this one predator
effected many trophic levels of the
ecosystem.
Cause and Effect chain map for Yellowstone
Main Cause:
Effect/ Cause
Effect/ Cause
Effect/ Cause
Effect/ Cause
1926 Hunting
Of wolves kills
them all off
Effect/ Cause
Overall Effect: Loss of Biodiversity in Yellowstone
Conclusion Questions
1. Define what a keystone species is and
give an example of one and why it is
one?
2. Explain a trophic cascade?
3. Summarize what this case study was
about? Analyze the data to help make
your summary more in depth.
4. Compare and contrast this case study
with the wolf and deer case study you did
before.