Chapter 41: Species Interaction Data Analysis
https://www.natureworkseverywhere.org/asset/Wolves_of_Yellowstone_Teacher_Guide_FINAL_NWE.
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Based on the graphs above, it appears that wolves were reintroduced to Yellowstone in 1995, with an
initial population of 20 wolves. Their reintroduction leads to a steady decline of the Elk population, while
the wolf population steadily rises until about 2003. By 2010, the wolf population had dropped to
approximately 50 individuals, and the Elk population hit a low point of around 7,000 individuals. Because
the wolf population is low at this point, the elk population should begin steadily increasing in size as long
as the wolf population stays low. This data is important because it shows how a population of only 20
predators can so drastically effect a prey population of almost 20,000 individuals.
As the elk population made a steady population decline starting in 1995, number of Cottonwood trees,
Willow Tree Ring area and mean Aspen heights all increased in response to the dwindling population of
elk. The introduction of only 20 wolves into Yellowstone caused the Elk population to decline greatly,
which causes tree growth to increase substantially. Many other animals in Yellowstone get their food
from these trees, so the decline of the elk population benefitted other species, specifically beavers.
The reintroduction of wolves did not directly affect beaver population at first, as the number of colonies
remained at 1 until 1999, but soon the declining elk population increased tree growth, giving the
beavers food and material for their shelter and dams. This allowed more beaver colonies to arise out of
the first one. The number of beaver colonies increased very steadily (more steadily than any other
species population) from 1999 to 2010. Based on the graphs we can infer that wolves aren’t natural
predators of beavers, as the reintroduction of the first wolf colony didn’t eliminate the single beaver
colony in 1995. The beavers actually benefitted from the reintroduction of wolves, because the elk
population was making the tree population scarce, as the elk had no predators in the park before 1995.
These graphs show the Canis lupus’ (wolf) role as the keystone species in Yellowstone. Before they were
introduced, the Elk population was rising rapidly, causing a decline in tree and beaver populations that
may have driven the populations in Yellowstone to extinction. A mere 20 wolves reintroduced into the
park rebalanced the park’s species populations, allowing the elk, beavers and Aspens to coexist. Because
Yellowstone was declared a national park and hunting was outlawed in the park, the wolves allow
Yellowstone’s ecosystem to thrive as it once did, before hunters killed off all of the wolves.