Trophic Cascades: What Happened When Wolves Were Reintroduced to Yellowstone Park?
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How Wolves Have Affected the Yellowstone National Park Ecosystem
Wolves were reintroduced in Yellowstone National Park with the hope of re-establishing the ecosystem's
biodiversity. The hypothesis was that, when all the wolves were hunted down, the food web was disrupted and
the ecosystem deteriorated. In the absence of wolves, too many elk in the park had negative implications. For
example, uncontrolled browsing of many plants by elk caused the disappearance of other animals because their
habitats and food supply were destroyed by the elk. As I learned in the Background section, a trophic cascade is
an event that has consequences throughout a food web. It is caused by adding or taking away a component in a
food web—in this case, the top predator, the wolves. A food web describes trophic relationships among
components of an ecosystem; and within a food web we can trace many food chains.
Overall, the investigation shows that the reintroduction of wolves was positive for the Yellowstone Park
ecosystem. These effects are consistent with a trophic cascade, where reintroduction of a predator has had
cascading or chain effects on other animals and plants. The reintroduction of wolves into Yellowstone National
Park had a negative impact on elk. The wolves are the top predators and they prey on the elk; these two animals
have a predator-prey relationship. By controlling the elk population, other organisms in the ecosystem, like
aspen trees and berry bushes, recovered. The elk consume plants—the berry bushes and the aspen trees. The elk
and the bears compete for the berries, and the wolves indirectly favor the bears by eating the elk. This allows
more berries to be produced, which are available for the bears to eat before hibernating.
The population dynamics between wolves and elk shows an out-of-phase undulating relationship
between these two species. As the prey, elk, are readily available, the predator's population, wolves, increases
because there is plenty of food to eat. My results in investigation 1 (snapshot) show that in 1995 there were
16,791 elk—plenty of food for the 21 wolves. After 1995, the number of wolves increased, from 21 up to 185 in
2004 and, as a response, the population of elk decreased. But eventually the wolf population also started to
decrease, perhaps because there were fewer elk to eat. There are no data after 2017, but if we extrapolate the
trend, we may see that the reciprocal relationship continues. There is a second piece of evidence that shows the
same wolf-elk relationship trend (Investigation 3 snapshot). In the presence of wolves, the elk density was only
0-1 elk per square kilometer compared to the high elk density of 16 per square kilometer when the wolves were
absent. Taking these two pieces of data together, my results suggest that the reintroduction of wolves reduced
the number of elk because wolves killed them.
One interesting consequence of the wolves eating the elk is that the plants that the elk eat increased. The
two plants under study are aspen trees and berry bushes. My results show that between 2003 and 2011, as the
number of wolves increased, the number of elk decreased, and the height of aspen trees increased (Investigation
2 snapshot). After 2011, the number of wolves reached a plateau; however, the aspen trees kept growing. This
result can be explained by the observation that elk can reach leaves only to a height of about 100 centimeters. If
the tree gets taller than that, then it is safe from elk and keeps growing. A similar trend is observed between the
wolves, elk, and berry bushes. There are not only more berry bushes, but they get taller and produce more
berries (Investigation 3 snapshot). The relationship between these species, herbivores and plants, also shows a
reciprocal relationship.