Today, we are going to study about an ecosystem by doing a systems analysis upon the Yellowstone ecosystem.
All systems have working and moving parts that are connected - meaning that one part affects another. One such connection may be based on matter and energy such as food, (which is matter that has chemical energy) - as is the case in an ecosystem. (Hence the name Eco - System).
The parts of any ecosystem are all of the biotic and abiotic factors that keep that area alive and functioning well.
In your studies today, you're going to read a story about the ecosystem of Yellowstone National Park - and how that system was affected when one part of the system was removed, then later returned.
How Wolves affect the Ecosystem of Yellowstone
Aspen trees are biodiversity hot spots in the west. They are home to a variety of birds which feast upon seeds and insects. In 1997 Aspen trees were on the decline in Yellowstone and no one knew why. William J. Ripple, a scientist, went to Yellowstone to try to solve the mystery.
He took core samples from 98 aspen trees and discovered that only two had begun to grow after the 1920’s - around the time the last substantial populations of wolves were killed or driven off. He found big trees and tiny trees but nothing in between, because nothing new grew from the 1930’s to the 1990’s. It was the first concrete evidence of a “wolf effect.”
The wolf-effect theory holds that wolves kept elk numbers at a level that prevented them from gobbling up every aspen or willow tree that poked its head above ground. When the wolves were exterminated in the park as a menace, elk numbers soared, and the hordes consumed the vegetation (shrubs & grasses), which drove out many other species of plants and animals.
For instance, without young trees, such as willow & aspen on the range, beavers had little or no food, and indeed they had been absent since at least the 1950’s. Without beaver dams and the ponds they create, fewer succulents (a type of water plant) could survive, and these plants are critical food for grizzly bears when they emerge from hibernation, as bears depend on succulent to replenish themselves after hibernation. Without both succulent and the beaver, the bear had to depend on other articles of food to feast on.
In 1995 the wolves were re-introduced, if the wolf-effect hypothesis is correct, and wolves are greatly reducing elk numbers since then, the vegetation should be coming back for the first time in seven decades. They have indeed found Aspens and willows rebounding in Yellowstone as wolf numbers have climbed - but that is only part of the change occurring in the park.
Other changes accompany the re-growth of vegetation. In the river there is a small beaver dam-one of the first documented in 50 years. Because of the re-growth, beavers have something to eat again. Also as more vegetation (shrubs & grasses) grows along the river, it stabilized the banks and stopped some erosion.
The wolf seemed to have an effect on the Yellowstone food web as well. One of its most dramatic effects has been on the coyotes. Coyotes have sacrificed a great deal to make room for the much larger wolves. The number of coyotes in the park is down 50% because the wolves kill coyotes.
With fewer coyotes, their prey- such as the voles (which eat mostly grass roots and seeds), mice, (which eat seeds, grass roots and insects), and other rodents like beavers and rabbits -have exploded in number. This has greatly benefited the red fox, which also prey upon rabbits, beavers and birds as well. The concern, however, is that with more foxes in the ecosystem could mean a possible reduction of birds in the future. Nevertheless, the increase of voles, mice & rabbits also increased the number of hawks, weasels and badgers.
The bear population also increased due to the berries (vegetation) growing from the regenerating shrub vegetation, and in turn, they compete with the wolves for the deer population. With the rise of the bear population, the wolves have to be mindful of their pups – as bears are known to kill their young. Bears too also keep their bear cubs close by from the wolf predators.
But, are the wolves really the engine driving these changes? Most scientists think so. One scientist stated that wolves are the primary force shaping the ecosystem in Yellowstone.
Other researchers, however, are not convinced about the effects of the wolf. One researcher stated a strong correlation between the return of the wolves and the new growth is far from demonstrated. The ecosystem in Yellowstone is an interactive system, and there is never a single cause; for instance, as the wolves are coming back the climate is warmer. It will take 20 years or more before we know for sure. So the mystery remains unsolved for now. What do you think?
Other connections not mentioned in the article:
Yellowstone ecosystem (aka: food web)