Have you heard about the “butterfly effect,” the idea that one small change can bring about big changes over time?  This idea is important in the study of ecology, which deals with the interactions of living things and their environments.  Each element of an ecosystem has its place.  When one element is eliminated, it affects everything else.

The Yellowstone ecosystem centered in Yellowstone National Park provides a great example.  Late in the 20th century, biologists were worried about the aspen trees there.  Aspens occur in clusters that are actually clones growing up from shared root systems.  Some of the Yellowstone clones were hundreds of years old, but the old, dying trees weren’t being replaced by strong young shoots.  It looked like they might just die out, and no one was sure why.

When a severe drought in 1988 led to big wildfires in the park, the idea that fire might stimulate aspen growth proved wrong.  Perhaps the elimination of wolves from the region in the early 20th century was to blame.  Wolves?  New trees?  How could that be?  Without wolves, the behavior of the Yellowstone elk had changed.  No predators. No worry. So the elk became lazy, acting like cows, lying around in shaded areas along the rivers and creeks, munching contentedly on the juicy fresh growth of the willows and aspens.

In 1995, after much political battling, wolves were reintroduced into Yellowstone. The wolf population grew and the elk learned to be on the alert.  As the wolves’ favorite food, the elk had to change their behavior to survive—no more relaxing by a stream where wolves could easy sneak up and make a meal of them!  They had to move around and spend more time in open places where watching for hungry wolves was far easier.

The wolves are changing the Yellowstone landscape in positive ways.  The aspens and willows are coming back.  Beavers, which had almost disappeared from some parts of the park, are returning.  These rodents feed on aspens and willows and use them to build their dams and lodges.  Beaver dams create ponds, and the ponds provide homes for hundreds of species of plants and animals, from algae and water striders to ducks and muskrats.  The willows and aspen trees around the pond are nesting sites for songbirds and homes for insects and spiders, all thanks to the wolf.

​     Welcome back, wolves!

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This aspen shoot has been chewed to bits by elk. Dan Hartman

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These aspen shoots have been allowed to grow strong and tall. Dan Hartman

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Wolves on the hunt in Yellowstone. Dan Hartman

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Now that the aspens and willows are returning, so are the beavers. Dan Hartman

PictureDorothy Hinshaw Patent’s book, When the Wolves Returned: Restoring Nature’s Balance in Yellowstone, is an IRA/CBC Teachers’ Choices book, an ALA Notable Children’s Book, A Book Sense Pick, and an Outstanding Science Trade Book for Children, as well as receiving the Orbis Pictus Honor Book Award. Booklist calls it “A great choice for elementary units about science and environmental protection,” and Kirkus gave it a starred review. Click here to read the reviews.

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