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Features of Populations

Discusses population size, density, dispersion and growth.

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Quaking Aspens (Populus tremuloides) are an important component of North American ecosystems. They have been increasing in numbers in Yellowstone National Park since the reintroduciton of wolves (Canis lupus) in 1995.


Reintroduction of wolves to Yellowstone National Park

Why It Matters

A New Look At An Old Enemy

The relationship between humans and wolves is an old and varied one. After all, the dogs we live with are so close to wolves they can successfully breed with them, but wolves aren't dogs. They are extremely social and intelligent animals, but they're not dogs. Sometime between when dogs were domesticated and now, wolves acquired a rather bad reputation with many humans. It seems this change occurred mainly in agricultural societies and was due to wolves feeling rather correctly that domesticated livestock were easy prey, but it's hard to pin down with absolute certainty the source of such attitudes as there are often multiple sources. What is certain is that when Europeans came to North America, they actively sought out and killed wolves indiscriminately. Wolves were declared vermin and the government paid people to seek them out. With incentive like that the program of eradication was extremely successful (it is estimated that 1 million wolves were killed between 1850 and 1900), and wolves were placed on the endangered species list in 1973. However, this program of eradication has proven a tad short-sighted as we are now learning about the importance of top level carnivores in ecosystems, and the cascading effects that arise from their removal.

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Use the resources below to answer the following questions:

  1. What technique was used to determine when Aspen trees (Populus tremuloides) started to decline in Yellowstone National Park?
  2. Does the correlation of lack of reproduction by Aspen trees and the removal of wolves ('Canis lupus) prove that removal of wolves was the cause of the change in Aspen trees? What evidence supports lack of wolves as the cause of Aspen tree decline?
  3. How has the return of wolves to Yellowstone helped the North American beaver (Castor canadensis)? How is this relationship an example of the interconnectedness of species in an ecosystem?
  4. Where has the most robust recovery of Aspen trees occurred? How does this pattern support wolves as the cause of the recovery?
  5. Explain the trophic cascade of which the wolves of Yellowstone are a part. Be specific and remember the life histories of the organisms involved as you explain this cascade. What other factors besides the animals involved could influence this cascade? How would you go about determining if the trophic cascade was the key factor in the decline in aspen trees in Yellowstone?

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