Introduction to Modeling

Qualitative modeling can be introduced by using the now classic example of the direct and indirect effects of wolf introduction to Yellowstone National Park.

(Say the following in your own words:) In Yellowstone national park from 1926 – 1995, there was also very little regeneration of new aspen, willow, and cottonwood trees. Elk populations began to show a marked increase in lame and sick megafauna. With no fear of wolves, elk could graze wherever they wanted, and for decades they proceeded to browse and kill nearly all the young cottonwood trees growing along the banks of the streams. This had a consequence of streambank erosion and higher water temperatures. 

Draw diagram number 1 (below).

Explain that you are using an arrow with a decreasing sign on the cottonwood tree end and an increasing sign at the elk end because elk are over-browsing the cottonwood trees.

Show the group all of the symbols used in qualitative modeling

By the 1990s, the Federal government had reversed its views on wolves.  Scientists decided to bring them back to the area in 1995. The U.S. Fish and Wildlife Service brought in 15 gray wolves from Canada. The wolves thrived and there are now over 300 of their descendents living in the Greater Yellowstone Area. Wolves now influence elk directly; they have been hunting and eating elk. There has been an increase in regeneration of aspen, cottonwood, and willow, which illustrates how wolves also influence elk through indirect means. The fear of attack by wolves apparently prevents elk from browsing on young trees in wolves’ core areas, and also from streams.

Using diagram number 1, add these new elements: Introduce the idea of a feedback loop by showing how the introduction of wolves helped to decrease the elk herd’s influence on cottonwood and aspen trees. Introduce indirect effects; the introduction of wolves indirectly helped decrease streambank erosion.  Develop diagram number 2 (below).