The model consisted of the following components:
 

Grasses

Willows

Beaver

Elk

Wolves

The first four species we introduced were: elk, beavers, willows, and grasses.  These all  had existing populations in the park before the 1995 reintroduction of  wolves . Summer estimates of elk in Yellowstone were between 25,000-31,000 in the 1980's.  Estimates of the winter population were between 13,00-21,000 elk. (Schullery, 1996)   Recent estimates for the beaver population were much lower than in the 1920's. ( Glick, D. et al 1991)

Beavers help maintain riparian areas.  Their dams increase the amount of water in an area which helps willow tree regeneration.  An increasing elk population and a low beaver population could ultimately make a willow population crash because the elks browse on the young willow shoots.

Using EcoBeaker, we designed models to show how reintroducing wolves would affect the other four species. We wanted a model that would show that all five species could coexist and that reintroducing wolves could help the willow population.  Our models contained two adjacent habitats called lowlands and highlands. The lowlands were set as the preferred habitat for beavers because they contained wetlands. We manipulated variables for each of the five species.  At times our models worked and at times they did not.  When our models did not behave as we thought they would,  we made changes in variables to see how these would affect our food web populations.  We wanted to demonstrate that the wolves would help increase the beaver population by decreasing the number of elk which were depleting willow tree saplings.

We found that the results of a model depended on both deterministic factors such as migration rates and on random factors such as the location of the settlement of immigrants in the model. In our final model, most of the simulation runs resulted in an increase in the willow population after wolves were introduced. This fit our expectations. However, some of the runs resulted in the wolf and beaver populations crashing and becoming extinct, necessitating a reintroduction of both species.
 
 

In the graph above, as the elk population initially increases, the willow population crashes at time step 50 .  Prior to wolf reintroduction at time step 100, the willow population decreased to its lowest level.  After wolves were released, this model demonstrates that the willow population will recover and cycle at higher levels. Though this simulation included a small population of beavers, they crashed immediately due to their pattern of settlement.  Apparently, elk still were able to out-compete the beavers and depleted willow stands in their immediate areas.

This model assumes that wolves can help control the population of elk in Yellowstone and stabilize the willow population.  It could not take into account the additional complexity of the potential importance of beavers to maintaining the riparian ecosystem of the park. Without beaver ponds, willow habitat can suffer and willows can decline. (U.S.News Online Sep.16, 1996)