STELLA Surprises:

Coloring the Grizzly Bears of Yellowstone

By:

Andrew Ford

Professor of Environmental Science

Washington State University

http://www.wsu.edu/~forda 

Have you ever made a simple change in a STELLA model and found surprising improvement in the subsequent discussions of the model? Perhaps you rearranged the stocks and flows to show the flow of material in a clearer manner? Sometimes a simple change can pay big dividends in communicating our theories about a system.

I've had such pleasant surprises in my own work, and I've heard stories from colleagues who were surprised by the improvement in communication that followed upon a relatively simple change in a model. The "STELLA Surprises" column will share such stories in The Connector. The surprises will involve simple changes that readers could easily implement in their own models.

Today's surprise comes from a recent project on the Grizzly Bear population of the Greater Yellowstone Ecosystem (GYE) area. Managing agencies believe that the Yellowstone grizzly bear population has recovered from its endangered status and will remain viable into the future. But controversy surrounds the decision to de-list, and some scientists are concerned about the management of the GYE habitat. Others are concerned about the management of the "problem bears" which become habituated to human sources of food and garbage.

These concerns prompted Rosemary Jackson to develop a STELLA model of the population. (The model was developed for her Masters thesis at WSU.) Rosemary selected STELLA to provide a clear visualization of the interaction between different categories of bears and their habitat. She used three stocks to keep track of the male bears and eleven stocks to keep track of the female bears. A key part of her theory is the separation of bears into foraging groups. For example, adult males and non-lactating females are assumed to forage in the areas remote from humans. On the other hand, the cubs, sub-adults and lactating females are more likely to forage closer to the human area, and they are more likely to come into unfortunate, problematic contact with the humans.

Keeping track of the bears by their age, sex and foraging groups became increasingly complicated as Rosemary added more and more variables to her model. After several months of work, the model was becoming increasingly cluttered. The clarity she was hoping for was becoming lost in the clutter of so many categories. What could be done to retain her key assumptions and keep the model clear enough for managing agencies to understand?

Figure 1. Image from one of the interface screens of Yellowstone Grizzly Bear model.

Rosemary's approach was simple and effective. She used STELLA's painting tool to display the dominant bears in red and the secondary bears in blue. The colors improved the clarity of displays on the "interface layer," as shown in Figure 1. The color scheme also proved useful on the "model/map layer" where it helped Rosemary check that the STELLA model was consistent with her theory of the bear behavior.

Coloring the Grizzly Bears was a pleasant "STELLA Surprise" --- a simple change that made a big difference in communicating the results of a model.

Andrew Ford is Professor of Environmental Science and Regional Planning at Washington State University. He teaches modeling with an emphasis on environmental problems in the west, and he is the author of the Island Press text on Modeling the Environment. He uses the system dynamics approach to modeling, and he is the recipient of the Jay W. Forrester Award for the outstanding contribution to the field of system dynamics. Ford's recent research concentrates on the transition from regulation to competition in the electric industry. He uses System Dynamics and Stella to simulate the problems that have emerged in the restructured electricity system in California and in the western USA. Dr. Ford's students have developed models to aid our understanding of a variety of environmental problems. Recent examples include Stella models of the Yellowstone grizzly bear population and restoration of salmon habitat in the Pacific Northwest.

A book by Dr. Ford: Modeling the Environment: An Introduction to System Dynamics Modeling of Environmental Systems A highly readable introduction to environmental modeling. What distinguishes the book from other environmental science and environmental modeling works is its interdisciplinary treatment. In particular, the models integrate the physical world and the world of human behavior. Far too many environmental models fail to close the feedbacks between human behavior and the state of the environment, instead taking waste inputs or resource use as exogenous. This book helps students learn to model human behavior (social and economic) as an integral part of the ecological system. The models and software mean the book encourages active learning, and enable students to explore important issues on their own if they choose.