The Consequences of Extinction

Edward Murray
1991 Woodrow Wilson Biology Institute


This activity is designed to help students to understand and appreciate the reasons why each type of organism, no matter how small or seemingly insignificant, can be important in the functioning of an ecosystem.


Our students often have a markedly egocentric view of the world which leads them to ask us hard questions. "Why do we need pelicans anyhow? So what if they all died; what difference would it make?" A question of this type verges on being unanswerable but it does deserve an answer. After all, if dandelions do make a difference (or snail darters or pelicans) then we should be able to explain why. In fact, not until we can give a reasonable answer to this question can we expect our students to see us as more than sentimental environmentalists. If we want our students to change their world view from EGOcentric to ECOcentric then we need to offer them rational and reasonable reasons why the ECO is more important than the EGO.


Students are bussed to the site of an older style bridge which is clearly constructed of steel girders bolted together. (It is important that you go to the trouble of physically transporting the students to the site, even if it is nearby and completely familiar. An essential element of this exercise is to provide them with a different and memorable experience that will survive beyond exams.) Offload the students and settle them in a comfortable spot with the bridge in full, easy view. Introduce them to the bridge, pointing out that it is constructed of small pieces bolted together to form a complete, stable, working system. Begin the discussion by asking what difference it would make if one of the bolts were removed. Would they still be willing to travel across? How about two bolts? Three? How many bolts would have to be removed before they would begin to feel uneasy about traveling on it? If the students don't soon point-out the fact, mention that as each bolt is removed, the bridge may shift imperceptibly to neutralize the stress but an uninformed observer/traveler would be completely unaware of the danger. They all will agree that eventually, if bolts continued to be removed, the bridge would be unable to redistribute the stresses and it would collapse. Extend the analogy to ask if the collapse of the bridge could be stopped if we acted as soon as we noticed it beginning to fall? Students easily make the connections and relate to the analogy. The result is predicable and reliably positive. They are especially impressed by the idea of the "ultimate insult" which causes the apparently healthy structure to suddenly collapse. For the great majority of students this is a new and powerful idea.


This exercise relates directly to the idea of how the complexity of food webs determines the stability of ecosystems. It is used to best advantage when students have a good understanding of how matter and energy flow through communities and have had at least an introduction to the the ecosystem concept. It works especially well as a group discussion placed at the end of a unit on ecosystem ecology because it provides a sense of closure for that topic. The same basic thing can be accomplished in the classroom setting but does not have the same effect. The inherent excitement of getting outdoors combined with the group intimacy and the visual reinforcement of the structure looming before them turns just another classroom topic into a big picture experience.

A bridge is not a requisite part of this exercise. Paul Ehrlich used a similar analogy in his book EXTINCTION* when he compared an ecosystem to a commercial jet airplane from which one rivet was removed, then two, etc. If convenient, you could take students to a nearby airport and explore this idea. Any one of several different analogies is usable. What makes the activity work is not the appropriateness of the analogy but the uniqueness of the experience, a seemingly unecessary field trip to an already familiar place for the simple purpose of having a discussion. What topic could be more deserving of the small expense?

* Ehrlich, P.R. and A.H.; Extinction: The Causes and Consequences of the Disappearance of Species. New York; Ballentine, 1983.

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