Students are bussed to the site of an older style bridge which is clearly constructed of steel girders bolted together. 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. 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.
Students easily make the connections and relate to the analogy.
The result is predicable and reliably positive. 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.