In 1962 Rachel Carson published 'Silent Spring' which drew attention to how the use of pesticides was indirectly decimating bird populations by causing thinning of egg-shells. This model illustrates how DDT in near-shore waters accumulates in a 'food chain' of phytoplankton, zooplankton, smelt, salmon, and pelicans.
Garrett Hardin (1968) described how multiple individuals acting in their own interests, while exploiting a common resource, will inevitably overexploit the resource. This model illustrates the concept in a common field where four farmers are grazing cattle. As farmers add cattle beyond the carrying capacity of the pasture, their individual gains increase even as overall milk production goes down. Students can explore how different economic systems affect how many cows a farmer 'should' graze, and also privatize the land into four quadrants.