Demo Dozen

 

RIDDLE OF THE LILY PAD

Donella Meadows, in her collection of essays "The Global Citizen" (Island Press, 1991), tells of a French riddle in which a lily pad grows on the surface of a pond until it completely covers the pond.  With the size of the lily pad doubling each day, it takes 29 days for the pad to cover half the pond, at which point you decide to take action.  After watching the pad grow for 29 days, how long do you now have to prevent the second half of the pond from being covered?  The answer is only one day!  As much growth will take place in that last day as in the preceding 29 days of the month; that's the power of exponential (or compound) growth.

 

This exercise is a slight modification of the scenario presented in that essay.  Here you represent the Homeowners' Association for the owners whose land borders a pond.  The pond has a rapidly growing population of lily pads (starting with one lily pad at the start of the growing season).  If the lily pads should completely cover the surface of the pond (that would take 100,000 lily pads), they will cut off the deeper water from dissolved oxygen and all the fish in the pond will die -- not a desirable outcome for you as Manager!

 

Can You Keep the Fish Alive in the Pond?

 

In this simulation, your job will be to decide if, and if so, when, during a 26 week growing season, you wish to apply herbicide to the pond.  State environmental law allows you to do so only ONCE each year, and the permitting process requires an environmental review period of four weeks from the time of your request until the application takes place.  When the weed-killer is applied, all but 500 of the lily pads are killed (unless you apply the herbicide before there are 500 lily pads, in which case half of the pads are killed).  In either case the survivors begin immediately to grow again.

 

Below is output from the simulation that reflects three different “decisions” to apply herbicide, each made a week apart. Notice that in two of the three cases the lily pads fully cover the pond and the fish are killed!  How good would you be in managing the pond?