Minnesotans For Sustainability©


Sustainable:  A society that balances the environment, other life forms, and human interactions over an indefinite time period.




The Pond Lilly Parable

Dell Erickson
September 24, 1999


The "parable" (not really a parable) is an amusing yet serious exercise which can stimulate thought and provide valuable insights into long term carrying capacity and population growth issues. The scenario should facilitate a discussion of compounding growth and living standards, while integrating sustainability issues. The concept of population momentum and time to reach a stationary population are core concepts.

Lilly Pond Parable & Population Growth

a.) If a pond Lilly doubles everyday and requires 30 days to completely cover a pond, on what day will the pond be 1/4 covered?

b.) 1/2 covered?

c.) Continuing with the initial pond (and 30th day), if three more similar ponds were found, how many more days would it take before they were also covered?

d.) Does the size of the pond make a difference?

e.) What will begin to happen at one minute past the 30th day?

f.) Regarding human populations, what kind of environmental, social, and economic changes can be anticipated as the 30th day approaches?

g.) At what point (what day) would preventive programs become necessary to avoid unpleasant events? Would remedial actions be possible or as satisfactory?

h.) How would the situation change if the pond Lilly doubling time were reduced from 30 days to 15 days because additional Lilies were transplanted from other ponds?

i.) If this pond were your city, state or country, and the Lilly, people, considering all other life, at what day would you or your family like to live, i.e., have the highest quality of life (or the quality of life you aspire to)? Would it be possible in the following hour? The following day?

j.) What are the implications if the doubling time, i.e., each "day," is 25 years? 50 years?


The question that needs to be addressed is "what day is it"? As will be learned, it is the combination of doubling time and time to achieve a stationary population and then carrying capacity that become the instructional themes.

Selected observations:

* at the beginning of day 27 the pond is only 1/8 (12.5%) covered; 87.5% open water yet there are only three of 30 days to go.

* at the beginning of day 28 the pond is still only 1/4 (25%) covered; 75% open yet only two days of 30 remain, two population doublings to completely cover the pond.

* at the beginning of day 29 the pond is only 1/2 (50%) covered; however, there is less than 24 hours remaining before being fully covered.

* the size of the pond makes no difference because it is the doubling time that is relevant.

* finding three empty ponds is the equivalent of adding two days, two doubling's (same size). What is the real world analogy?

Throughout the early days it is evident that the rapid population growth of Lilies will not be noticed or seen as threatening until the consequences of successive doublings approach a climax.

In short, even when there appears to be "lots of empty space", even when, for example, only 1/2 covered, there is diminished probability of saving at least some biodiversity even if preventive measures are promptly implemented.

Not evident from the simple arithmetic is that as the number of Lilies increase, the resources to support those Lilies are being used up --space (open water), access to sunshine, and minerals are examples. Diminishing the resource base beyond its carrying capacity will actually reduce the number of Lilies that could be sustained over some longer period of time. It is possible that as resources are exhausted a large number of Lilies will die. The increasing Lilies would also tend to crowd out and use resources necessary for other lifeforms. Although it is subject to debate, as intended, it is apparent that the U.S. and the planet is approaching or in a later day.

Arriving at an understanding and a general agreement:

It can be difficult for some Americans (and world citizens) to understand the relationships between U.S. (and world) resources, environmental, social, and economic problems and population growth.

Some Americans wonder if the U.S. (or the world) has a population problem; others believe overpopulation a concern of other countries. Until the later stages, the final doubling, change is accommodated; in many respects, then, the degree of overpopulation is a matter of perspective:

* some drivers can "park" on a crowded freeway and not understand population is the determining factor;

* natural resource "shortages" drive rising prices yet few see the relationships;

* some vacationers can be denied a permit to enter a park and not understand population is a factor;

* some people see a sinister agenda where none is evident nor intended;

* some Americans say to remedy our problems we should, ...(select a favorite) increase gas mileage, reduce income inequality, empower women, provide more jobs, more highways/mass transit, more high rise apartments (i.e., stop "sprawl"), catch and release fish, reduce water use, reduce pollution," etc.

As inferred above, a good first definition of overpopulation is one that illustrates how one is personally impacted. For example, to an outdoor enthusiast a possible definition could be overpopulation is when a reservation (a means of rationing) is required to visit Yosemite or the Boundary Waters Canoe Area Wilderness. That's simple enough, but think about it.

Are ebbing resources another factor in determining a sustainable U.S. (or world) population? Earlier in its history, the U.S. had bountiful resources which when combined with developments led to the nation seen today. However, the U.S. now imports about 65% of its oil with the percent increasing each year in a world where, even assuming the discovery of substantial new oil fields, the world's oil production will peak within ten years. The U.S. is becoming a nation dependent on foreign energy resources and economically running a large and inexorably growing balance of payments deficit to pay for it.

Water is also becoming a worsening problem as is evident in many areas. Even in Minnesota, for example, a state known for water, local water problems are becoming more evident as water use restrictions are spreading.

What would be your definition?

Then ask the ecologist's mantra, "what then"?

* "Testimony Of Dell Erickson Before the State of Minnesota. House Committee on Civil Law Regarding Illegal Aliens in Minnesota" September 24, 1999.
Re: Concluding Comments, at p12, regarding sustainability —The Pond Lilly

Used with permission of the author.


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