Daisyworld is a model derived by James Lovelock to explain part of his Gaia hypothesis. Briefly stated, Gaia (the Greek goddess equivalent to our idea of "mother nature") is a view of the Earth as a single living organism which actively maintains conditions for life, as opposed to a view of the Earth as a rock on which it happens that it is possible for things to live. Parts of the hypothesis, such as the fact that plants and animals are largely responsible for the composition of the atmosphere, are not in contention, while other parts are. The DaisyWorld model is a simple attempt to show that it is theoretically possible for living organisms to control the temperature of the planet through evolution.

As with other revolutionary ideas, Gaia forces us to view things in new ways. In the Gaian world, evolution is not as much a mechanism for change (in the organisms) as it is a mechanism for stasis (as in homeostasis) of the physical conditions for life.

The DaisyWorld model is a simple one. It envisions a planet with a simple atmosphere (no greenhouse gasses, even heat transfer, etc.) and only one species of life - daisies. The daisies can be of various colors, however, and the different colors will affect the temperature of the planet. Light colored daisies will reflect heat back to space, and cool the planet, while dark daisies will absorb heat and warm the planet. Each color of daisy may have a unique optimum temperature, and growth is only possible in the range 4o - 40o C. Each daisy color comprises a separate population, and these populations compete for space (but not nutrients, sunlight, etc.). The growth of each population assumes a logistic model, with growth slowing as the carrying capacity of the planet is approached (the sum of all daisy populations). Growth is at a maximum for any population near its optimum temperature. Mutations (changes in color) are always arising in each population.

My implementation of the DaisyWorld model is slightly different than Lovelock's. In the model you will use, there are only two populations, white and black. The ground has an intermediate albedo (look it up). The carrying capacity of the planet is limited to 1,000 (this capacity is never reached, incidentally). In our model, the amount of sun reaching the planet may be constant (set by you), increasing at a steady rate, or decreasing at a steady rate. You are free to choose the optimum temperatures for the black and white plants, as well as the pattern of solar activity (fixed, declining, increasing).

The Defaults

Lovelock's model envisions a world where solar output is slowly increasing over time. Further, he hypothesizes that the black plants will be at an advantage at cooler temperatures than the white plants, since they are warmed by the sun. The optimum temperature internally for both types of plants is the same, probably about 20-25o C, just as it is for most living plants; note that you are specifying the optimum external temperature for the plants. If you find this confusing, think about cars. A white car will be cooler than a black car, and is a better idea in the summer, but a black car will keep you warmer in the winter, although in both cases the ideal interior temperature is the same - about 70o F.

To see what happens in Lovelock's standard model, start the program. This will select default values of 10o C for the black daisies, 25o C for the white daisies, and increasing solar radiation (just as our own sun is acting). As the model runs, note that there are four separate displays. At the bottom right of the screen, there is a textual listing of the temperature of the planet if there were no plants, the temperature that exists with the plants, and how many of each type of daisy there is, as well as how many years the model has 'run'. In the middle is a vertical scale with two bars. The scale represents temperature in degrees C, while the green bar represents the temperature with the plants, and the red the temperature that would be reached without plants. The distance between the tops of the bars thus represents the effect of the plants on temperature.

The main graph displayed in the upper left plots both temperature (the red and green lines) using the left vertical axis, and population (the white and black daisies plus overall population), using the left vertical axis. Where values are close together, one line may overly the other and "hide" it. A dotted line represents the 40o C maximum temperature for life; the horizontal axis is time in billions of years. At the upper right is an "orbital view" of the planet. A central circle gets larger as the number of daisies (both white and black) gets larger; the circle changes color to represent the dominance of either white or black in the total population. You can print the screen at any point by clicking on the appropriate button. To CLOSE the graph, you must first click the BREAK button.

Using the defaults, notice how the temperature initially moves up as the solar radiation increases, with both temperature values close together since there are few plants. As the temperature approaches 10o, the optimum for the black plants, the black plant population grows very quickly. Since the black plants absorb a lot of heat, the planet also gets hotter than you might expect, causing the green line to move above the red line. This warmer temperature, however, is more beneficial to the white plants, so the black plants slowly die off and the white plants increase in number. Eventually the white plants control the temperature to the extent that despite constantly increasing solar radiation, the temperature remains fairly constant, even over a 10 billion year period. This temperature stability is a large part of Lovelock's Gaia hypothesis.

1. What happens if you make the optimal temperature for black 7o C and white 5o C (increasing solar radiation)?

2. Is the above situation realistic? Explain.

3. Are there any other ways (without changing the population values) to obtain a result similar to that obtained in #1?

4. What happens if you set the solar radiation constant at 5o or 35o, and leave the optimal temperatures at their defaults (black 10o, white 25o)? (NOTE: run the model at BOTH 5o AND 35o).

5. What happens if solar radiation is decreasing? Can you make extinction come earlier or later by manipulating the optimal temperatures? (Bonus point to whoever can move the extinction point the most).

6. What can you conclude about the overall temperature stability of DaisyWorld under physiologically realistic conditions?

7. Explain the results from the default model and #1 in terms of natural selection, alleles, fitness, etc.

8. Think of a practical, every-day, local application for knowledge you have gained from this assignment and write a paragraph about it.