Why is a butterfly like a steel rivet?

He offers another example: "Suppose there were 2,000 blue whales left in the world, and they were all one population mixed together. You might be able to take 500 whales per generation without pushing down its size. But if there are four different populations of 500 each, you couldn't take nearly as many out without doing permanent damage."

Having distinguished the three separate populations of checkerspots, Ehrlich set out to solve the next mystery: What was controlling the fluctuations in their population size? Recalls the professor: "We couldn't figure it out, but we knew it had something to do with the caterpillar stage in the cycle."

Every spring on Jasper Ridge, adult checkerspots lay masses of eggs on Plantego erecta,m a native annual plantain. The caterpillars (or larvae) hatch, eat the plantego, and go into a resting period during the dry California summer. In the winter months the rains return, the plantego blooms, and the larvae "wake up" and feed. They form their pupae in the early spring, hatch, mate, lay eggs. And so the cycle continues.

During the winter "caterpillar season," soggy Jasper Ridge is unbearably soggy and offers three distinct varieties of mud to choose from. Hip boots are advisable. "I decided if I was going to solve the caterpillar mystery I had to find a graduate student pre-adapted to the climate," says Ehrlich. "I chose Michael Singer from England, where it's muddy most of the time. He went out and lay around in the mud for two years on Jasper Ridge, and came back with the answer."

The news Singer brought back was simple but starling. He found that although the checkerspots laid their eggs on plantego and were raised in the lab on plantego, the vast majority of the caterpillars which reached adulthood did so by switching in the summer to an owl's-clover, Orthocarpus densiflorus.m "I had been fooled all along," says Ehrlich, who had puzzled over why the checkerspots around Stanford were found only around the serpentine soil when the plantego, the checkerspots' food plant, was found elsewhere. The answer: the owl's-clover , the secondary food source, was restricted to the serpentine soil.

This revelation and others led Ehrlich to focus on something he called "coevolution," which he defines as "the reciprocal evolution of two or more ecologically intimate species." For example, a predator evolves better ways to catch its prey, while the prey evolves better ways to escape. In the case of Jasper Ridge, his term refers to the complex cat-and-mouse interplay between the checkerspots and the plants they feed on.

The ramifications of the "coevolution" principle in world food production are revolutionary. Traditionally, agriculturalists have believed that if they could keep one side of the food equation constant, by creating a pestless "miracle" crop or finding the ultimate pesticide, their problems were over. Ehrlich's "coevolution" principle holds that both sides of the equation are constantly in flux, and that maximum-yield agriculture is achieved by constantly selecting new strains of crops in response to the evolution of new strains of pests. According the Ehrlich, there are no long-term "miracles" in agriculture.

"The cotton industry is working to artificially create a cotton plant that will be forever pest-resistant. But if you know how plants and insects interact , you know it's only a matter of time before a new pest evolves. the average life of a new wheat strain in the West is five to six years. Almost every time a new pesticide is made, it promotes a new organism to pest status. Now, DDT won't kill anything. In fact, insects love it," says Ehrlich.

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