In 1960 Nelson Hairston, Frederick Smith, and Lawrence Slobodkin (HSS) proposed that vegetation patterns are determined primarily by patterns of food consumption by herbivores. They further suggested that the act of predation shaped herbivory. Therefore, herbivores unchecked by predators would have a great influence on vegetation. Known as the green world hypothesis, their ideas provided an ingeniously simple explanation for why the earth is green. This hypothesis resulted from a seminar at the University of Michigan, in which scientists met to discuss central issues in terrestrial ecology and arrived at this remarkable conclusion. Their ideas were inspired by Charles Elton's food pyramid.
As a new professor at the University of Washington, Paine tested the green world hypothesis. The idea for a simple experiment that essentially involved playing God in the rocky intertidal zone came to him during a visit to the Scripps Institution Wharf in California, as he stood watching the carnivorous sea star Pisaster ochraceus devour the mussel Mytilus californianus. What if he removed sea stars to see what would happen? The next year, National Science Foundation funding in hand, he settled into the University of Washington and began his research. Month after month he traveled to Mukkaw Bay, at the northern tip of the Olympic Peninsula. There, on a rocky crescent of shore, he hurled sea stars into the ocean. In his control plot he did nothing. As he continued removing stars, the assemblage of species on the rocks gradually began to change. Within one year the sinister implications of his experiment became all too graphically obvious. Where Pisaster flourished, so did the vegetation. Where Pisaster had been removed, the mussels took over, crowding out other species and eating all the vegetation, until little more than a dark carpet of mussels and barnacles remained. The paper Paine produced for the American Naturalist about this research turned out to be one of the most influential journal articles in the history of ecology. It provided one of the first examples of an ecosystem that had been transformed by trophic cascades.
Mentored by Frederick Smith of HSS fame, Paine was also influenced by Charles Elton and Charles Darwin. He thinks the green world hypothesis may have had its genesis during Smith's 1957 course at the University of Michigan on the natural history of freshwater invertebrates, on the kind of spring day when professors don't really feel like teaching and students don't really feel like sitting in class. That day Smith looked out the window of the zoology building, which faced a courtyard, and said to his students, "We're not going to have a field trip today. I want to teach you how to think instead. Tell me, what do you see out there?"
"A tree," said some bright person.
"What color is the tree?" asked Smith.
"Well, it's got green leaves," said another student.
"And why aren't the grazers out there—the insects—eating all those leaves? What makes the leaves green?" asked Smith.
"Well, it's photosynthetic pigment," said someone.
That wasn't quite what Smith was after; he wanted them to explore ecological relationships between organisms. And so the conversation continued, back and forth. For Smith had been thinking about these ideas for quite some time: he was a deep thinker. So he looked out the window, and this argument of immense consequences followed linearly fromthat. What came to be known as the green world hypothesis was first articulated on that bright spring day in Smith's zoology class, and it fueled the fires of enthusiasm in the graduate students at the University of Michigan and rocked science.
The Architecture of a Trophic Cascade: Predator-Herbivore-Vegetation
When HSS formulated their hypothesis, the scientific community commonly accepted that bottom-up processes, mostly related to competition between species, were the primary forces shaping populations. The green world hypothesis provided an alternative view of population regulation driven by top predators, via carnivory. It enabled predation and grazing to play roles equally important to resources or habitat, with predation having the key role for some populations and resources having the dominant role in others.