The green world hypothesis, or HSS, proposes that predators are the primary regulators of ecosystems: they are the reason the world is 'green', by regulating the herbivores that would otherwise consume all the greenery.
The green world hypothesis was likely first proposed in a 1957 course by Frederick Edward Smith at the University of Michigan.[1][2]
In 1960, Nelson Hairston, Smith, and Lawrence Slobodkin published a paper laying out the green world hypothesis. The name HSS derives from the first letters of each of their surnames.[3][4]
Robert T. Paine did experiments in 1966[5] with Pisaster ochraceus which illustrated their role as a keystone species in regulating Mytilus californianus.[6]
James Estes and John Palmisano did similar experiments with otters, sea urchins, and kelp, where otter presence increased kelp presence in a trophic cascade.[5]
John Terborgh examined Venezuelan valleys with and without predators in 2006, demonstrating the green world hypothesis on land.[7][8]
The plant self-defense hypothesis proposes that plants are not entirely consumed by herbivores primarily because of their adaptations against it (thorns, toxicity, cellulose, etc.).[2]