What is the green world hypothesis?

The theory, which was first put forth in 1960 by American scientists Nelson Hairston, Frederick Smith, and Lawrence Slobodkin, holds that predators, not plant defenses, are what prevent herbivores from spreading too far, which would ultimately result in the widespread extinction of plant life.

The impossibility of eliminating all predators from a test site makes it difficult to test in the wild, but this was demonstrated at a hydroelectric project site in Venezuela, where flooding caused by the dam created several islands, and on those islands or mainland patches where herbivores and predators coexisted, there was a greater destruction of green cover than on those where predators were absent.

The Green World Hypothesis postulates that the presence of many plant species and the intricate interactions between herbivores and their predators allow herbivores to consume a significant portion of plant tissue without reducing the number of plants. In other words, the hypothesis implies that herbivore populations are kept in check by predators, which prevents overgrazing and creates a "green world" where plants are the dominant species in the ecosystem.