Green world hypothesis

The green world hypothesis posits that predators primarily regulate ecosystems by controlling herbivore populations, preventing them from overconsuming vegetation. Also known as the HSS hypothesis after Hairston, Smith, and Slobodkin, who published a seminal paper in 1960, it suggests that predators maintain biomass and biodiversity by acting as a check on herbivores. This balance allows flora to thrive despite the presence of herbivores. Predators, along with plant defense mechanisms, limit herbivore populations, ensuring vegetation persists.

Trophic cascades, where changes at one trophic level affect others, are central to this hypothesis. For example, predators can influence plant abundance by controlling herbivores, while herbivores may reduce plant populations if left unchecked. These interactions can operate from the bottom-up, through resource availability, or top-down, through predator control. Aquatic ecosystems often show stronger trophic cascades than terrestrial ones, though overregulation in any system can disrupt these processes.

Key studies support the hypothesis. In 1966, Robert Paine demonstrated that the sea star Pisaster ochraceus regulated mussel populations and maintained biodiversity in intertidal zones. James Estes and John Palmisano found that sea otters controlled sea urchin populations, allowing kelp forests to flourish. Without these predators, ecosystems like kelp forests or intertidal zones degrade. John Terborgh’s study of Venezuelan valleys with and without predators further validated the hypothesis on land.

However, the plant self-defense hypothesis offers an alternative explanation, suggesting that plants’ own adaptations, such as toxicity or thorns, primarily prevent overgrazing by herbivores rather than predator activity.