The green world hypothesis 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. It is also known as the HSS hypothesis, after Hairston, Smith and Slobodkin, the authors of the seminal paper on the subject.
Although plenty of herbivores exist that would potentially diminish the vegetation of the world, many researchers find themselves asking the question of how biomass and biodiversity are able to be maintained. The natural order to allow for the persistence of all species and ecosystems requires an opposite force acting upon these herbivores. A system of checks and balances is proposed in allowing the flourishing of flora in various ecosystems, as suggested by the green world hypothesis. In addition to plant defense mechanisms, predators assist in the regulation of these herbivore population numbers, limiting the amount of vegetation that is consumed. Several ecosystems are characterized by a trophic cascade system, in which all levels interact and impact the persistence of one another (Siliman and Angelini, 2012). For example, the herbivores reduce plant populations, but are kept in check by carnivorous consumers that limit population growth beyond what's allotted given resource availability.
The study of trophic cascades is highly important to understanding the green world hypothesis. One way that trophic cascades can impact ecosystems is through the limitation of net primary productivity, which determines energy flow, through various resources. This bottom-up approach results in the abundance of unpalatable plant species due to various environmental conditions (Siliman and Angelini, 2012). Additionally, energy in a given system can be determined by predators at the highest trophic level, or the carnivores that consume other carnivores. This top-down approach is characterized by high consumer densities, and in many cases, weedy plant systems, without strong initial defense mechanisms in place (Siliman and Angelini, 2012). These processes of the maintenance of trophic cascades often operate simultaneously. A general consensus is that trophic cascades tend to have a larger effect in aquatic ecosystems compared to terrestrial. However, overregulation in any of these communities has the potential to result in the degradation of the trophic cascade within the system, preventing growth across many species of all levels (2012).
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