HSS' hypothesis was based on a terrestrial environment mostly involving insects and their predators, with the Kaibab mule deer irruption the sole example presented of a mammalian system. This provoked a strong rebuttal by wildlife ecologist Graeme Caughley, who suggested that because the factors that may have resulted in this irruption were "hopelessly confounded," a case study of the Kaibab provided an ineffective example of top-down control. These factors included herbivory by livestock and unreliable deer counts. HSS based evidence for top-down effects on things that control insects and on the connections between insects and plants. They observed that plants flourished in the absence of insects when insects were kept down by predation. Researchers working in various marine and mammalian systems subsequently tested the green world hypothesis. In all systems, whether pertaining to insects or mammals, they found a strong positive correlation between predator removal, plant community simplification, and reduced energy flow, which goes back to Paine's dictum about being able to change the world by removing one species.
Ecologists Lauri Oksanen and Stephen Fretwell proposed that if we treat trophic levels as units, systems having four or more trophic levels may have more than one level representing predation. In this scenario a predator at the fourth level will dominate a predator at the third level, and this will release the herbivore population at the second level frompredation, causing its numbers to increase. This in turn will cause overgrazing of plant communities at the first level. Trophic cascades in which a top predator controls its herbivore prey via top-down forces will therefore always have an odd number of trophic levels. Removal of the top trophic level in such systems will have a radical effect on lower levels, causing herbivore irruption and overconsumption of vegetation. In any food chain, energy flow alternates; in odd-linked systems, plants will be limited by resources available to them (top-down control); in even-linked systems, plants will be limited by grazers (bottom-up control). Thus systems with an odd number of levels will be green, while systems with an even number of levels will be brown or barren.
In the 1990s Estes saw a three-level trophic system flip into a four-level system in coastal Alaska. The apex carnivore Orcinus orca, the killer whale, began preying on sea otters, reducing their numbers significantly. Killer whales added a fourth level, reversing the flow of energy from top-down to bottom-up. The resulting cascade rippled through the North Pacific, causing an upsurge in sea urchin numbers and kelp consumption and denuding the ocean floor.
Researchers have studied how the green world hypothesis holds up under different levels of ecosystem productivity, called net primary production. In 1981 Oksanen and his colleagues developed the exploitation ecosystems hypothesis (EEH). In their model herbivory is greatest in relatively unproductive environments, with predation more important and the impact of herbivory reduced as ecosystem productivity increases. Deserts and arctic regions provide examples of systems where plant production is so scant that it can fail to support herbivores. At slightly higher productivity levels, such as on prairies and savannas, an ecosystem becomes able to support a consumer trophic level, with herbivory increasing as productivity increases. As productivity continues to increase, as in boreal forests, plant biomass (the total mass of living matter of a particular type) increases, herbivore biomass increases, and ecosystems become capable of sustaining a third trophic level—the predators—with this level controlling herbivores. Because it uses environmental productivity as a key driver of ecosystemdynamics, EEH incorporates both bottom-up and top-down forces.
The Keystone Species Concept
The keystone species concept lies at the heart of the HSS debate. When Robert Paine introduced it in 1969, he envisioned its mechanisms as a dominant predator consuming and controlling the abundance of a particular prey species and a prey species competing with other species in its trophic class and excluding them from the community. As long as one keeps these two processes in mind, the whole idea of keystone species fits into place—like the keystone of an arch. And when the keystone is removed, arches and ecosystems fall apart. This mechanism explains the ecological collapse Paine observed on Mukkaw Bay. One year after he began removing Pisaster, seven of the fifteen species he had inventoried at the start of his star-throwing experiment were gone, with the others declining rapidly. However, some scientists suggest that ecosystems may not exactly "collapse" when a keystone predator is removed. An ecosystem thus altered continues to function, albeit in a different manner, by moving into what is referred to in ecology as an alternative stable state.