The authors highlight 5 lines of reasoning to underscore the importance of predators as top-down controls:
1) the rate of planetary fossil fuel accumulation over time has not been minuscule as compared to the rate of photosynthesis in the same systems;
2) given this, decomposers must be food-limited; otherwise, fossil fuels would build up at higher rates;
3) in terrestrial systems, plants are typically not herbivore-controlled, nor are they regularly destroyed by weather, but are controlled by bottom-up factors such as light, water, and nutrients;
4) terrestrial herbivores are therefore typically not limited by their food supply, even in areas where the primary consumers are overabundant;
5) herbivore populations are therefore controlled by predators.