Productivity, organism size, and the trophic structure of the major terrestrial biomes

I present a hypothesis suggesting that terrestrial biomes' productivity determines the size of the major organisms and plant physiognomy in them and consequently determines their trophic structure. I suggest a more comprehensive set of patterns over current hypotheses, which consider productivity alone. Based on my hypothesis, I predict that the number of trophic levels decreases with increasing productivity from four in hot deserts to two in productive grasslands and that there are three to four trophic levels in all types of forested biomes. I also suggest that productivity is the limiting factor for the number of trophic levels at the lower end of the terrestrial productivity gradient, herbivore size is the limiting factor at intermediate productivities, and plant physiognomy is the limiting factor in the high productivity range. This contradicts existing hypotheses that predict either an increase (Am Nat 118:240-261, 1981; Am Nat 155:703-723, 2000) or no change (Am Nat 142:379-411, 1993) in the number of trophic levels with the increase in biome productivity.