How coupling between green and brown food webs alters trophic structure

In 1960 Hairston, Smith and Slobodkin began their Green World Hypothesis with consideration of detritus and decomposers. However, most work since then has ignored the decomposer “brown world” and concentrated on the living producer “green world,” with a general paucity of work incorporating both producers and decomposers. When recent syntheses indicated that trophic cascades structure many freshwater systems but few terrestrial ones, numerous possible mechanisms relating to the green web were offered to explain these differences, including autotroph edibility and heterotroph metabolism. Another possible explanation is cross-system differences in the relative connectedness of detritus-based and producer-based food webs. For example, many freshwater lakes where top-down forces predominate are actually structured by top predators subsidized heavily from benthic prey, and include herbivores that can feed from both webs. In contrast, terrestrial systems tend to have specialist herbivores and predators that may not switch as freely between green and brown webs. We developed a food web model comprising two initially separate food chains based on autrotroph and detritus pools connected via a single nutrient pool. We then examined how altering the number and strength of linkages between the brown and green chains altered the strength of bottom-up and top-down forces. By also altering model parameters relating to heterotroph metabolisms (assimilation efficiencies) and autotroph edibility (egestion fractions), we compared green-brown linkages to the other major hypotheses put forth to explain cross-system differences. Finally, we consider how these theories may all contribute to creating the fast and slow channels within many food webs.