The brown-world role of insectivores: Frogs reduce plant growth by suppressing detritivores in an alpine meadow

Predators of plant-suppressing herbivores have long been known to indirectly enhance plant biomass, while more recent work has revealed that predators of plant-facilitating detritivores can have the opposite effect on plant biomass. Generalist predators, such as frogs that typically facilitate plant growth in green food webs, may potentially negatively affect plant growth by consuming prey from brown food webs. In a marshy Tibetan alpine meadow, we tested the hypothesis that the locally abundant frog Rana kukunoris could negatively affect plant growth through suppression of dung-decomposing detritivores that promote plant growth by enhancing nutrient recycling. We conducted a factorial experiment (presence/absence of predators x presence/absence of dung) using replicate field enclosures over a growing season. Where dung was present, frogs significantly reduced the number of dung-feeding beetles and dung-feeding flies (including fly eggs and maggots) per dung pat, thereby decreasing dung mass loss and, indirectly, aboveground plant biomass (by 22%) surrounding dung pats. Where dung was absent, frogs did not affect plant biomass. Moreover, the number of dung beetles was positively associated with dung mass loss and soil soluble N concentration (but not total N concentrations), which in turn positively correlated with aboveground plant biomass. These results indicate that a generalist predator species standing in green food webs may play a contrasting role in brown food webs and indicate that a more nuanced appreciation of the functional role of predators in tri-trophic systems is required to accurately predict their cascading effects. Future studies must assess the relative strength of cascading effects mediated through brown and green channels in order to assess the net cascading effects of generalist predators in natural food webs.