Top-down effects on biological soil crust function

Biological soil crusts (BSCs) are communities of microorganisms, mosses, and fungi that control ecosystem functions in drylands. Despite their importance, little is known about how trophic interactions affect BSC function. We conducted a series of mechanistic experiments to tease out the direct (i.e., consumption) and indirect (i.e., fecal and mucus deposition) pathways by which crustivores (i.e., consume BSCs) and detritivores affect BSC functions— complemented by a manipulative field experiment exploring the integrative effect of these pathways. We showed that detritivore feces, mucus, and grazing increased the BSCs’ CO₂ respiration. Detritivore feces also increased BSC N content by 9% compared to BSCs not exposed to snail consumers. Crustivorous snail feces increased BSC CO₂ respiration, and their mucus decreased BSC %C and %N. In the field, detritivorous and crustivorous snails increased BSC %C by 15% and 17%, respectively, but did not affect BSC CO₂ respiration. Combined, our findings highlight that macro-invertebrate consumers exert top-down regulation on BSC function, opening the door for a new avenue of trophic research.