Top-down effects on biological soil crust function

Biological soil crusts (BSCs) are communities of microorganisms 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 crustivore (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 efflux. Detritivore feces also increased BSC %N. Crustivorous snail feces increased BSC CO2 efflux, and their mucus decreased BSC %C and %N. In the field, both snail species increased BSC %C and did not affect BSC CO2 efflux. Combined, our findings highlight that macro-invertebrate consumers exert top-down regulation on BSC function, opening the door for a whole new avenue of trophic rese...