Crustivorous macro-arthropods regulate the microtopography and carbon dynamics of biological soil crusts

Biological soil crusts (biocrusts) play key roles in dryland ecosystem processes by mediating soil surface conditions. How consumption by macro-arthropods affects biocrust surface roughness and carbon cycling remains largely unknown. In two separate experiments, we addressed this knowledge gap by exposing biocrusts to varying levels of desert isopod crustivory (i.e. grazing intensity), and quantifying the consequences for microtopography, CO₂ efflux and carbon fixation. Biocrust surface roughness peaked under intermediate crustivory pressure, implying that varying levels of crustivory may have opposing consequences for ecosystem processes such as carbon cycling, water infiltration, runoff generation and soil erosion. However, crustivory had a monotonic negative effect on biocrust carbon cycling. Biocrust CO₂ efflux decreased with increasing crustivory, but recovered after several wetting events. Crustivory had a negative effect on biocrust C fixation, but only after the CO₂ efflux recovered to pre-crustivory levels. Our findings suggest that macro-crustivores may play a pivotal role in regulating biocrust functioning, introducing a whole new line of trophic research that may transform our understanding of ecosystems dynamics in drylands.