The battle between bioturbation and biocompaction: Biotically driven vegetation mosaics

Grazing ecosystems ranging from the arctic tundra to tropical savannas are often characterized by small-scale mosaics of herbivore-preferred and herbivore-avoided patches, promoting their biodiversity and resilience. The three leading explanations for bistable patchiness in grazed ecosystems are: i) herbivore-driven nutrient cycling, ii) plant growth-water infiltration feedback under aridity, and iii) irreversible local herbivore-induced abiotic stress (topsoil erosion, salinity). However, these insufficiently explain the high temporal patch dynamics and wide-ranging distribution of grazing mosaics across productive habitats. Here we propose and experimentally test a fourth possibility where alternating patches are governed by the interplay of two important biotic processes: bioturbation by soil fauna that locally ameliorates soil conditions, promoting tall plant communities, alternating with biocompaction by large herbivores that locally impairs soil conditions, and promotes lawn communities. This study provides novel and important insights into the mechanisms whereby the aggregation and activities of organisms promote their own resources.