When plant communities suffer the stress of limited resources, for instance adverse environmental conditions such as extreme aridity, the spatial homogeneity of the biomass is lost and self-organized patterns may arise. Here, we report the observation of spiral-shaped patterns in the biomass of grass (genus deyeuxia), under highland arid conditions in the north of Chile. The spiral arms are a few meters long and a few centimeters wide. These dynamic structures are observed in the grazing area of an herbivore member of the South American camelids, the vicuna, on the border of highland wetlands. These spirals cannot be explained by the well-established mathematical models which describe other vegetation patterns (that emerge from a Turing-type of instability) such as stripes, rings, or fairy circles. We attribute the formation of spirals to the coupling between the growth of vegetation in semiarid regions and the grazing of vicunas. The mathematical analysis of this coupling reveals an excitable behavior, i.e. small perturbations of the equilibrium generate large trajectories before coming back, that is the origin of the spirals.
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