We report for the first time on the formation of spirals like vegetation patterns in isotropic and uniform environmental conditions. The vegetation spirals are not waves and they do not rotate. They belong to the class of dissipative structures found out of equilibrium. Isolated or interacting spirals and arcs observed in South America (Bolivia) and North Africa (Morocco) are interpreted as a result of curvature instability that affects the circular shape of localized patches. The biomass exhibits a dynamical behaviour with arcs that transform into spirals. Interpretation of observations and of the predictions provided by the theory is illustrated by recent measurements of peculiar plant morphology (the alfa plant, or Stipa tenacissima L.) originated from northwestern Africa and the southern part of the Iberian Peninsula. This article is part of the theme issue ‘Dissipative structures in matter out of equilibrium: from chemistry, photonics and biology (part 2)’.

Les habitats utilisés par le Tadorne casarca Tadorna ferruginea dans la zone humide d'Aguelmam Sidi Ali ont été étudiés durant la période allant du mois d'octobre 2010 au mois de septembre 2011. Ces habitats sont divisés en 5 types, selon leurs caractéristiques: les bords des lacs, les eaux profondes, les prairies avec ruisseaux de Taânzoulte, les marécages temporaires et la cédraie. Les prairies avec ruisseaux, les marécages et les bords des lacs qui sont riches en ressources alimentaires, facilement accessibles, sont les préférés par le Tadorne casarca. Les eaux profondes constituent un refuge en cas de dérangement et aussi pour la sauvegarde des poussins. La forêt du cèdre est devenue l'unique endroit pour sa nidification. Une délimitation des endroits pour chaque activité humaine, en tenant compte des habitats est à prévoir pour réduire les menaces humaines sur le Tadorne casarca au niveau de la zone d'étude.

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We consider a generic interaction-redistribution model of vegetation dynamics to investigate the formation of patchy vegetation in semi-arid and arid landscapes. First, we perform a weakly nonlinear analysis in the neighborhood of the symmetry-breaking instability. Following this analysis, we construct the bifurcation diagram of the biomass density. The weakly nonlinear analysis allows us to establish the condition under which the transition from super- to subcritical symmetry-breaking instability takes place. Second, we generate a random distribution of localized patches of vegetation numerically. This behavior occurs in regimes where a bare state coexists with a uniform biomass density. Field observations allow to estimate the total biomass density and the range of facilitative and competitive interactions.