Sheep Spatial Grazing Strategies at the Arid Patagonian Monte, Argentina

Abstract

We asked what vegetation traits influence sheep in selecting foraging paths on the range. We obtained 40 000 records of positions of six ewes (Ovis aries) collared with Global Positioning System receivers during several seasons in a paddock of 1 250 ha at the Patagonian Monte shrublands, Argentina. We classified the vegetation through ground-truth floristic analyses and remotely sensed imagery, and overlaid the ewes’ positions onto a digital map of vegetation units. For each vegetation unit, we assessed the cover of main life forms and preferred plants, the visibility range at ewe’s head height, and several structural/chemical traits of dominant shrubs (leaf mass/area, lignin-phenolics-nitrogen concentration in leaves, presence of thorny stems and spiny leaves). Ewes followed diverse paths across the paddock but always selected among a limited number of vegetation units. Selected vegetation units were those with structural traits allowing wide ewes’ visibility ranges and low structural antiherbivore defenses, irrespective of their local abundance, relative cover of preferred plants, or distance to the watering point. Within preferred vegetation units, ewes further selected those with high cover of preferred plants and/or reduced structural/chemical antiherbivore defenses. We concluded that sheep selectivity at our study paddock resulted from compromises among different structural/visual cues related to visual impairment, antiherbivore physical/chemical defenses, and the offer of preferred plants. In a hierarchy of decisions, the abundance of preferred plants was not a sufficient condition for a high selection of vegetation units. Monitoring animal movements within shrubby paddocks of the Patagonian Monte with high visual impairment can supply criteria to assess the relevance of nonnutritional environmental traits on grazers’ decisions. This information is valuable in identifying and predicting spots of potential land degradation, and planning the distribution of flocks within paddocks in the context of sustainable management practices for shrubby rangelands.