Selective grazing by domestic livestock is a major control of plant community structure and dynamics in drylands. However, grazing impact predictions supporting management decisions are frequently based on average biomass consumption, neglecting selectivity. We evaluated the relative importance of grazing pressure, total and each species density, and plant dead biomass proportion as drivers of selective defoliation by sheep in three dominant native grass species in Patagonian steppes. Species were Poa ligularis, Festuca pallescens, and Pappostipa speciosa, which a priori present different preference degree by sheep. The relevance of these drivers for differently preferred species has not been simultaneously studied. We recorded the defoliation frequency and degree of the three species (dependent variables) throughout 112 field surveys. Besides, we recorded grazing management and vegetation structure descriptors (independent variables). Poa ligularis was highly defoliated (90% of plants), and grazing pressure was the leading driver (asymptotic exponential relationship). For F. pallescens, almost 70% of plants were defoliated, and defoliation non‐linearly increased as grazing pressure rose and linearly decreased as both its dead biomass proportion and P. ligularis density increased. For P. speciosa, defoliation was low (20% of plants) and linearly decreased as both its dead biomass proportion and the density of the other two species increased. Grazing pressure played a negligible role in this species. These patterns confirmed that P. ligularis, F. pallescens, and P. speciosa, respectively, present high, intermediate, and low preference degree by sheep. In conclusion, our findings suggest that (1) selective defoliation can be satisfactorily predicted as function of grazing pressure, species densities, and plant dead biomass proportion; (2) grazing pressure becomes a more relevant driver as species preference rises and its effect on defoliation is markedly non‐linear; (3) the dead biomass proportion and the abundance of highly preferred species are the leading factors determining less preferred species defoliation; and (4) grazing pressure management by itself is not enough to reduce the high defoliation of preferred species and increase the defoliation of non‐preferred species. This knowledge is critical for developing effective management practices to control forage species defoliation in rangelands worldwide where species with different preference by herbivores coexist.

中文翻译：

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干旱牧区绵羊对草料选择性落叶的控制

家畜选择性放牧是干旱地区植物群落结构和动态的主要控制。但是，支持管理决策的放牧影响预测通常基于平均生物质消耗量，而忽略了选择性。我们评估了在巴塔哥尼亚草原的三种主要原生草种中，放牧压力，总和每种物种密度以及植物死生物量比例作为绵羊选择性落叶的驱动因素的相对重要性。物种为轻皮早熟禾（Poa ligularis），白桦（Festuca pallescens）和番木瓜（Pappostipa speciosa），这是先验的羊表现出不同的偏好程度。这些驱动因素与其他偏爱物种的相关性尚未同时进行研究。我们记录了112个现场调查中这3个物种（因变量）的落叶频率和程度。此外，我们记录了放牧管理和植被结构描述符（独立变量）。舌状Poa高度落叶（占植物的90％），放牧压力是主要驱动因素（渐近指数关系）。对于淡色镰刀菌，近70％的植物有落叶，随着放牧压力的增加，落叶的非线性增加，而死生物量和草密度的增加则线性减少。对于P. speciosa，落叶的比例很低（占植物的20％），并且随着其死生物量比例和其他两个物种密度的增加而线性降低。放牧压力在该物种中的作用微不足道。这些模式证实了P. ligularis，F。pallescens和P. speciosa，绵羊分别表现出高，中和低偏好度。总之，我们的发现表明：（1）选择性放叶可以作为放牧压力，物种密度和植物死生物量比例的函数令人满意地预测；（2）放牧压力随着物种偏好的增加而变得更加相关，并且其对落叶的影响明显是非线性的；（3）死生物量比例和高度优先物种的丰富度是决定次要优先物种落叶的主要因素；（4）仅靠放牧压力管理不足以减少首选物种的高度落叶和增加非首选物种的落叶。