放牧对大针茅草原建群种与优势种空间分布关系的影响

为探讨放牧下大针茅草原建群种大针茅(Stipa grandis)和优势种羊草(Leymus chinensis)及糙隐子草(Cleistogenes squarrosa)种群空间分布特点及关系,本实验以内蒙古自治区锡林郭勒盟毛登牧场大针茅草原建群种大针茅、优势种羊草和糙隐子草为研究对象,通过野外试验,以地统计学为基础,采用半方差函数、分形维数及克里格插值法,分析了围封(CK)和放牧(G)样地大针茅、羊草及糙隐子草种群小尺度空间分布关系。结果表明:围封样地大针茅、羊草及糙隐子草植株密度分别为10.94株/m~(2 )、12.95株/m~(2 )、13.60株/m~(2 ),放牧样地植株密度分别为16.84株/m~(2 )、48.28株/m~(2 )、28.63株/m~(2 ),放牧显著增加大针茅、羊草及糙隐子草种群密度(P0.05);半方差函数进行模型拟合发现,大针茅、羊草及糙隐子草种群空间分布函数关系均符合高斯模型;通过对空间分布函数关系分析,围封和放牧样地结构比为G (93.3%) CK(60.4%),表明放牧样地大针茅种群空间分布主要受结构性因素影响,而围封大针茅种群则受随机性因素影响较大;围封和放牧样地分形维数值为CK(1.796) G(1.361),表明放牧样地大针茅、羊草和糙隐子草所形成的空间分布格局相比围封样地较为复杂,大针茅空间分布对羊草和糙隐子草空间分布的依赖性较弱;通过立体图分析,在放牧利用过程中,羊草处于中低密度时,及糙隐子草处于高密度时,均与大针茅种群间的关系从围封中的竞争关系变为放牧过程中的亲和关系,可见放牧导致种间关系改变。     

草地利用方式对典型草原大针茅种群空间异质性的影响

以内蒙古锡林郭勒盟典型草原为研究对象,以野外实地调查的数据为基础,采用半方差函数和克里格插值法,分析了割草(M)、放牧(G)和围封(CK)样地中大针茅种群(Stipa grandis)小尺度空间分布异质性。结果表明:大针茅种群密度表现为割草样地最高,放牧样地次之,围封样地最低(P0.05);其中围封样地大针茅种群的结构比为95.3%,具有较强的空间自相关性,而刈割和放牧样地大针茅种群的结构比分别为70.2%和62.5%,属于中等程度的空间自相关性,大针茅种群的空间异质性表现为割草样地放牧样地围封样地;通过拟合半变异函数图,得到围封、放牧和割草样地大针茅种群空间分布最优模型分别为高斯模型、球状模型和球状模型;围封样地大针茅空间变异主要受结构因素的影响而趋于简单化分布格局,割草和放牧样地大针茅空间变异受到结构因素和随机因素的共同作用而表现为密度较大的斑块和密度较小的斑块镶嵌分布格局。     

放牧和水淹干扰对松嫩平原碱化草地空间格局影响的分形分析

对东北松嫩平原碱化草地植物群落空间格局的分形性质进行了分析,分别用边长面积指数和Ｋｏｒｃａｋ相对斑块化指数估计了斑块边界复杂性和斑块面积大小频率分布格局。结果表明：随放牧强度增加,占优势的羊草（Ｌｅｙｍｕｓｃｈｉｎｅｎｓｉｓ（Ｔｒｉｎ．）Ｔｚｒｅｌ）斑块的斑块化加剧,而斑块的边界在中度放牧时最不规则;在水淹地,占优势的羊草斑块和次优势的碱茅（Ｐｕｃｉｎｅｌｉａｓｐｐ．）斑块、獐茅（Ａｅｌｕｒｏｐｕｓｌｉｔｏｒａｌｉｓ（Ｇｏｕａｎ）Ｐａｒｌ．ｖａｒ．ｓｉｎｅｎｓｉｓＤｅｂｅａｕｘ）斑块的斑块边界复杂性和斑块化程度比未水淹地都低,说明水淹降低了群落复合体内的异质性;在水淹地,优势斑块的边长面积指数和Ｋｏｒｃａｋ指数均低于次优势种,但在重度放牧地结果正好相反,可能是由于两种样地处于不同的演替阶段;而且,斑块边界复杂性符合同一尺度规律,在现有的面积范围内没有尺度转换,斑块面积大小的频率分布格局则存在尺度转换点。     

不同干扰类型下羊草种群的空间格局

生态系统各层次的空间格局与生态学过程的相互关系一直是生态学的研究热点之一,这一问题的关键是选取合适的观察尺度。羊草(Leymus chinensis)作为呼伦贝尔草原的地带性植被,研究其种群空间格局对理解种群生态过程和草地资源可持续利用具有重要意义。利用Ripley's K函数和Monte Carlo随机模拟方法,对两块样地3种干扰下的羊草种群格局进行定量分析。结果表明:(1)刈割、围封、放牧3种干扰对羊草草甸草原恢复改良样地(LM)和贝加尔针茅试验样地(SM)的羊草种群空间格局具有显著影响,空间格局在不同尺度上存在差异。(2)在较小的空间尺度内,羊草种群倾向于非随机分布,在LM样地、SM样地分别为均匀分布和聚集分布;当空间尺度大于临界值后,羊草种群在围封(30-100cm)、放牧干扰(84-100cm)下倾向于随机分布,刈割干扰下仍为聚集分布。这主要与羊草的繁殖对策、群落地位以及各种干扰下对资源的竞争能力有关。(3)空间结构的规律性表面上取决于观察尺度,但更小尺度的数据对局域空间结构的生物学解释可能更有效。     

恢复演替中草地斑块动态及尺度转换分析

研究了９ａ草地恢复油潜系列中斑块边界形状和斑块面积分布动态,并进行了凡度转换分析。獐茅斑块的边界分维数和斑块化指数最高,羊草斑块的较低,碱蓬斑块的斑块化指数略小于羊草斑块,边界分维数大于洋草斑块。共他类型斑块的两种指数基本上介于獐茅斑块之间,斑块化指数的年限变动滞后于斑块边界分维数,斑块边发维数在整个试验的尺度范围内符合同一自相似规律,斑块的面积分布格局在不同的尺度上有不同的自相似规律。     

不同放牧强度下羊草草原群落斑块植被-土壤特征

植物群落斑块化是天然放牧草地最基本的特征之一,影响着草地生态系统结构和功能。本研究以不同放牧强度下典型羊草草原植物群落斑块为研究对象,对植物群落多样性特征、初级生产力、土壤理化性质、植物养分含量及其相关关系进行研究。结果表明:随着放牧强度的增加,斑块群落内植物种类组成发生改变并趋于简单化,放牧敏感物种消失,物种多样性降低;高放牧强度显著降低了植物群落的地上净初级生产力;初级生产力与土壤氮含量具有显著的正相关关系;随着放牧强度增加,土壤碳、氮、磷含量呈现增加趋势;由于放牧强度及斑块结构的影响,土壤碳、氮、磷含量存在空间分布差异;同时,斑块群落中植被碳、氮、磷含量与土壤碳、氮、磷含量之间没有显著的相关性。研究表明,放牧能够改变草地群落结构和功能,引起植被-土壤养分之间的非同步性发展。     

利用方式对大针茅草原优势种糙隐子草空间异质性的影响

为探讨不同草地利用方式下糙隐子草种群空间分布的变化特点,以内蒙古自治区锡林郭勒盟毛登牧场大针茅草原优势物种糙隐子草(Cleistogenes squarrosa)为研究对象,通过野外试验,以地统计学为基础,采用半方差函数、分形维数及克里格插值法,分析了围封、放牧和刈割样地糙隐子草种群小尺度空间分布异质性。结果表明:糙隐子草种群密度分别为放牧(28.63株·m~(-2))围封(13.60株·m~(-2))刈割(3.85株·m~(-2))(P0.05),放牧显著增加糙隐子草种群密度(P0.05),而刈割显著降低糙隐子草种群密度(P0.05);通过半方差函数进行模型拟合,围封、放牧和刈割样地糙隐子草种群分布分别符合指数模型、球状模型和球状模型;空间分布格局分析表明,各样地糙隐子草种群的结构比分别为刈割(91.9%)放牧(75.3%)围封(71.1%),放牧与刈割样地的空间自相关性均较高,主要受结构性因素影响,而围封样地空间自相关处于中等程度,受随机性因素影响较大;结合分形维数、2D平面图及3D立体图分析,3块样地空间结构良好,空间分布简单,说明合理放牧及刈割不会对种群分布格局造成很大影响,但与围封样地相比,放牧增加了糙隐子草种群空间异质性,而刈割降低了种群异质性。     

草地利用方式对草甸草原植被空间异质性的影响

研究不同利用方式下植物群落中物种的分布特征,有助于解释植物在不同利用方式干扰下的演替规律。本研究以内蒙古呼伦贝尔草甸草原为研究对象,通过野外调查,基于幂函数法则,探讨围封、放牧和刈割利用方式下草地植被物种分布状况和群落整体空间分布的变化。结果表明:在放牧和刈割利用及围封下,草甸草原植被物种分布规律符合幂函数法则;与围封相比,放牧增加了群落整体的空间分布异质性,而刈割则降低了这种变化,刈割和放牧草地物种异质性指数高于群落整体异质性指数的物种增多;物种出现频率发生变化,如对糙隐子草(Cleistogenes squarrosa)种群而言,围封地中糙隐子草的出现频率为0.010,刈割和放牧增加了糙隐子草的出现频率(0.151和0.521),刈割和放牧降低了狭叶青蒿(Artemisia dacunculus)的出现频率(0.132和0.028)。     

Plastic responses of stem and leaf functional traits in Leymus chinensis to long-term grazing in a meadow steppe

植物对不同功能性状进行权衡, 通过表型可塑性达到对异质生境的适应是植物的一种生态对策。羊草(Leymus chinensis)是欧亚温带草原东缘的主要优势植物, 研究其对放牧的表型反应对揭示草原生态系统的放牧响应机制具有代表意义。该文以内蒙古呼伦贝尔草甸草原为例, 通过设置不同放牧压力与围封的长期试验, 研究了羊草茎叶功能性状对放牧的可塑性响应模式。结果表明: 1)与长期围封相比, 长期放牧导致羊草茎叶性状显著小型化, 其中, 株高和个体地上生物量分别降低76.82%和89.88%, 但3年短期围封对茎性状影响不显著, 说明羊草表型矮小化现象具有一定的保守性; 2)通过排序构建羊草性状可塑性变化谱, 发现茎质量、总质量、茎高、株高、叶面积等为对放牧响应的敏感性状, 而叶片数、茎粗、叶宽等较为稳定, 为惰性性状; 3)放牧干扰下, 羊草性状可塑性程度与其变异性之间符合y = y₀ + ae^bx拟合关系, 随着植物性状的响应强度增大, 其变异性增强; 4)偏最小二乘法分析发现茎长、株高、叶面积、叶长等性状的投影重要性指标大于1, 对地上生物量变化的解释率为68.6%, 是导致长期放牧下羊草个体生物量降低的主要因子。研究认为, 矮化型变是羊草的避牧适应对策, 在亚稳态下, 通过不同性状的权衡, 充分利用环境资源完成其生活史。     

草甸草原羊草茎叶功能性状对长期过度放牧的可塑性响应

植物对不同功能性状进行权衡, 通过表型可塑性达到对异质生境的适应是植物的一种生态对策。羊草(Leymus chinensis)是欧亚温带草原东缘的主要优势植物, 研究其对放牧的表型反应对揭示草原生态系统的放牧响应机制具有代表意义。该文以内蒙古呼伦贝尔草甸草原为例, 通过设置不同放牧压力与围封的长期试验, 研究了羊草茎叶功能性状对放牧的可塑性响应模式。结果表明: 1)与长期围封相比, 长期放牧导致羊草茎叶性状显著小型化, 其中, 株高和个体地上生物量分别降低76.82%和89.88%, 但3年短期围封对茎性状影响不显著, 说明羊草表型矮小化现象具有一定的保守性; 2)通过排序构建羊草性状可塑性变化谱, 发现茎质量、总质量、茎高、株高、叶面积等为对放牧响应的敏感性状, 而叶片数、茎粗、叶宽等较为稳定, 为惰性性状; 3)放牧干扰下, 羊草性状可塑性程度与其变异性之间符合y = y₀ + ae^bx拟合关系, 随着植物性状的响应强度增大, 其变异性增强; 4)偏最小二乘法分析发现茎长、株高、叶面积、叶长等性状的投影重要性指标大于1, 对地上生物量变化的解释率为68.6%, 是导致长期放牧下羊草个体生物量降低的主要因子。研究认为, 矮化型变是羊草的避牧适应对策, 在亚稳态下, 通过不同性状的权衡, 充分利用环境资源完成其生活史。     

A hierarchical model for analysing the stability of vegetation patterns created by grazing in temperate pastures

Questions: Does vegetation structure display any stability over the grazing season and in two successive years, and is there any correlation between the stability of these spatial patterns and local sward composition? Location: An upland grassland in the French Massif Central. Method: The mosaic of short and tall vegetation stands considered as grazed and ungrazed patches respectively is modeled as the realization of a Boolean process. This method does not require any arbitrarily set sward‐height thresholds to discriminate between grazed and ungrazed areas, or the use of additional variables such as defoliation indexes. The model was validated by comparing empirical and simulated sward‐height distributions and semi‐variograms. Results: The model discriminated between grazed and ungrazed patches at both a fine (1 m²) and a larger (500 m²) scale. Selective grazing on legumes and forbs and avoidance of reproductive grass could partly explain the stability of fine‐scale grazing patterns in lightly grazed plots. In these plots, the model revealed an inter‐annual stability of large‐scale grazing patterns at the time peak biomass occurred. At the end of the grazing season, lightly grazed plots showed fluctuating patch boundaries while heavily grazed plots showed a certain degree of patch stability. Conclusion: The model presented here reveals that selective grazing at the bite scale could lead to the creation of relatively stable patches within the pasture. Locally maintaining short cover heights would result in divergent within‐plot vegetation dynamics, and thus favor the functional diversity of vegetation.     

Effects of grazing on patch structure in a semi‐arid two‐phase vegetation mosaic

Abstract. Question: What are the grazing effects in the spatial organization and the internal structure of high and low cover patches from a two‐phase vegetation mosaic? Location: Patagonian steppe, Argentina. Methods: We mapped vegetation under three different grazing conditions: ungrazed, lightly grazed and heavily grazed. We analysed the spatial patterns of the dominant life forms. Also, in each patch type, we determined density, species composition, richness, diversity, size structure and dead biomass of grasses under different grazing conditions. Results: The vegetation was spatially organized in a two‐phase mosaic. High cover patches resulted from the association of grasses and shrubs and low cover patches were represented by scattered tussock grasses on bare ground. This spatial organization was not affected by grazing, but heavy grazing changed the grass species involved in high cover patches and reduced the density and cover of grasses in both patch types. Species richness and diversity in high cover patches decreased under grazing conditions, whereas in low cover patches it remained unchanged. Also, the decrease of palatable grasses was steeper in high cover patches than in low cover patches under grazing conditions. Conclusions: We suggest that although grazing promotes or inhibits particular species, it does not modify the mosaic structure of Patagonian steppe. The fact that the mosaic remained unchanged after 100 years of grazing suggests that grazing does not compromize population processes involved in maintaining patch structure, including seed dispersal, establishment or biotic interactions among life forms.     

Composition and diversity of herbaceous patches in woody vegetation: The effects of grazing,soil seed bank,patch spatial properties and scale

Herbaceous plants contribute much to plant diversity in Mediterranean-type ecosystems though mostly occupying relatively small patches within the dense woody vegetation. While studying species diversity in the herbaceous patches, we hypothesized that grazing, soil seed bank, and spatial properties of the patch affect plant diversity and composition at different spatial scales. The study site was in an LTER site located in the Mediterranean region in north Israel. We determined herbaceous species composition in: (1) randomly sampled quadrats in herbaceous patches in grazed and un-grazed plots; (2) soil seed bank samples taken from the same patches and germinated under optimal greenhouse conditions; (3) quadrats in the same patches sown with a homogenous mixture of local soil samples. Using GIS methods, we determined small-scale spatial characteristics of the herbaceous patches. Alpha and beta diversities were calculated at the patch and plot scales using Shannon's entropy H. Grazing increased alpha diversity of local untreated seed bank samples but decreased alpha diversity of the artificial homogenous soil seed bank mixture at both patch and plot scales. Positive relation between alpha diversity and patch area was detected only under grazing. Grazing increased beta diversity in all three treatments at the patch scale. Grazing decreased the similarity in species composition between above-ground vegetation and soil seed bank. The results indicate that moderate cattle-grazing affects species diversity in the herbaceous patches within the dense maquis. This effect is scale-dependent, and interacts with the effects of soil seed bank and patch spatial-properties: without grazing soil seed bank plays a more important role than patch spatial properties, but under grazing the size and the accessibility of the patch are more important in the determination of herbaceous species composition.     

Effects of grazing pattern and nitrogen availability on primary productivity

A major part of the impact of grazing on primary productivity results from the joint action of tissue removal and nutrient return to the soil via dung and urine. Grazing, however, is not uniformly distributed in space: grazed grasslands show a matrix of grazed and ungrazed patches, which in turn, may or may not be affected by faecal or urine deposition. This paper investigates the effects of grazing spatial pattern and nitrogen availability on primary productivity. We propose that grazed plants located at the edge of a grazed patch are more shaded by their taller ungrazed neighbours than plants at the center. Since the border effect is less important as patch size increases, the effects of grazing will be more positive, or less negative, when grazing pattern is coarse-grained than when it is fine-grained. We also propose that nitrogen availability will affect this response to grazing through its effects on the intensity of competition for light and on the amount of compensatory growth. We performed a field experiment in a grassland community of the Flooding Pampa, Argentina, in which we compared the productivity of undefoliated controls and defoliated patches of different size, with and without nitrogen application. Defoliation reduced primary productivity and this effect was greater in the smallest, fertilized patches. Productivity was highest at patches of intermediate and large sizes. Nitrogen addition increased productivity by two-fold. The integrated photon flux density reaching the base of the canopy was affected by defoliation and by patch size: it was lower in controls than in defoliated patches and increased with patch size. Our results showed that (a) the size of the defoliated patch modified the response of this grassland to defoliation, (b) this response was correlated with light availability, and (c) nitrogen addition, simulating urine depositions, increased primary productivity and affected the response to defoliation of the smallest patches.     

放牧与围栏羊草草原生态系统土壤呼吸作用比较

采用静态箱式法,比较分析了内蒙古典型羊草草原放牧与围栏生态系统土壤呼吸作用及其与环境因子的关系．结果表明,围栏和放牧样地的土壤呼吸作用日动态均呈单峰型曲线,高峰值一般出现在13：00～15：00,围栏样地土壤呼吸作用日动态与地表温度相关性最好,而放牧样地与地下5cm温度相关性最好．在整个观测期内,6、7月份是植物生长的旺盛期,围栏样地土壤呼吸作用明显大于放牧样地约2．7倍;到植物生长后期的8、9月份,二者差异不大,与地下生物量的变化相似,可能与牲畜的采食对不同物候期的植物影响不同及周围环境因子的改变有关,说明人类活动的干扰不一定增加土壤呼吸作用．围栏样地和放牧样地土壤呼吸作用季节动态都与0～10cm的土壤含水量相关性最好,相关系数分别是0．853和0．741,而围栏样地土壤含水量与土壤呼吸作用季节动态的相关性大于放牧样地;围栏和放牧样地不同层次土温、土壤含水量与土壤呼吸作用日、季动态的关系均表现出浅层的相关性普遍大于深层．     

Effects of fencing and grazing on the temporal dynamic of soil organic carbon content in two temperate grasslands in Inner Mongolia,China

This study aimed to investigate the impact of long-term grassland management on the temporal dynamic of SOC density in two temperate grasslands. The top soil SOC density, soil total nitrogen density and soil bulk density (0–20 cm) under long-term fencing and grazing treatments, the aboveground net primary productivity of fenced plots and the associated climatic factors of Leymus chinensis and Stipa grandis grasslands in Inner Mongolia were collected from literatures and analyzed. The results showed that the SOC density increased linearly with fenced duration but was insensitive to grazed duration in both grasslands. Compared with long-term grazing, fenced plots had larger potential for carbon sequestration, and the accumulation rate of SOC density was 29 and 35 g Cm^–2y^–1 for L. chinensis and S. grandis grasslands. Fenced duration and mean annual temperature jointly contributed large effect on temporal pattern of SOC density. Climate change and grazed duration had little influence on the inter-annual variance of SOC density in grazed plots. Our results confirmed the enhancement effect of long-term fencing on soil carbon sequestration in degraded temperate grassland, and long-term permanent plot observation is essential and effective for accurately and comprehensively understanding the temporal dynamic of SOC storage.     

放牧对内蒙古典型草原生态系统植物及土壤δ15N的影响

植物和土壤中的¹⁵N自然丰度值(δ¹⁵N)是评价生态系统N循环的一个重要指标, 而放牧是草原生态系统的主要土地利用方式, 对草原生态系统的N循环过程的改变起着重要作用。该研究测定了内蒙古锡林河流域放牧和围封条件下草原群落主要优势植物和土壤的δ¹⁵N值, 探讨放牧对草原N循环的影响。研究中所测定的8种植物叶片δ¹⁵N变化很大(-4.04‰-4.34‰), 但与植物功能型有一定的相关性。放牧显著降低了大针茅(Stipa grandis)、杂类草和小半灌木木地肤(Kochia prostrata)的δ¹⁵N值。具有潜在共生固氮能力的豆科植物δ¹⁵N偏低负值(-4.04‰ - -1.90‰), 但在放牧和围封条件下无显著差异; 而被认为具有联合固氮能力的羊草(Leymus chinensis), 放牧后δ¹⁵N显著增加, 一定程度上表明了豆科植物和羊草生物固氮能力的存在。所有植物中, 除无菌根侵染的木地肤外, 其他有丛枝菌根真菌侵染记录的物种δ¹⁵N值较低, 通常接近0或为负值, 说明在N限制的内蒙古草原, 菌根转运N可能也是一种重要的N源途径。放牧显著降低了0-20 cm土壤δ¹⁵N值, 这也与过去的研究结果不同。δ¹⁵N的测定为生态系统提供了一个整合时空N循环过程的综合指标, 反映出放牧改变了草原生态系统的N循环。     

Disturbance intensity and above- and belowground herbivory effects on long-term (14 y) recovery of a semiarid grassland

The importance of disturbance intensity and herbivory by cattle and white grubs, or the larvae of June beetles (including Phyllophaga fimbripes), to recovery of shortgrass steppe ecosystems in Colorado, U.S.A. were evaluated over a fourteen year time period. Disturbance intensity was defined by survival of the dominant grass species (Bouteloua gracilis) after an outbreak of root feeding activity by white grubs. Sixteen patches of vegetation consisting of four pairs of adjacent ungrazed-grazed by cattle locations with two replicates that were recently affected by white grubs were selected in 1977. Disturbance intensity was determined in 1977 by the area in each patch that contained live tillers of B. gracilis. Permanent plots were located both within and outside of each patch. Plant basal cover and density by species were estimated at time of peak aboveground biomass in six different years on each plot.Successional dynamics on patches was similar to areas affected by other types of disturbances, however, rate of recovery was faster for patches affected by grubs. Grazing by cattle was infrequently important to plant recovery, a result similar to effects of grazing on other aspects of shortgrass steppe ecosystems. Disturbance intensity was important to recovery of B. gracilis since tiller survival in 1977 was linearly related to cover in each year of sampling. For ungrazed patches, initial conditions were important to recovery of B. gracilis for as many as 14 years. For grazed patches, initial conditions decreased and grazing increased in importance through time. Changes in resource quality and a more uniform distribution of roots due to grazing likely resulted in more complete mortality of plants by grubs under grazed compared to ungrazed conditions.Persistence of shortgrass ecosystems in spite of disturbances with different intensities are determined at least in part by characteristics of disturbances interacting with the ability of plants to respond, and in part by the evolutionary history of the system. Although white grubs affect shortgrass communities infrequently, they have large and important effects on plant community structure through time, and represent an important class of disturbance defined by intensity.     

Effect of grazing on plant patterns in arid ecosystems of Patagonian Monte

Our objective was to assess the relationship between the spatial patterning of perennial grasses (total, grazed, and non‐grazed) and shrub patches in rangelands under different grazing pressures of the Patagonian Monte. We selected three grazed paddocks with the usual stocking rate for the area, where previous studies showed that a piosphere formation is common. At each paddock, we analysed the grain of heterogeneity at sites located at two distances from the single watering point (near, far), using high‐resolution aerial photographs. At these sites, we also assessed in the field the density, size, cover, and spatial patterning of grazed and non‐grazed perennial grasses and shrub patches. The grain of heterogeneity of shrub patches was coarser in sites near the watering point than in those distant from it, as a consequence of the increase in size of both, bare soil and shrub patches. Field sampling showed that a coarse grain of heterogeneity relative to fine‐grained sites resulted from changes in species composition, increased bare soil areas and reduced perennial grass cover. In coarse‐grained sites, lower perennial grass cover resulted from lower density and/or smaller size of grass bunches than in fine‐grained sites. We did not find significant differences among sites in the proportion of perennial grazed grasses. Since the density and cover of perennial grasses was higher in fine‐ than in coarse‐grained sites, we suggested that fine‐grained sites are more important as feeding stations than coarse‐grained sites. The consequences of this differential use could lead to degradation of fine‐grained sites and to higher homogeneity in spatial plant structure and floristic composition within paddocks with respect to the condition observed at present, increasing the size of the highly degraded zone within the piosphere. At the patch level, we found that at about one third of the sampled transects, both total and non‐grazed perennial grasses were spatially aggregated with shrub patches. However, in most transects grazed perennial grasses were indifferently distributed in relation with shrub patches, showing that grazers display high selectivity of foraging sites at macro level (i.e. high and low grazing pressure sites at the paddock level), but random occupancy of vegetation units (randomness in the distribution of grazed perennial grasses at the patch level). The intensity of the positive association between non‐grazed grasses and shrub patches was higher in fine‐grained than in coarse‐grained sites and may be attributed to higher protection against herbivores associated to denser shrub patches in fine‐ relative to coarse‐grained sites. We concluded that a feedback exists between the spatial distribution of species preferred by grazers and the spatial patterning of use of these species.