Vertical structure of wet grasslands under grazed and non‐grazed conditions in Tierra del Fuego

Abstract. We studied the vertical structure of wet grazed grasslands in Tierra del Fuego (southern Argentina). A point quadrat method was developed using a fine needle graduated in cm. The vertical and horizontal frequency of species and organs was quantified in samples collected from non‐grazed and grazed plots in the field. There was vertical stratification in both types of samples, but only in the first eight cm above the ground in grazed samples, with a dominance of Caltha sagittata. In non‐grazed samples graminoids grew taller than forbs and their inflorescences were an important element of the canopy structure. In both treatments, vertical species diversity was maximum in the lower part of the canopy, although diversity was significantly higher in grazed pots. In grazed samples, Caltha sagittata was the dominant species in 46% of samples and its leaves occupied 35% of the upper canopy. In the non‐grazed samples, Hordeum pubiflorum and Festuca magellanica were dominant in 63% of the samples, with H. pubiflorum leaves occupying 55% of the upper canopy. Comparing species by pairs, significant differences in vertical position were maintained in non‐grazed versus grazed pots. It is concluded that vertical stratification occurs even in the shortest communities. In this community, forb species grew close to the ground in the grazed areas, while forbs grew in the gaps and grasses above them in the non‐grazed areas. The main differences were in the relative dominance of forb and grass species and the presence of inflorescences.     

放牧家畜的践踏作用研究评述

放牧家畜的践踏作用是家畜作用于草地的 3个途径之一 ,相对于采食和排泄物的作用具有作用时间长、直接作用的草地组分多和效果持久的特点。家畜践踏损伤牧草、埋实种子、促进萌发 ,减少凋落物的现存量 ,增加土壤的紧实度、容重 ,降低土壤孔隙度、水稳性团聚体、透水性和透气性 ,导致雨后水涝和植物根区缺氧 ,引发水土流失。国内外使用的 4种践踏强度指标分别存在不确定性大、忽视践踏强度的时间依赖性和畜种间差异、没有考虑放牧行为及其时间分配模式等问题 ,分析认为践踏强度应为家畜放牧行为、不同行为的持续时间、家畜体重、放牧地面积和坡度的函数。家畜的践踏作用因草地的健康状况差异而与之存在正或负的反馈机制以及践踏强度阈值 ,可能在草地退化和健康维护中起主导作用。今后需建立严格的研究方法和科学的践踏强度指标 ,重视草地植被对践踏的耐受阈限、践踏对牧草更新的调控机理及其对凋落物产生和分解的影响 ,以及土壤水分与践踏作用的关系 ,确定放牧系统的践踏强度阈值。     

Winners and losers in a long-term study of vegetation change at Moor House NNR: Effects of sheep-grazing and its removal on British upland vegetation

We analysed data collected between 1954 and 2000 from nine long-term experiments designed to assess the effects of sheep-grazing versus no-sheep-grazing at Moor House NNR, an Environmental Change Network site. The experiments were set up between 1954 and 1972 across a range of vegetation types typical of much of upland Britain. Data from this type of experiment are often difficult to analyse and we describe the procedures undertaken to prepare the data for analysis. We fitted the resultant data to the British National Vegetation Classification and used ordination techniques to assess the relative positions of the experiments to each other. Finally, we used Generalized Linear Mixed-effects Modelling within a Bayesian framework to model change of species taxonomic/physiognomic groups through time in both sheep-grazed and ungrazed treatments across all nine experiments; variables included species diversity, Shannon–Weiner index and derived data on occurrence and abundance of species groups based on taxonomy and physiognomy. Hurdle analysis was used to model the species groups; this analysis separated the change through time in both probability of occurrence (binomial distribution) and abundance (Poisson distribution).In the sheep-grazed plots (the “business-as-usual” treatment hence here designated the “control”) there was a reduction in species diversity and a decrease in abundance of vascular plants, grasses, lichens, liverworts and mosses; whereas herbs, sedges and shrubs increased. When probability of occurrence was considered, there was a reduction in number of presences of both lichens and liverworts. Thus, the status quo management of continuous sheep-grazing, even though reduced since 1972, has resulted in a marked change in species composition of these plant communities, with some winners and some losers, but overall they support the concept of biotic homogenization. It is likely that some of these changes were driven by external factors such as elevated atmospheric nutrient deposition. Removal of sheep grazing had some positive benefits; with the herbs, mosses, sedges and shrubs increasing, but faster reductions in grasses and liverworts. Sedges + rushes were stable. It suggested that future monitoring schemes might use either the probability of occurrence of liverworts and lichens, or the abundance of lichens, liverworts, grasses and mosses as sensitive indicators of change in upland Britain.Moreover, during the period that Moor House has been protected as a nature reserve some key plant species groups have declined in spite of reductions in grazing pressure. To reverse this trend requires some form of interventionist management. In order to increase the diversity of vascular plants some form of disturbance will probably be needed, but for bryophytes and lichens this remains a research question. If the results from these small-scale experiments are replicated at the landscape scale a reduction of sheep grazing pressure, for example in rewilding schemes, will have little effect on species composition over a 28–44 year period.     

Grassland dynamics under sheep grazing in an Australian Mediterranean type climate

Grassland dynamics in a degraded disclimax grassland dominated by Danthonia caespitosa Gaudich. are examined using both demographic and multivariate approaches in an experiment designed to determine the effect of grazing intensity and exclosure on pasture dynamics. The experiment ran for 20 years from 1949 to 1968, using permanent quadrats at 3 grazing intensities and within exclosures. Demographic studies of some perennial grass species demonstrated markedly different responses to grazing; Danthonia caespitosa was unaffected by grazing but responsive to seasonal rainfall differences. Enteropogon acicularis survived only on protected sites. Numerical classification of total species set (121 species) for six observation periods demonstrated that community types were sensitive to differences in winter rainfall, and time since the start of experiment. Principal component analysis of permanent quadrat observations for individual years demonstrates quadrat trajectories which confirm this and indicate progressive divergence of the successional trends of the grazed and ungrazed quadrats. Repeated analysis on grazed quadrats only, shows that three components of pasture dynamics can be recognized; these are trend (succession?) and seasonal differences, each of which account for about 20% of the variance, and differences due to soil heterogeneity in the experimental paddock (8% of variance accounted for). No effect of grazing intensity was detected. Multivariate techniques can provide a clear partitioning of types of dynamic behaviour present in grassland communities. It is concluded that partitioning of environmental heterogeneity prior to demographic studies would increase their sensitivity.

     

Directional and non‐directional vegetation changes in a temperate salt marsh in relation to biotic and abiotic factors

Abstract. The effects of reduction and cessation of sheep grazing on salt‐marsh vegetation were studied on a formerly intensively grazed salt marsh in northern Germany. Plant species cover was recorded in 45 permanent plots from 1992 to 2000. In 1995, physical and chemical soil parameters were analysed. Results of Redundancy Analysis (RDA) indicated that salinity and the depth of anoxic conditions below the surface were the most important soil factors related to the spatial vegetation pattern. Furthermore, plant species distribution was influenced by present and past grazing intensity, by soil grain size and nitrogen content. Vegetation changes over 9 yr were analysed by non‐linear regression. The cover of Aster tripolium, Atriplex portulacoides, and, to a lesser extent, Artemisia maritima and Elymus athericus increased due to reduced grazing pressure, whereas the cover of Salicornia europaea decreased. After a strong increase in the first years Aster decreased 2 to 6 yr after abandonment. In the mid salt‐marsh zone Puccinellia maritima was replaced by Festuca rubra. The cover of Puccinellia, Festuca, Suaeda maritima, Glaux maritima and Salicornia fluctuated strongly, probably due to differences in weather conditions and inundation frequency. Species richness per 4 m² generally increased while vegetation evenness decreased during the study period. Only in the high salt marsh abandoned for 9 yr did the number of species decrease slightly. Thus far, cessation of grazing did not lead to large‐scale dominance of single plant species.     

Successional changes in the morphology and ecological responses of a grazed pasture ecosystem in Central Spain

Morphological characteristics related to spatial occupation, reproduction and adaptations to grazing were used to characterize the most frequent species in a therophytic pastureland of Central Spain.Periodic ploughing is a traditional practice in these pastures and allows observation of successional change. In the present study, four neighbouring slopes of differing time since last ploughing were chosen. Species biomass was measured at different times during the annual growing season for two different slope positions.Grazing pressure is an important environmental factor affecting ecosystem organization, the most palatable plants tending to show increasing biomass with succession. In the most mature stages, there is a predominance of species characterized by horizontal occupation of space and sprouting after mowing or grazing.During succession segregation of the different morphological characteristics occurs in slope sectors related to geomorphological dynamics. Similarly, phenological development tends to be later in pastures in the lowest slope zones, due probably to their greater summer soil moisture content.Nomenclature follows: Tutin et al., 1964–1980. Flora Europaea.     

Plant response to variations in nitrogen availability in a desert shrubland community

Spatial variations in nitrogen availability were studied in a desert community codominated byLarrea tridentata (DC.) Cov. andProsopis glandulosa Torr. Measurements of natural ¹⁵N values in tissues suggested thatProsopis obtains approximately half of its nitrogen through direct symbiotic fixation. Soils were collected under 1)Prosopis shrubs, 2)Larrea shrubs 2 m fromProsopis (LP), and 3)Larrea 2 m from otherLarrea but> 5 m from the nearestProsopis (LL).Prosopis soils showed significantly higher rates of nitrogen mineralization than LL soils in both A and B horizons. Rates of mineralization in LP soils were significantly higher than rates in LL soils only in the B horizon and were not significantly different from rates inProsopis soils. Leaf nitrogen concentrations were significantly higher in LP shrubs (2.06%) than in LL shrubs (1.78%), although ¹⁵N values did not differ between the two shrub types. Nitrogen concentrations inPerezia nana Gray, a perennial herb, were greater in plants underProsopis shrubs (2.09%) than under LP shrubs (1.93%) or LL shrubs (1.67%). Despite apparent differences in nitrogen availability, biomass ofLarrea and density ofPerezia did not differ significantly among these sites.     

Vegetation change along gradients from water sources in three grazed Mongolian ecosystems

Foliar cover of plant species; grass, forb and total herbaceous biomass; soil P, K, N and C; and percent coarse fraction of soils were sampled over two years along grazing gradients from livestock water sources in three grazed Mongolian steppe ecosystems of varying productivity. Samples within each of the three systems (mountain-steppe, and desert-steppe) were classified into plant communities using TWINSPAN and species-environment relationships in each system were examined using CCA. Community classifications were driven by the presence/absence of ruderal species and highly palatable grasses in the steppe and mountain-steppe and by the presence/absence of salt-shrub or Caragana shrub species and associated Iris species in the desert-steppe. Ordinations were largely driven by soil nutrient concentrations, particularly P and K, in all three zones. Bulk density and percent coarse fraction to 10 cm were also important in the desert-steppe. Distance from water, which we assumed to be inversely related to grazing pressure, was a significant driving factor in steppe and mountain-steppe ordinations, and was negatively correlated with P and K. We speculate that elevated nutrient concentrations near water sources result from livestock redistributing nutrients in the landscape by voiding urine and feces in the areas where they congregate. Livestock may thus influence species composition in these systems both through the direct effects of defoliation and trampling, and the indirect effects of nutrient enrichment and depletion over the broader landscape. This hypothesis deserves further testing under controlled conditions.     

Spider biodiversity potential of an ungrazed and a grazed inland salt meadow in the National Park 'Neusiedler See-Seewinkel' (Austria): implications for management (Arachnida: Araneae)

To assess the biodiversity potential of an ungrazed and a grazed inland salt meadow in the Seewinkel (Eastern Austria), spider assemblages were recorded by pitfall trapping for 1 year. Both species assemblages consisted, to a large extent, of rare species of conservation interest. The species assemblage of the grazed site was dominated by Pardosa agrestis, but highly specific halotopobiontic species also occurred in higher numbers. Halotolerant species were also present in the ungrazed meadow, but their individual number was much lower. The species composition of this site reflects the more balanced microclimatical situation of the high sward. Comparison of the two assemblages with 207 other meadow spider assemblages of Central Europe shows a separated position, especially of the grazed site assemblage. High similarities with assemblages of meadows with a similar vegetation structure indicate a high importance of management. Considering the high proportion of rare species on both sites, the best management of the salt meadow and pan shores of the Seewinkel should combine areas of light grazing with ungrazed areas. However, the proportion of these parts and the intensity of grazing still remains to be determined by quantitative experiments.     

Foraging Behavior Patterns of Sheep and Horses Under a Mixed Species Grazing System

The research objective was to assess the behavior patterns of the Polish Konik horse breed and the Uhruska variety of the Polish Lowland Sheep breed under a mixed-grazing system, and their relationship with climatic factors. The observation included 4 adult horses, 27 ewes with lambs and 10 primiparous ewes. The behavior of the animals and the weather conditions were recorded at 60-min intervals. Horses and sheep displayed similarities in both species-specific behavioral patterns and timing of grazing activity, and the duration of foraging sessions was mostly influenced by time of day, lower temperature and relative humidity. In addition, the two species showed more interest in the watering place in the afternoon, and drinking frequency was mainly dependent on air temperature. The animals that pastured together gradually began to mix, but at first, they remained within their species group. Both animal species can be used for environmental protection and landscape conservation making rational use of pastures within protected areas.     

Comparison of biodiversity and ground cover between a commercial rotationally grazed property and an adjacent nature reserve in semi‐arid rangeland

Continuous livestock grazing can have negative effects on biodiversity and landscape function in arid and semi‐arid rangelands. Alternative grazing management practices, such as rotational grazing, may be a viable option for broad‐scale biodiversity conservation and sustainable pastoral management. This study compared ground cover, plant species composition and floristic and functional diversity along gradients of grazing intensity between a pastoral property rotationally grazed by goats and an adjacent nature reserve (ungrazed by commercial livestock) in semi‐arid south‐eastern Australia. Understorey plant species composition differed significantly between the rotationally grazed property and the nature reserve, with a greater proportion and frequency of palatable species recorded in the nature reserve. Understorey plant species richness, diversity, functional biodiversity measures and ground cover declined with increasing grazing pressure close to water points under commercial rotational grazing management. However, at a whole‐paddock scale, there were few differences in plant biodiversity and ground cover between the rotationally grazed property and the nature reserve, despite differences in overall plant species composition. Flexible, adaptive, rotational grazing should be investigated further for its potential to achieve both socio‐economic and biodiversity conservation outcomes in semi‐arid rangelands to complement existing conservation reserves.     

Soil-foraging animals alter the composition and co-occurrence of microbial communities in a desert shrubland

Animals that modify their physical environment by foraging in the soil can have dramatic effects on ecosystem functions and processes. We compared bacterial and fungal communities in the foraging pits created by bilbies and burrowing bettongs with undisturbed surface soils dominated by biocrusts. Bacterial communities were characterized by Actinobacteria and Alphaproteobacteria, and fungal communities by Lecanoromycetes and Archaeosporomycetes. The composition of bacterial or fungal communities was not observed to vary between loamy or sandy soils. There were no differences in richness of either bacterial or fungal operational taxonomic units (OTUs) in the soil of young or old foraging pits, or undisturbed soils. Although the bacterial assemblage did not vary among the three microsites, the composition of fungi in undisturbed soils was significantly different from that in old or young foraging pits. Network analysis indicated that a greater number of correlations between bacterial OTUs occurred in undisturbed soils and old pits, whereas a greater number of correlations between fungal OTUs occurred in undisturbed soils. Our study suggests that digging by soil-disturbing animals is likely to create successional shifts in soil microbial and fungal communities, leading to functional shifts associated with the decomposition of organic matter and the fixation of nitrogen. Given the primacy of organic matter decomposition in arid and semi-arid environments, the loss of native soil-foraging animals is likely to impair the ability of these systems to maintain key ecosystem processes such as the mineralization of nitrogen and the breakdown of organic matter, and to recover from disturbance.     

Ingestion and digestion of epilithic algae by larval insects in a heavily grazed montane stream

1. Epilithic algae grown on elevated or non-elevated ceramic tiles were exposed (to produce assemblages with different grazing histories) in a heavily grazed, montane stream in New Mexico, U.S.A. to Ameletus nymphs (Ephemeroptera) and Ecclisomyia larvae (Trichoptera) and the algal composition in insect faeces was compared to that on the tiles. Differences in grazing and digestion efficiency between grazers were then assessed and also differences in susceptibility to ingestion and digestibility among common algae. 2. Ordination of tile and faecal samples, using the relative abundance of common algae, revealed that: (i) algal assemblages on elevated vs. non-elevated tiles differed only slightly; (ii) the taxonomic composition of algae in faeces of both caddis and mayflies differed substantially from that on the tiles, indicating low grazing efficiency for some algal taxa; and (iii) the algal composition of faeces produced by caddis larvae and mayflies was similar, indicating little difference in grazing efficiency between them. However, some algal taxa were more susceptible to ingestion by caddisfly larvae when occurring on elevated tiles than on non-elevated tiles, suggesting that previous exposure to caddis grazing influenced assemblage attributes. 3. Although Ameletus and Ecclisomyia differed little in grazing efficiency, the percentage of diatoms that were dead after passage through the gut was greatest in the mayfly treatment, suggesting that mayflies digested diatoms more efficiently than the caddis. Analyses of differences in the condition of chloroplasts within diatoms in tile and faecal samples showed that losses of ’live‘ diatom cells (i.e. those containing full chloroplasts) during gut passage through mayflies equalled the increase, in faeces, of ’dead‘ (empty frustules) cells of all common diatoms. In contrast, some diatoms were digested inefficiently by caddis larvae. 4. Algae on elevated tiles contained a higher proportion of dead diatoms than those on non-elevated tiles, possibly because mayflies visited raised tiles more often and, consequently, ingested and defaecated cells at a higher rate in the absence of caddis larvae. Moreover, diatom taxa differed in the percentage of cells that were dead within tile assemblages, with populations of typically grazer-resistant taxa (e.g. Achnanthidium minutissimum, Planothidium lanceolatum and Cocconeis placentula var. euglypta) containing significantly more dead cells than grazer-susceptible taxa [e.g. small, chain-forming Fragilaria (= Staurosirella)]. This result suggests that a trade-off exists between ingestion vs. digestion resistance of microalgae. Both the ingestion and digestion efficiency of algivorous macroinvertebrates could influence the structure and function of algal assemblages. In heavily grazed systems, where algal cells are probably processed through grazer guts repeatedly, differential resistance to digestion among algae may be particularly important.     

The desert vegetation of El Pinacate,Sonora, Mexico

The Pinacate region is part of the Gran Desierto, one of the driest deserts in North America. The presence-absence of perennial plant species, together with soil and landform characteristics, were registered in 110 sampling sites within this region. A classification and ordination of plant communities showed soil and landform units to be good predictors of plant variation. Plant distribution and species richness were also strongly related to altitude and rockiness. A Generalised Linear Model was used to fit the response curves of individual species.The environmental factors related with plant distribution are indicators of the water regime within a given site. Plant communities repeat along topographic gradients the large-scale biogeographic variation of the Sonoran Desert. Microphyllous shrubs colonise the drier bajada slopes, while more diverse communities, dominated by cacti and drought-deciduous trees, grow on the wetter pediments and in pockets within rocky soils.     

Small mammal abundance and seed predation across boundaries in a restored‐grazed woodland interface

Passive restoration is an effective tool for the maintenance and conservation of biodiversity. Often areas in recovery are immersed in a matrix of land uses, in which the expansion and intensification of human activities exert new visible pressures at their boundaries. The degree of connectivity between these areas and their peripheral lands can be analyzed by mobile link species, organisms that actively move in the landscape by connecting areas to one another through their functional roles. We focus our design on the interface generated by the long‐term restoration area and surrounding grazing lands. We analyze the changes on boundary structure, small mammal abundance, and on the function of native seed dispersal by these vertebrate species. We captured small mammals and determined seed removal of Prosopis flexuosa at three distances inside and outside a fence that delineates passively restored and currently grazed areas. Our results indicate that small rodents find more suitable habitats at the site under restoration than in grazing lands. The restored‐grazing interface shows a decrease in small mammal abundance from the protected area to the grazed lands. From a functional perspective, an increase in small mammal abundance results in an increase in their seed removal activity with implications for seed fate, because the long‐term recovery of vegetation could enhance seed predation on a native tree species.     

Development of a new piosphere in arid chenopod shrubland grazed by sheep. 2. Changes to the vegetation

The initial development of a piosphere created by stocking a new watering trough with ca. 200 sheep at a near-pristine site in arid chenopod shrubland near Whyalla, South Australia, was studied. The following variables were monitored for up to 8 years using fenced plots as controls:shrub mortality; forage biomass and defoliation of the dominant shrubs and short-lived grasses; linear dimensions of Rhagodia ulicina; the density of short-lived species; and the invasion of exotics. Patterns in these variables were looked for which reflected the attenuation of stocking pressure with distance from water; that is, ‘piosphere patterns’. Piosphere patterns were detected for many variables. Those with values increasing towards the trough were mortality of the main forage shrub Atriplex vesicaria. the density of three short-lived taxa, viz. the bassias (Sclerolaena obliquicuspis, S. patenticuspis. Maireana sclerolaenoides), Tetragonia tetragonoides, and annual Zygophyllum spp. (Z. crenatum. Z. ovatum), and growth of short-lived grass species when sheep stocking pressure was low. Variables with values decreasing towards the trough were grass biomass, percentage of A. vesicaria individuals in flower, the percentage foliation of A. vesicaria and of the forage shrub Maireana sedifolia, and shrub forage biomass of A. vesicaria and M. sedifolia. The rate at which these patterns were registered varied. After only 3 months, a piosphere pattern was evident for grass biomass. Marrubium vulgare, an introduced weed, invaded the trough site within the first 6 months. Almost no A. vesicaria shrubs had died after 2 years, but after 8 years there was a marked piosphere pattern in their mortality. The piosphere patterns in forage biomass and defoliation of A. vesicaria and M. sedifolia became more pronounced with time. Some variables did not exhibit a piosphere pattern, such as the dimensions of the shrub R. ulicina and the densities of successive cohorts of the forb Erodium spp., even though there was an overall effect of grazing in reducing their values. There was no mortality of two dominant shrubs, M. sedifolia and R. ulicina, during the first 8 years.     

Impacts of long-term snow climate change on a high-elevation cold desert shrubland, California, USA

Ecological responses to 50-year old manipulations of snow depth and melt timing were assessed using snow fences arrayed across 50 km of a shrub–conifer landscape mosaic in eastern California, USA. We compared how increased, decreased, and ambient snow depth affected patterns of vegetation community composition, fire fuel accumulation, and annual tree ring growth. We also tested the effect of snow depth on soil carbon storage based on total C content under the two co-dominant shrub species (Artemisia tridentata and Purshia tridentata) in comparison with open, intershrub sites. Increased snow depth reduced the cover of the N-fixing shrub P. tridentata but not the water-redistributing shrub A. tridentata. Annual ring growth was greater on +snow plots and lower on ?snow plots for the conifer Pinus jeffreyi but not for Pinus contorta. Graminoid cover and aboveground biomass indicated higher fire fuel accumulation where snow depth was increased. Dead shrub stem biomass was greater regardless of whether snow depth was increased or decreased. Results demonstrate community shifts, altered tree growth, feedbacks on carbon storage, and altered fire fuel accumulation as a result of changes in snow depth and melt timing for this high-elevation, snow-dominated ecotone under future climate scenarios that envision increased or decreased snow depth.     

Refuge effects of a cactus in grazed short‐grass steppe

Abstract. Question: Which factors influence the effectiveness of biotic refuges for harbouring grazing‐sensitive species in pastures with a long history of grazing by large herbivores? Previous research showed that spiny clumps of the cactus Opuntia polyacantha provided refuges from cattle grazing for plants and for inflorescence production on short‐grass steppe. In this paper, seven factors that may have a potential positive influence on the refuge effects of cactus at a landscape scale were assessed. Location: Short‐grass steppe of the Great Plains of North America. Methods: The study was conducted in eight long‐term grazed pastures and their respective ungrazed controls that were established 60 years ago. Results: Heavy grazing intensities were necessary for some positive effects of cactus to manifest, and some refuge effects changed to negative effects under lower grazing pressure. Refuge effects increased with plant community productivity due to greater abundances of grazing‐sensitive species, and greater grazing intensities in the more productive areas. Cover of cactus cladodes (spine‐covered pads) inside clumps appeared to be the main limiting factor for refuge effects, probably by limiting available space for grazing‐sensitive species in the clumps. Other factors such as size and density of cactus clumps, and the presence of large refuges in the proximity of clumps had minor influence on the effectiveness of cactus refuges. Conclusions: The effects of biotic refuges largely varied with ecological conditions and structural characteristics of the refuge. Refuge effects were mainly influenced by grazing intensity, plant community productivity, and structural characteristics of the biotic refuges. A conceptual model of factors influencing refuge effects at a local landscape scale in plant communities grazed by large herbivores is presented.