Patterns of plant species turnover along grazing gradients

Questions: How are plant species distributed along grazing gradients? What is the shape of species richness patterns? How can we test for the existence of potential discontinuities in species turnover pattern? Location: Semi‐deserts in the eastern Caucasus, Azerbaijan, Gobustan district. Methods: We studied the distribution of vascular plant species along transects 900‐m long, perpendicular to five farms, and estimated grazing intensity as current livestock units per distance. We modelled species response curves with Huismann–Olff–Fresco (HOF) models and calculated species turnover by accumulating the first derivatives of all response curves. To test for potential discontinuities in changes of vegetation composition along the grazing gradient, we introduce a new null model based on the individualistic continuum concept that uses permutations of the observed pattern of species responses. Results: Most species show a sigmoidal negative response to grazing intensity, while a few species respond with a unimodal pattern. The monotonic decrease in species richness with increasing grazing intensity marks a process of overgrazing that leads to the complete extirpation of plant species. Although the species turnover pattern shows a clear peak, it does not deviate significantly from the null model of individualistic continuous changes. Conclusions: Our approach offers a method for differentiating between transition zones and continuous shifts in species composition along ecological gradients. It also provides a valuable tool for rangeland management to test state‐and‐transition concepts and gives deeper insights into ecological processes affected by grazing.     

Using ants to manage sustainable grazing: Dynamics of ant faunas along sheep grazing gradients conform to four global patterns

Ants are considered an important faunal group for the functioning of arid rangelands, they have a long history of use for environmental monitoring, and exhibit four global patterns in grazing lands: (i) soil and vegetation type are primary determinants of ant community composition, and have a far greater effect on ant community composition than grazing; (ii) grazing induces species compositional change, but does not necessarily affect species richness or abundance; (iii) a species response to grazing is not necessarily consistent across habitats; and (iv) approximately one‐quarter to one‐half of species that are common enough for statistical analysis have significant responses to grazing. Here we report the patterns of arid zone ant faunas as they exist after several decades of sheep grazing in southern Australia, and examine the extent to which they conform to the four global patterns. We measured ant faunas along grazing gradients (varying distance to water) in seven paddocks containing two soil and two vegetation types on five pastoral properties. Total site abundance and richness of ants did not differ significantly with distance from water, but the abundance of 10 (34%) of the 29 most common species did differ; three were increasers, three were decreasers, and four had mixed responses dependent on soil/vegetation type. Rare species showed no trend with grazing intensity. The ant fauna of the more structurally complex vegetation types appeared to be the most vulnerable to grazing effects. Multivariate analysis showed soil type was the primary factor influencing ant faunal composition, followed by vegetation structure; however, grazing treatment effects were present. This study fully supports the recently identified global patterns of ant responses to grazing. It also shows that sampling regional ant faunas using widely dispersed traps can detect ant faunal patterns comparable to studies that use smaller‐scale grids of traps.     

A fence-line contrast reveals effects of heavy grazing on plant diversity and community composition in Namaqualand,South Africa

Changes in plant species richness and community composition were investigated across a fence separating heavily grazed communal and lightly grazed commercial farming systems in Namaqualand, South Africa. No significant differences in plant species richness between communal and commercial farming systems were detected either locally within individual plots or overall across all plots. Within-plot, richness of species tolerant of grazing, such as annuals and geophytes, has increased, while the richness of large palatable shrub species has decreased on the communal rangeland. In terms of plant cover, species' responses to grazing were strongly associated with growth form. Annuals and geophytes formed the majority of grazing increasers, while large, presumably palatable, shrubs and leaf succulents were characteristic grazing decreasers. An investigation into population processes of five shrub species revealed that heavy grazing on the communal rangeland has resulted in: reduced size of palatable shrub species; reduced flower production and seedling recruitment of palatable species; increased density and recruitment of the unpalatable shrub, Galenia africana. Reductions in shrub volume, reproductive output and seedling recruitment were most marked in the palatable shrub Osteospermum sinuatum and were in the order of 90%. The results are further discussed in terms of their relevance to rangeland dynamics and the current land use practices of the region.     

Does the intensive grazing and aridity change the relations between the dominant shrub Artemisia kopetdaghensis and plants under its canopies?

The interspecific plant interactions along grazing and aridity stress gradients represent a major research issue in plant ecology. However, the combined effects of these two factors on plant–plant interactions have been poorly studied in the northeast of Iran. To fill this knowledge gap, 144 plots were established in 12 study sites with different grazing intensities (high vs. low) and climatic characteristics (arid vs. semiarid) in northeastern Iran. A dominant shrub, Artemisia kopetdaghensis, was selected as the model species. Further, we studied changes in plant life strategies along the combined grazing and aridity stress gradients. In this study, we used relative interaction indices calculated for species richness, Shannon diversity, and species cover to determine plant–plant interactions using linear mixed‐effect models (LMM). The indicator species analysis was used to identify the indicator species for the undercanopy of shrub and for the adjacent open areas. The combined effects of grazing and aridity affected the plant–plant interactions and plant life strategies (CSR) of indicator species. A. kopetdaghensis showed the highest facilitation effect under high stress conditions (high grazing, high aridity), which turned into competition under the low stress conditions (low grazing, low aridity). In the arid region, the canopy of the shrub protected ruderals, annual forbs, and grasses in both high and low grazing intensities. In the semiarid region and high grazing intensity (low aridity/high grazing), the shrubs protected mostly perennial forbs with C‐strategy. Our findings highlight the importance of context‐dependent shrub management to restore the vegetation damaged by the intensive grazing.     

Species response curves along environmental gradients. A case study from SE Norwegian swamp forests

Abstract. Vegetation science has relied on untested paradigms relating to the shape of species response curves along environmental gradients. To advance in this field, we used the HOF approach to model response curves for 112 plant species along six environmental gradients and three ecoclines (as represented by DCA ordination axes) in SE Norwegian swamp forests. Response curve properties were summarized in three binary response variables: (1) model unimodal or monotonous (determinate) vs. indeterminate; (2) for determinate models, unimodal vs. monotonous and (3) for unimodal models, skewed vs. symmetric. We used logistic regression to test the influence, singly and jointly, of seven predictor variables on each of three response variables. Predictor variables included gradient type (environmental or ecocline) and length (compositional turnover); species category (vascular plant, moss, Sphagnum or hepatic), species frequency and richness, tolerance (the fraction of the gradient along which the species occurs) and position of species along each gradient. The probability for fitting a determinate model increased as the main occurrence of species approached gradient extremes and with increasing species tolerance and frequency and gradient length. Appearance of unimodal models was favoured by low species tolerance and disfavoured by closeness of species to gradient extremes. Appearance of skewed models was weakly related to predictors but was slightly favoured by species optima near gradient extremes. Contrary to the results of previous studies, species category, gradient type and variation in species richness along gradients did not contribute independently to model prediction. The overall best predictors of response curve shape were position along the gradient (relative to extremes) and tolerance; the latter also expressing gradient length in units of compositional turnover. This helps predicting species responses to gradients from gradient specific species properties. The low proportion of skewed response curves and the large variation of species response curves along all gradients indicate that skewed response curves is a smaller problem for the performance of ordination methods than often claimed. We find no evidence that DCA ordination increases the unimodality, or symmetry, of species response curves more than expected from the higher compositional turnover along ordination axes. Thus ordination axes may be appropriate proxies for ecoclines, applicable for use in species response modelling.     

Does cattle dung cause differences between grazing increaser and decreaser germination response?

Although grassland species are usually classified as grazing increasers or decreasers, the response mechanisms to herbivore action in these two groups have yet to be clarified. One such action is dung deposition. The present study tested the hypothesis that increaser species perform more efficiently than decreasers during germination and/or establishment in the presence of dung leachates. A phytotron experiment was conducted on fourteen Mediterranean grassland species from five families, with an equal number of increaser and decreaser species chosen for each family to control for phylogeny. Seeds were germinated and grown in different concentrations of cattle dung leachate. We monitored germination percentages, median germination time and root length, measured five days after germination. To analyse the response and identify potential thresholds, we fitted piecewise mixed models using family/species as nested random factors. Differences in germination parameters between the two groups peaked in favour of increasers under intermediate leachate concentrations. Root growth was also negatively affected by leachates in decreaser species and promoted in increasers. Livestock dung deposition can increase the germination and colonising performance of increasers over decreasers, and may therefore be one of the mechanisms that explain the different species composition of grazed and ungrazed environments.     

Fungal-mediated mortality explains the different effects of dung leachates on the germination response of grazing increaser and decreaser species

Depending on their response to grazing, grassland species can be categorized as grazing increasers or decreasers. Grazing by livestock includes several different activities that can impact species differently. Recent evidence suggest that one of these actions, dung deposition, can reduce the germinative performance of decreaser species, thus favouring increasers. The present study tested the hypothesis that decreased germinative success of decreaser species is caused by a greater activity of fungal pathogens under the influence of dung leachates. We performed a phytotron experiment analysing the germination and fungal infections of fourteen species from Mediterranean grasslands. Species were grouped into phylogenetically-related pairs, composed of an increaser and a decreaser species. Seeds of each species were germinated under four different treatments (control, dung leachate addition, fungicide addition and dung leachate and fungicide addition), and the differences in germination percentage, germination speed and infection rate between each increaser species and its decreaser counterpart were analysed. Decreaser species were more affected by mortality than increaser ones, and these differences were higher under the presence of dung leachates. The differences in germinative performance after excluding the effect of seed mortality did not differ between treatments, showing that the main mechanism by which dung leachates favour increaser species is through increased mortality of the seeds of decreaser species. Drastic reductions in the number of dead seeds in the treatments including fungicide addition further revealed that fungal pathogens are responsible for these differences between species with different grazing response. The different vulnerabilities of increaser and decreaser species to the increased activity of fungal pathogens under the presence of dung leachates seems the main reason behind the differential effect of these leachates on species with different grazing response.     

Changes in semi-arid plant species associations along a livestock grazing gradient

In semi-arid ecosystems, vegetation is heterogeneously distributed, with plant species often associating in patches. These associations between species are not constant, but depend on the particular response of each species to environmental factors. Here, we investigated how plant species associations change in response to livestock grazing in a semi-arid ecosystem, Cabo de Gata-Níjar Natural Park in South East Spain. We established linear point-intercept transects at four sites with different grazing intensity, and recorded all species at each point. We investigated plant associations by comparing the number of times that each pair of species occurred at the same spatial point (co-occurrences), with the expected number of times based on species abundances. We also assessed associations for each shrub and grass species by considering all their pairs of associations and for the whole plant community by considering all pairs of associations on each site. At all sites, the plant community had a negative pattern of association, with fewer co-occurrences than expected. Negative association in the plant community increased at maximum grazing intensity. Most species associated as expected, particularly grass species, and positive associations were most important at intermediate grazing intensities. No species changed its type of association along the grazing gradient. We conclude that in the present plant community, grazing-resistant species compete among themselves and segregate in space. Some shrub species act as refuges for grazing-sensitive species that benefit from being spatially associated with shrub species, particularly at intermediate grazing intensities where positive associations were highest. At high grazing intensity, these shrubs can no longer persist and positive associations decrease due to the disappearance of refuges. Spatial associations between plant species and their response to grazing help identify the factors that organize plant communities, and may contribute to improving management of semi-arid ecosystems.     

pH optima for Danish forest species compared with Ellenberg reaction values

Species distribution depends on the physiological and ecological niche where a species can exist and regenerate in resource competition with other species (niche limitation). The realized niche is influenced by local biotic processes that influence species behaviour and the shape of the response curves relative to environmental gradients. Processes on larger scales also influence the species niche through source-sink mechanisms (dispersal limitation) and the species richness of an area (pool limitation). Despite the growing evidence of skewed or irregular species response curves along gradients, many ecologists still assume symmetric, unimodal response curves along gradients in ecological interpretation. Ellenberg’s indicator system is probably the most common example. However, the assumption is not ecologically or statistically valid, due to the many different processes affecting the distribution of plant species. Here I present the results of Huisman-Olff-Fresco (HOF) regressions for 209 Danish forest species. HOF modelling is chosen to avoid the classical drawbacks of assuming symmetric, unimodal response patterns. I calculate the optima for all species with unimodal responses to soil pH and compare these with the Ellenberg indicator values for reaction (R), which are often used as a substitute for soil pH measurements. I demonstrate that the assumption of symmetric, unimodal species behaviour is violated in 54% of the cases and that pH optima and R indicator values for species are not always compatible. Ellenberg reaction scale has been used byEwald (Folia Geobot. 38: 357–366, 2003) as an indicator of which species are calcicole, i.e., whether they can grow and reproduce on calcareous soils. Such affinities of species, however, are related to both local niche properties and processes on large scales and cannot be generalized from a single empirical variable such as pH, nor from Ellenberg semi-ordinal indicator scale. I conclude that while the determination of whether species are calcicole or calcifuge requires more research, it is evident that Denmark contains a fairly balanced number of calciphytic and acidophytic species. This is probably due to the nearly equal areas with acidic and alkaline soils in Denmark, which also contribute to the high species richness of more than 500 vascular plant species in Danish forests.     

Grazing impacts on the diversity and composition of alpine rangelands in Northwest Yunnan

Aims The eastern Himalayan region of Southwest China represents the world's most biodiverse temperate region as well as a cultural hotspot undergoing rapid cultural and ecological change. This area represents the center of origin for many plant groups including horticulturally valuable species of Pedicularis, Rhododendron and Primula. Alpine meadows here also provide summer pastures for Tibetan yak herders and the source for important medicinal plants. Stocking levels for livestock here have quadrupled over the last five decades and shrubs are encroaching into many historical rangelands. Yak herders voice concerns over both shrub encroachment and shrinking grasslands. In this study, we sought to determine: (i) Are alpine rangelands in Deqin County overgrazed and degraded? (ii) What are the local impacts of grazing on plant diversity and community composition? And (iii) which environmental variables covary with these differences in species composition across the grazing gradient?Methods To examine the ecological impacts of grazing in Deqin County, northwest Yunnan and assess its long-term sustainability, we used county records to determine historical population and livestock numbers, confirmed the results of interviews by Haynes (2011) and surveyed plant species richness and cover along a grazing gradient extending away from herder huts. Along these transects, we sampled 1?m 2 quadrats at 5 m intervals, noting species present and percent cover for vascular plants, grasses, sedges, rushes, moss, lichen, exposed rock, bare ground and feces. We also measured the average and maximum plant height within each quadrat. We then computed species richness for each of the 38 transects, calculating alpha and beta diversities. We used one-way ANOVAs to compare mean species richness values and average and maximum plant height across grazing intensities. To chart changes in composition along the 100m gradient, we also plotted the percent cover of graminoid, forb, shrub and bare ground versus distance from the hut. We applied NMS ordination to relate community patterns to environmental variables and grazing intensity using distances to determine species groupings.Important findings Livestock impacts are clearly evident with proportions of grass and bare ground decreasing, shrubs increasing and forbs maintaining even cover with increasing distance from the huts. In comparison with earlier surveys of sites farther from huts, we found reduced plant cover and diversity. Plant species richness almost doubles with increasing distance from herder huts from 9.9 to 19.3 species per 1 m 2 quadrat. An ordination of species and environmental variables demonstrates that grazing strongly affects plant community com position across these plots with strong impacts on palatable plants. If herd sizes remain large and suitable areas for grazing continue to decline, the cumulative impacts of grazing appear likely to degrade the rich diversity of the region and reduce rangeland quality, threatening its ability to sustain current grazing levels.     

Plant–community responses to shrub cover in a páramo grassland released from grazing and burning

Shrub encroachment can follow grazing or burning release in páramo grasslands. While encroachment decreases herbaceous species richness in some grassland systems, the effects of this process on the herbaceous community in páramo grasslands are currently unknown. We collected data on shrub cover, herbaceous‐species cover and species composition in a páramo grassland 12 years after release from burning and cattle grazing near Zuleta, Ecuador. Topographic and soil measures were also included as predictor variables of differences in community composition. Contrary to studies in other systems, shrub cover did not have a significant effect on herbaceous‐species richness, whereas shrub‐species richness significantly increased with shrub cover. However, shrub cover was associated with significant shifts in herbaceous–community composition. Most notably, there was an increase in some shade‐tolerant forbs and tall‐statured wetland grasses with increasing shrub cover, and a corresponding decrease in some short‐statured grasses and early successional forbs. These results could indicate that the ameliorative effects of shrubs (e.g. frost and wind protection) in harsh alpine environments may partially compensate for the expected competitive effect of shrubs due to shading.     

Ecological stoichiometry explains larger-scale facilitation processes by shrubs on species coexistence among understory plants

Plant facilitation (positive plant–plant interactions) strongly influences biodiversity, structure, and dynamics in plant communities, and the topic has received considerable attention among ecologists. Most studies of facilitation processes by shrubs have been conducted at small spatial scales between shrubs and their neighboring species. Yet, we know little about whether facilitation processes by shrubs at a small scale (i.e., a patch scale) also work at a larger scale (i.e., a site scale) in terms of the maintenance of biodiversity. Here, we report that the facilitative effects of shrubs on plant diversity at a larger scale can be explained by changing ecological stoichiometry. The soil fertility showed unimodal shape along shrub cover gradient, suggesting that the facilitative effects of a shrub do not necessarily increase as the shrub develops. The unimodal shape of dependence of plant species richness on shrub cover probably was generated by the unimodal dependence of soil fertility on shrub cover. Soil nutrient enrichment by shrubs shifted low N:P ratios of plant communities with low levels of shrub cover to more balanced N:P ratios at intermediate levels of shrub cover. At the peak N:P ratio along the gradient in shrub cover, the maximum species richness and functional richness were observed, which was consistent with the unimodal relationship predicted by the resource balance hypothesis. Thus, our findings showed that facilitation processes by shrubs at a patch scale also work at a larger scale in terms of the maintenance of biodiversity. Because observed larger-scale facilitation processes are enhanced at some intermediate levels of shrub cover, this study offers practical insight into the need for management practices that allow some intermediate levels of grazing by livestock for optimizing the role of larger-scale facilitation processes in the maintenance of biodiversity and ecosystem functioning in arid and semi-arid rangelands.     

Food selection by the guanaco (Lama guanicoe) along an altitudinal gradient in the Southern Andean Precordillera (Argentina)

Wild ungulates like the guanaco are exposed to important changes in climate and plant diversity along altitudinal gradients in the Andes Mountains, such as in the Southern Andean Precordillera where three phytogeographic provinces are present in altitudinal belts. The guanaco’s diet and food availability were seasonally analyzed using microhistological analysis and point-quadrat transects at six sampling sites, representative of the phytogeographic belts along the altitudinal gradient. Plant cover and diversity decreased with growing altitude. Richness of plant species was poorer at the summit than in the lower altitudes, whereas the proportion of species eaten by guanacos increased with altitude. The diet included 77 species. Grasses were preferred and shrubs were avoided all year round. The grass Poa spp. occupied more than 50 % of the diet at all altitudes. Grasses were the main dietary item even at low altitudes, where shrubs constituted the main food available. Decreasing generalism with descending phytogeographic belts agrees with the prediction for altitudinal gradients. The increase of diversity in the diet during the winter decline of plant cover at high and middle altitudes follows that expected from the optimal foraging theory. The winter decline of vegetation and the dietary shift from grazing to browsing proved to be stronger as altitude increases and the climate become more rigorous. Plant species richness, food scarcity, and climate severity are relevant variables to explain altitudinal and seasonal changes in the diet of adaptive ungulates in mountain environments, such as the guanaco in the Southern Andean Precordillera.     

Explaining patterns of primary production from individual level traits

Question: Do species traits explain differences in productivity in grazed and ungrazed plots? Location: Río de la Plata grasslands, Uruguay (31°54′S, 58°15′W). Methods: In a greenhouse experiment, we measured the relative growth rate (RGR) of grasses with contrasting responses to grazing (increasers and decreasers). We performed six harvests at weekly intervals in order to calculate the RGR and assess 12 plant traits. We compared the RGR between increaser and decreaser species after 2 and 5 weeks using t‐tests. Linear and potential regression models were fitted to time versus natural logarithm of total dry biomass relationships. The RGR temporal trajectories of increaser and decreaser species were obtained from the derivatives of the best‐fit functions. Principal component analysis (PCA) was used to sort species according to their traits. Results: The RGR of decreaser grasses was higher than that of increasers at the second week, while at the fifth week the opposite was observed. The RGR of decreasers dropped through time, while the RGR of increaser species was constant. The PCA separated increaser from decreaser species. The attributes related to increaser species were: high specific leaf area, tillering rate, green leaf rate, total leaf number, root weight ratio and leaf weight ratio; while those associated with decreaser species were: high dead biomass, senescence rate, reproductive biomass, leaf elongation rate and total biomass. Conclusions: Traits possessed by decreasers reduce light availability and increase the reproductive investment, explaining the drop in RGR. Specific differences in RGR seem to scale up to the ecosystem level and would explain the pattern in aboveground net primary production observed in the field under contrasting grazing regimes.     

Moderate grazing promotes the root biomass in Kobresia meadow on the northern Qinghai–Tibet Plateau

Grazing is an important modulator of both plant productivity and biodiversity in grassland community, yet how to determine a suitable grazing intensity in alpine grassland is still controversy. Here, we explore the effects of different grazing intensities on plant biomass and species composition, both at community level and functional group level, and examines the productivity–species richness relationship under four grazing patterns: no grazing (CK), light grazing (LG), moderate grazing, (MG) and heavy grazing (HG), attempt to determine a suitable grazing intensity in alpine grassland. The results were as follows. The total aboveground biomass (AGB) reduced with increasing grazing intensity, and the response of plant functional groups was different. AGB of both sedges and legumes increased from MG to HG, while the AGB of forbs reduced sharply and the grass AGB remained steady. There was a significant positive relationship between productivity and species richness both at community level and functional group level. In contrast, the belowground biomass (BGB) showed a unimodal relationship from CK to HG, peaking in MG (8,297.72 ± 621.29 g/m²). Interestingly, the grassland community tends to allocate more root biomass to the upper soil layer under increasing grazing intensities. Our results suggesting that moderate levels of disturbance may be the optimal grassland management strategy for alpine meadow in terms of root production.     

Multi-scale responses of vegetation to removal of horse grazing from Great Basin (USA) mountain ranges

Although free-roaming equids occur on all of the world’s continents except Antarctica, very few studies (and none in the Great Basin, USA) have either investigated their grazing effects on vegetation at more than one spatial scale or compared characteristics of areas from which grazing has been removed to those of currently grazed areas. We compared characteristics of vegetation at 19 sites in nine mountain ranges of the western Great Basin; sites were either grazed by feral horses (Equus caballus) or had had horses removed for the last 10–14 years. We selected horse-occupied and horse-removed sites with similar aspect, slope, fire history, grazing pressure by cattle (minimal to none), and dominant vegetation (Artemisia tridentata). During 1997 and 1998, line-intercept transects randomly located within sites revealed that horse-removed sites exhibited 1.1−1.9 times greater shrub cover, 1.2–1.5 times greater total plant cover, 2–12 species greater plant species richness, and 1.9–2.9 times greater cover and 1.1–2.4 times greater frequency of native grasses than did horse-occupied sites. In contrast, sites with horses tended to have more grazing-resistant forbs and exotic plants. Direction and magnitude of landscape-scale results were corroborated by smaller-scale comparisons within horse-occupied sites of horse-trail transects and (randomly located) transects that characterized overall site conditions. Information-theoretic analyses that incorporated various subsets of abiotic variables suggested that presence of horses was generally a strong determinant of those vegetation-related variables that differed significantly between treatments, especially frequency and cover of grasses, but also species richness and shrub cover and frequency. In contrast, abiotic variables such as precipitation, site elevation, and soil erodibility best predicted characteristics such as forb cover, shrub frequency, and continuity of the shrub canopy. We found species richness of plants monotonically decreased across sites as grazing disturbance increased, suggesting that either the bell-shaped diversity-disturbance curve of the intermediate-disturbance hypothesis does not apply in this system or that most sites are already all on the greater-disturbance slope of the curve. In our study, numerous vegetation properties of less-grazed areas and sites differed notably from horse-grazed sites at local and landscape scales during a wetter and an average-precipitation year. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Restoration of grazing management and its effect on vegetation in an upland grassland

Question: How are plant species and functional group composition, and potential sward height affected by implementation of different grazing regimes on previously abandoned semi-natural grassland? Location: The Jizerské mountains, northern Czech Republic. Methods: We established a randomized block experiment with the following treatments: unmanaged control (U), intensive (IG) and extensive (EG) continuous grazing, first cut followed by intensive (ICG) and first cut followed by extensive (ECG) continuous grazing for the rest of the growing season. The percentage cover of all vascular plant species was recorded in 40 permanent plots. Results: Total plant species richness increased in all managed treatments, whereas species number was reduced in U at the end of the experiment. Tall forbs (Aegopodium podagraria, Galium album, Anthriscus sylvestris, Cirsium arvense) as well as tall grasses (Elytrigia repens and Alopecurus pratensis) were more abundant in U. Species associated with both grazing treatments (IG, EG) were Dactylis glomerata, Festuca rubra agg. and Phleum pratense. Agrostis capillaris, Taraxacum spp., Trifolium repens, Ranunculus repens and Cirsium vulgare were promoted by ECG and ICG. Abundance of tall grasses and tall forbs reflected the intensity of management in the order U>EG, ECG>IG, ICG. Prostrate forbs, on the other hand, increased their cover with increasing intensity: ICG>IG>ECG>EG. Conclusions: Plant species composition of semi-natural grasslands is affected by the defoliation regime. Continuous grazing on abandoned grassland alters the sward structure towards a permanent pasture with short, light-sensitive grasses and prostrate forbs. To maintain or enhance plant species richness in semi-natural grasslands, understanding the effects of different grazing regimes on plant species composition is necessary.     

Hippopotamus and livestock grazing: influences on riparian vegetation and facilitation of other herbivores in the Mara Region of Kenya

Riparian savanna habitats grazed by hippopotamus or livestock experience seasonal ecological stresses through the depletion of herbaceous vegetation, and are often points of contacts and conflicts between herbivores, humans and their livestock. We investigated how hippopotamus and livestock grazing influence vegetation structure and cover and facilitate other wild herbivores in the Mara region of Kenya. We used 5 km-long transects, each with 13 plots measuring 10 × 10 m², and which radiate from rivers in the Masai Mara National Reserve and adjoining community pastoral ranches. For each plot, we measured the height and visually estimated the percent cover of grasses, forbs, shrubs and bare ground, herbivore abundance and species richness. Our results showed that grass height was shortest closest to rivers in both landscapes, increased with increasing distance from rivers in the reserve, but was uniformly short in the pastoral ranches. Shifting mosaics of short grass lawns interspersed with patches of medium to tall grasses occurred within 2.5 km of the rivers in the reserve in areas grazed habitually by hippos. Hence, hippo grazing enhanced the structural heterogeneity of vegetation but livestock grazing had a homogenizing effect in the pastoral ranches. The distribution of biomass and the species richness of other ungulates with distance from rivers followed a quadratic pattern in the reserve, suggesting that hippopotamus grazing attracted more herbivores to the vegetation patches at intermediate distances from rivers in the reserve. However, the distribution of biomass and the species richness of other ungulates followed a linear pattern in the pastoral ranches, implying that herbivores avoided areas grazed heavily by livestock in the pastoral ranches, especially near rivers.     

·荒漠草地植物多样性对草食动物采食的响应机制

草食动物采食对草地植物多样性和生态系统功能的影响机制是放牧生态学研究的核心问题。该研究以内蒙古锡林郭勒盟苏尼特右旗荒漠草原的长期放牧控制实验为平台, 从既有草地植物多样性和动物偏食性两个层面系统地研究了荒漠草地植物多样性对草食动物采食的响应机制。结果显示: 1)荒漠草地植物对草食动物采食呈现4种响应模式: 放牧“隐没种”、放牧“敏感种”、放牧“无感种”、“绝对优势种”; 2)在群落尺度上, 物种多样性指数随放牧强度增加而减少, 与不放牧小区相比, 重度放牧(HG)与适度放牧(MG)小区植物多样性均下降, 且这一规律同样适用于功能群多样性, 灌木半灌木这一功能群内物种多样性对放牧干扰较敏感; 3)在草地既有植物的基础上, 以不放牧小区为参考系, 草食动物对植物功能群偏食性的排序为: 一二年生草本(AB) >多年生杂类草(PF) >灌木半灌木(SS) >多年生禾草(PG), 且偏食性物种主要分布于AB和PF中; 4)植物多样性与动物偏食性基本呈显著负相关关系(p < 0.05)。     

Variation in grazing tolerance among three tallgrass prairie plant species

Three tallgrass prairie plant species, two common perennial forbs (Artemisia ludoviciana and Aster ericoides [Asteraceae]) and a dominant C(4) perennial grass (Sorghastrum nutans) were studied under field and greenhouse conditions to evaluate interspecific variation in grazing tolerance (compensatory growth capacity). Adaptation to ungulate grazing was also assessed by comparing defoliation responses of plants from populations with a 25-yr history of no grazing or moderate ungulate grazing. Under field conditions, all three species showed significant reductions in shoot relative growth rates (RGR), biomass, and reproduction with defoliation. In the two forbs, clipping resulted in negative shoot RGR and reductions in both number and length of shoot branches per ramet. Sorghastrum nutans maintained positive RGR under defoliation due to a compensatory increase in leaf production. Defoliation reduced rhizome production in A. ericoides and S. nutans, but not in A. ludoviciana. Clipping significantly reduced sexual reproductive allocation in all three species, although S. nutans showed a smaller reduction than the forbs. All three species showed similar responses to defoliation in burned and unburned sites. Under greenhouse conditions, a similar clipping regimen resulted in smaller reductions in growth and reproduction than those observed in the field. For all three species, the grazing tolerance indices calculated under natural field conditions were significantly lower than those estimated from greenhouse-grown plants, and the interspecific patterns of grazing tolerance were different. Aster ericoides exhibited the highest overall defoliation tolerance under greenhouse conditions, followed by S. nutans. Artemisia ludoviciana, the only study species that is typically not grazed by ungulates in the field, showed the lowest grazing tolerance. In the field experiment S. nutans showed the highest grazing tolerance and the two forbs had similar low tolerance indices. These patterns indicate that, despite high compensatory growth potential, limited resource availability and competition in the field significantly reduce the degree of compensation and alter interspecific differences in grazing tolerance among prairie plants. In all three species, defoliation suppressed sexual reproduction more than growth or vegetative reproduction. Significant interactions between plant responses to defoliation and site of origin (historically grazed or ungrazed sites) for some response variables (root/shoot ratios, rhizome bud initiation, and reproductive allocation) indicated some degree of population differentiation and genetic adaptation in response to a relatively short history of ungulate grazing pressure. The results of this study indicate that patterns of grazing tolerance in tallgrass prairie are both genetically based and also environmentally dependent.