Ecosystem engineering by foxes is mediated by the landscape context—A case study from steppic burial mounds

In intensively used landscapes, remnant grassland fragments are often restricted to places unsuitable for agricultural cultivation. Such refuges are the ancient burial mounds called “kurgans,” which are typical landscape elements of the Eurasian steppe and forest steppe zone. Due to their hill‐like shape, loose soil structure and undisturbed status kurgans provide proper habitats for burrowing mammals. Accordingly, grassland vegetation on kurgans is often exposed to bioturbation, which can influence the habitat structure and plant species pool. In our study, we explored the effect of fox burrows and landscape context on the habitat properties and vegetation composition of small landscape elements, using kurgans as model habitats. We surveyed the vegetation of fox burrows and that of the surrounding grassland on five kurgans situated in cleared landscapes surrounded by arable lands and five kurgans in complex landscapes surrounded by grazed grasslands. We recorded the percentage cover of vascular plants, the amount of litter, and soil moisture content in twelve 0.5 m × 0.5 m plots per kurgan, in a total of 120 plots. We found that foxes considerably transformed habitat conditions and created microhabitats by changing the soil nutrient availability and reducing total vegetation cover and litter. Several grassland specialist species, mostly grasses (Agropyron cristatum, Elymus hispidus, and Stipa capillata) established in the newly created microhabitats, although the cover of noxious species was also considerable. We found that landscape context influenced the sort of species which could establish on kurgans by affecting the available species pool and soil moisture. Our results revealed that foxes act as ecosystem engineers on kurgans by transforming abiotic and biotic conditions by burrowing. Their engineering activity maintains disturbance‐dependent components of dry grasslands and increases local environmental heterogeneity.     

Isolation and patch size drive specialist plant species density within steppe islands: a case study of kurgans in southern Ukraine

The fragmentation and isolation of preserved habitat patches within intensive agricultural landscape threaten species survival in steppes. In Ukraine, <5 % of the original steppe area has survived, mostly in nature reserves, loess ravines and kurgans (burial mounds). The latter are small, but relatively numerous objects. The aim of this study was to investigate whether plant species density (= the number of species per plot) within small steppe patches on kurgans differs from that within large steppe enclaves in southern Ukraine. Another aim was to determine the influence of patch area, isolation and heat load index on species density on kurgans. Presence-absence data for vascular plant species were collected on plots of a size of 100 and 1 m². Thirty kurgans and ten sites within large steppe enclaves were sampled. Every noted species was classified as specialist or generalist. For kurgans data on area and degree of isolation were collected. Additionally the heat load index for plots on kurgans was calculated. The density of habitat specialists was higher in larger steppe enclaves than on kurgans at both spatial scales. On kurgans the most important predictors of specialist density were isolation (negative) and kurgan area (positive). The density of generalists was positively affected by the heat load index. Kurgans can provide refuges for steppe plant species. However, our results show that over a longer time period, proximity to larger steppe refuges is important to maintain species diversity within these small patches.     

Cultural monuments and nature conservation: a review of the role of kurgans in the conservation and restoration of steppe vegetation

Steppe is among the most endangered biomes of the world, especially in Eastern Europe, where more than 90 % of original steppes have been destroyed due to conversion into croplands, afforestation and other human activities. Currently, steppe vegetation is often restricted to places unsuitable for ploughing, such as ancient burial mounds called kurgans. The aim of our study was to collect and synthesise existing knowledge on kurgans by a review of research papers and grey literature. The proportion of kurgans covered by steppe vegetation increases from west to east and from lowlands to uplands. Despite their small size, kurgans act as biodiversity hotspots and harbour many red-listed species. High overall species richness and a high proportion of grassland specialists are maintained by a pronounced fine-scale environmental heterogeneity. The main factors threatening the biodiversity of kurgans are intensified agriculture and construction works. We conclude that kurgans can play a crucial role in preserving steppe vegetation, especially in intensively used agricultural landscapes in the western part of the steppe zone. Despite the vital role of kurgans in sustaining steppe vegetation, we identified serious knowledge gaps on their distribution, vegetation, flora and fauna and their potential role in steppe restoration.     

The value of small habitat islands for the conservation of genetic variability in a steppe grass species

The habitat loss and fragmentation due to agricultural land-conversion affected the steppe throughout its range. In Ukraine, 95% of steppe was destroyed in the last two centuries. Remaining populations are confined to few refuges, like nature reserves, loess ravines, and kurgans (small burial mounds), the latter being often subject to destruction by archeological excavations.Stipa capillata L. is a typical grass species of Eurasian steppes and extrazonal dry grasslands, that was previously used as a model species in studies on steppe ecology. The aim of our research was to assess genetic diversity of S. capillata populations within different types of steppe refuges (loess ravines, biosphere reserve, kurgan) and to evaluate the value of the latter group for the preservation of genetic diversity in the study species.We assessed genetic diversity of 266 individuals from 15 populations (nine from kurgans, three from loess ravines and three from Askania-Nova Biosphere Reserve) with eight Universal Rice Primers (URPs).Studied populations showed high intra-population variability (I: 0.262–0.419, PPB: 52.08–82.64%). Populations from kurgans showed higher genetic differentiation (Φ_ST = 0.247) than those from loess ravines (Φ_ST = 0.120) and the biosphere reserve (Φ_ST = 0.142). Although the diversity metrics were to a small extent lower for populations from kurgans than from larger refugia we conclude that all studied populations of the species still preserve high genetic variability and are valuable for protection. To what extent this pattern holds true under continuous fragmentation in the future must be carefully monitored.     

High genetic diversity in fragmented Iris pumila L. populations in Ukrainian steppe enclaves

Habitat fragmentation can prevent gene flow between plant populations and lead to a loss of genetic diversity. However, such impact of fragmentation has not always been consistently confirmed by previous studies and the issue still needs further research. Particularly little is known about the impact of fragmentation on steppe plants. Steppe once covered vast, continuous areas, and nowadays is among the most fragmented biomes. In Ukraine, remnants of this habitat survived in large but few nature reserves and loess ravines as well as on kurgans (burial mounds of ancient nomadic people), which, despite their small size, are still numerous and scattered throughout the landscape.We studied the impact of fragmentation on the genetic diversity and structure of Iris pumila, a typical species of European steppes. Our main focus was to compare the genetic characteristics between kurgan populations (n = 8), and populations from larger refugia (n = 6). We assessed the genetic diversity of the studied populations with Universal Rice Primers.Our analyses revealed high genetic diversity across all investigated populations (mean He: 0.233; mean PPB: 58.57). However in kurgan populations genetic diversity was significantly higher than in larger refugia. Genetic diversity (He) was negatively correlated with population size. Most of the molecular variance (82%) was represented within populations, whereas genetic differentiation among populations was moderate (Φ_ST = 0.160), and low among refugia types (Φ_RT = 0.026).The maintenance of high genetic diversity despite two centuries of fragmentation may be related to the moderate disturbance occurring on kurgans, which enhances the sexual reproduction of the species. Moreover, we assume that species traits such as longevity and polyploidy might counterbalance genetic drift, while its self-incompatibility and presence of a soil seed bank allows for the replenishment of the gene pool. Overall, our results suggest that kurgans can protect genetic diversity of steppe species.     

The impact of grazing intensity on soil characteristics of Stipa grandis and Stipa bungeana steppe in northern China (autonomous region of Ningxia)

The effects of grazing intensity on selected soil characteristics in the feather-grass steppes of the autonomous region of Ningxia (northern China) were investigated by a comparison of non-grazed areas (grazing intensity 0), slightly grazed areas (grazing intensity I), moderately grazed areas (II), intensively grazed areas (III) and over-grazed areas (IV). Even in areas used only minimally for grazing activities (I), a serious increase (doubling) in soil hardness was apparent in the upper soil layer. A continual decrease in organic matter in the surface soil can be correlated directly to soil compaction. The content of organic matter in soil of degree IV amounts to only a third of the organic matter found in non-grazed areas. This decrease can be attributed partly to the poor living conditions for soil organisms in compacted soils, but also to a significant reduction in litter. This is because intensive grazing causes reduced vegetation cover leading to litter being blown away by wind or washed away by heavy rainfall. Thus in level III hardly any plant litter remained to be incorporated into the soil as humus. Likewise root density also suffered its largest decrease in areas with a grazing intensity level III. With regard to the content of nitrogen and phosphorous (total and available) hardly any difference between soils of grazing intensity 0 and I was observed, whereas a noticeable decrease was apparent between levels I and II. Available Potassium was similar for all grazing levels. The pH-value of the soil solution is not significantly affected by grazing. We did not observe differences in the soils of the two main types of steppe vegetation (Stipa grandis and Stipa bungeana steppe) in response to grazing. Only the amount of litter in the S. grandis-steppe in non-grazed or slightly grazed areas is noticeably higher than in the S. bungeana steppe.     

Spontaneous steppe restoration on abandoned cropland in Kazakhstan: grazing affects successional pathways

After the collapse of the Soviet Union in 1991, 12 million ha of cropland were abandoned in the steppe zone of Kazakhstan. At the same time livestock numbers crashed, leaving large areas of steppe without any grazing. We aimed to investigate, to which degree spontaneous succession on former cropland leads to the restoration of native steppe vegetation, and how this process is influenced by changing grazing patterns. We recorded biomass and vegetation characteristics as well as local soil and landscape variables in 151 quadrats of 100 m², covering 89 plots on former cropland abandoned in the early 1990s and 62 reference plots of near-natural steppe grassland that was never ploughed. About half of the plots of each category were located in a remote region where grazing has been absent for ca. 20 years, whereas the other half was located in a region with moderate livestock grazing. While there were no differences in the diversity, structure and plant life form composition of currently grazed and un-grazed near-natural steppe grasslands, corresponding successional plots on abandoned arable land exhibited significant differences. Grazed plots on former fields showed higher species richness and a higher cover of dwarf shrubs (mostly Artemisia spec.), ruderals and perennial herbs. At the same time, immigration of typical steppe species was much more successful. Contrary, in the absence of any grazing we found species-poor swards dominated by Stipa lessingiana and Leymus ramosus exhibiting an increasing frequency of wildfires due to litter accumulation. After 15–20 years, secondary steppe grasslands still differed substantially from their near-natural references. Our results suggest that grazing is mandatory to fully restore the original near-natural steppe vegetation and the underlying processes of pyric herbivory.     

Impact of grazing regime on a Mongolian forest steppe

Question: What is the impact of grazing regime on plant species abundance, plant growth form, plant productivity and plant nutrient concentrations in a forest steppe? Location: Hustai National Park in the forest steppe region of Mongolia. Methods: On the Stipa steppe we applied three different grazing regimes by using; (1) one type of exclosure which excluded grazing by large mammalian herbivores, mainly takh (Przewalski horse), (2) another type of exclosure that excluded both large and small (Siberian marmots) mammalian herbivores, and (3) control plots which were freely grazed. We measured species frequencies, tiller densities, plant biomass and nitrogen concentrations of the vegetation. Results: Exclusion from grazing by takh and marmots significantly increased plant standing crop, but marmot grazing and full grazing did not show significant differences. Protection from grazing decreased forage quality, shown by a lower N-concentration of the standing crop. However, this was solely the result of the lower live-dead ratio of the vegetation. The frequency of the rhizomatous Leymus chinensis decreased under reduced grazing, as did the frequency of the total of rhizomatous species. The frequency of Stipa krylovii increased under reduced grazing, as did its basal areas, tiller density and tussock height. Conclusion: Reduced grazing leads to a lower abundance of rhizomatous species and an increase in tussock species.     

Post-Soviet fire and grazing regimes govern the abundance of a key ecosystem engineer on the Eurasian steppe,the yellow ground squirrel Spermophilus fulvus

Aim

Grazing intensity and fire patterns across the Eurasian steppes have changed dramatically over the past decades due to the collapse of the Soviet Union in 1991, and Kazakhstan is now a global fire hotspot. The implications of these changes for ecosystem functioning are largely unclear. We aimed to understand the effects of changed grazing intensity and fire frequency on a key ecosystem engineer, the yellow ground squirrel Spermophilus fulvus, on a very large scale.

Location

Kazakhstan.

Methods

Ground squirrels were surveyed in an area of ca. 100,000 ha in the dry steppe of central Kazakhstan, using hierarchical distance sampling at more than 200 random points, stratified by fire frequency and livestock grazing intensity. We modelled abundance as a function of different variables, grouped at the landscape scale (fire and grazing), meso-scale (soil and vegetation structure) and at burrow scale (plant traits such as palatability, digestibility and nutrient content).

Results

Ground squirrels prefer areas of a high wormwood cover (Artemisia spp.) and high plant species richness, which are moderately grazed, preferably by cattle, with only rare fire occurrence. High squirrel densities were also related to the availability of nitrogen-rich plants of high nutritional value for herbivores.

Main Conclusions

Yellow ground squirrels seem to reach their density optima by balancing trade-offs between optimal foraging in areas of short, nutrient-rich vegetation and a good visibility of approaching predators. Post-Soviet changes in grazing pressure, resulting in higher fire recurrence rates due to grass encroachment and litter accumulation (i.e. fuel for wildfire), have likely affected the abundance of burrowing mammals and associated biodiversity across huge parts of the Eurasian steppes and semideserts.     

Burrowing activities of kangaroo rats and patterns in plant species dominance at a shortgrass steppe-desert grassland ecotone

Abstract. Our objective was to evaluate the effects of burrowing activities by banner-tail kangaroo rats (Dipodomys spectabilis Merriam) on plant community structure and species dominance for two patch types at the ecotone between shortgrass steppe and desert grassland in New Mexico, USA. 10 mounds produced by kangaroo rats were selected in patches dominated by Bouteloua gracilis (the dominant in shortgrass steppe communities) and 10 mounds were selected in patches dominated by B. eriopoda (the dominant in Chihuahuan desert grasslands). Plant cover and density by species were sampled from three locations associated with each mound: the mound proper, the edge of the mound in the transition area, and the off-mound vegetation. Similar cover of B. eriopoda for the edges of mounds in both patch types indicates the ability of this species to respond to animal disturbances regardless of the amount of cover in the surrounding undisturbed vegetation. By contrast, cover of B. gracilis was low for all mounds and mound edges in patches dominated by this species. Much higher cover of B. eriopoda on mound edges compared to the undisturbed vegetation in B. gracilis-dominated patches indicates that kangaroo rats have important positive effects on this species. Lower cover of perennial grasses and higher cover of forbs, shrubs, and succulents on the edges of mounds in B. eriopoda-dominated patches compared to patches dominated by B. gracilis indicate the importance of surrounding vegetation to plant responses on disturbed areas. Our results show that kangaroo rats have important effects on both species dominance and composition for different patch types, and may provide a mechanism for small-scale dominance patterns at an ecotone; thus providing further support for their role as keystone species in desert grasslands.     

Effect of grazing removal on aboveground vegetation and soil seed bank composition in sub-alpine grasslands of northern Iran

Background: Knowledge about vegetation and soil seed bank composition and the processes that contribute to vegetation recovery after the removal of heavy grazing is lacking in sub-alpine ecosystems.

Aims: In order to assess the effects of large herbivores on above-ground vegetation (AGV) and soil seed bank (SSB) characteristics, intensively sheep-grazed areas were compared to adjacent areas where grazing had been removed 10 years previously in a sub-alpine grassland of northern Iran.

Methods: A total of 40 4-m² (2 m × 2 m) plots were established in each treatment, and soil samples were collected from each plot within a depth of 10 cm. Plant species composition was determined for each plot during the flowering stage of herbaceous species in June 2011. The seedling emergence method was used to estimate SSB composition.

Results: A total of 45 species (23 annuals and 22 perennials) emerged from the soil samples of the grazed area, while the number of species emerged from the soil samples of the previously grazing area was 76 (37 annuals and 39 perennials). The removal of grazing led to a significant increase in species richness and seed density in the SSB. Species turnover of AGV was higher, and that of the SSB was similar for grazed areas compared with areas that were formerly grazed. Detrended correspondence analysis ordination of AGV composition showed a clearly separate structure between grazed plots and plots where grazing has been removed, while the segregation was less clear for SSB composition.

Conclusions: We concluded that restoration of locally degraded sites cannot rely on the SSB when grazing is stopped simultaneously and unvegetated gaps are colonised by vegetative growth rather than by seed.     

The dynamics of the central russian meadow steppes and the problem of their preservation

The structure and dynamics of vegetation of the zonal central Russian meadow steppes have been studied with the example of the Privolzhskaya Forest–Steppe State Nature Reserve. Under the influence of a completely protected mode, the vegetation of the studied meadow steppes on the watershed surfaces was subjected to varying degrees and mesophytization and sylvatization. It was noted that these processes occur more intensively in the meadow steppe landscapes of secondary moraine plains (Poperechenskaya steppe and Ostrovtsovskaya forest–steppe) than in the meadow steppes of erosion-denudation plains (Kuncherovskaya forest–steppe).     

Annual methane uptake by temperate semiarid steppes as regulated by stocking rates,aboveground plant biomass and topsoil air permeability

Overgrazing‐induced degradation of temperate semiarid steppes may affect the soil sink for atmospheric methane (CH₄). However, previous studies have primarily focused on the growing season and on single grazing patterns. Thus, the response of annual CH₄ uptake by steppes compared with various grazing practices is uncertain. In this study, we investigated the effects of grazing on the annual CH₄ uptake by two typical Eurasian semiarid steppes (the Stipa grandis steppe and the Leymus chinensis steppe) located in Inner Mongolia, China. The CH₄ fluxes were measured year‐round using static chambers and gas chromatography at 12 field sites that differed primarily in grazing intensities. Our results indicated that steppe soils were CH₄ sinks throughout the year. The annual CH₄ uptake correlated with stocking rates, whereas the seasonality of CH₄ uptake was primarily dominated by temperature. The annual CH₄ uptake at all sites averaged 3.7±0.7 kg C ha^?1 yr^?1 (range: 2.3–4.5), where approximately 35% (range: 23–40%) occurred during the nongrowing season. Light‐to‐moderate grazing (stocking rate≤1 sheep ha^?1 yr^?1) did not significantly change the annual CH₄ uptake compared with ungrazed steppes, but heavy grazing reduced annual CH₄ uptake significantly (by 24–31%, P<0.05). These findings imply that easing the pressure of heavily grazed steppes (e.g. moving to light or moderate stocking rates) would help restore steppe soil sinks for atmospheric CH₄. The empirical equations based on the significant relationships between annual CH₄ uptake and stocking rates, aboveground plant biomass and topsoil air permeability (P<0.01) could provide simple approaches for the estimation of regional CH₄ uptake by temperate semiarid steppes.     

Limited effect of hare grazing and short-term climatic variations on the most common alpine vegetation community in the Snowy Mountains,Australia

Background: In the alpine zone of the Snowy Mountains, grazing by mammals is limited. However, introduced European hare numbers have increased since the 1970s.

Aims: To estimate the density of hares and hence grazing pressure among years. To assess the response of biomass, vegetation height and composition to a cessation of hare grazing.

Methods: We used indices of hare abundance based on spotlighting and counts of hare pellets on a transect. The effect of hare grazing on tall alpine herbfield was assessed by using 15 paired exclosure and control quadrats for six years.

Results: The indices of hare abundance suggested densities similar to those in upland areas of Britain. Grazing did not affect the composition, cover of herbs or graminoids or, for 2010, vegetation height or biomass. Variation in vegetation and hare numbers among years was not correlated with climatic variables. Observations of selective grazing suggested that impacts on vegetation may be localised and restricted to certain species. Prior analyses of hare pellets indicated that hares might spread seed of native and exotic species.

Conclusions: Hares are having no general effect on tall alpine herbfield but may affect certain plant species via selective grazing or by spread of viable seed.     

Determinants of Caragana microphylla density distribution in the Mongolian steppe

Regional differences in Caragana microphylla density in the Mongolian steppes were explained by considering multiple abiotic and biotic factors collectively, including aridity gradients, grazing regimes, fire disturbance, and interspecific interactions. In the central and eastern Mongolian steppes, we collected vegetation data from 127 sites. Along 250-m line transects, the hit frequencies of C. microphylla and tall-grass species were recorded. Ancillary data included weather information, livestock populations, fire occurrence maps, and herder camp locations. Based on the steppe types and disturbance regimes, the sites were classified into 12 sub-groups. The data were statistically analyzed at the site, county, and sub-group levels. The natural C. microphylla density decreased with climatic aridity from forb-steppes to semi-desert steppes, but this pattern was not observed at grazed and burned sites. Livestock grazing decreased C. microphylla density, but this effect was considerably confounded by aridity effects, especially in the central steppes, making the relationship between C. microphylla and livestock densities complex. Although fire appeared to be an important factor in the eastern steppes, the mechanism of its effect on C. microphylla density was unclear, because the fire–shrub interaction is influenced by the tall-grass recovery process after fire. Based on our results, we propose that two different confounding effects, namely aridity versus grazing and fire versus interspecific interactions, play important roles in determining the spatial distribution of C. microphylla density in the central and eastern Mongolian steppes, respectively.     

Long‐term effects of the abandonment of grazing on steppe‐like grasslands

Question: What are the consequences of grazing abandonment on the Stipa lessingiana dominated steppe‐like grasslands? What is the relative importance of management and environmental factors in causing variation in species composition and abundance in the continuously grazed and abandoned grassland stands? Location: Transylvanian Lowland, Romania. Methods: Repeated vegetation mapping of a grassland stand, where grazing was abandoned 35 years ago; re‐sampling six grassland stands surveyed 29–57 years ago. For revealing long‐term changes in species composition and rank abundance PCoA ordination was applied. The relative importance of management and environmental factors in structuring vegetation were explored by CCA ordination. Diversity, evenness and the relative number and abundance of red‐listed species were compared between managed and abandoned stands. Results: Our results pointed out that grasslands which were formerly grazed and dominated by S. lessingiana, in the long‐term absence of grazing, have been transformed into a S. pulcherrima dominated type. Management, probably by creating bare surfaces and preventing litter accumulation, had the strongest effect on the species composition and abundance in the grasslands. Abandoned grassland stands had lower diversity and evenness compared to continuously grazed stands. While at the same time, the relative number of threatened, rare species did not differ between managed and abandoned sites. Conclusion: Maintaining extensively grazed, as well as un‐managed, Stipa dominated grasslands would be important in order to create various habitat conditions for plant species, especially threatened and rare species, and promote diversity on the landscape scale.     

Effects of livestock grazing intensity on soil arbuscular mycorrhizal fungi and glomalin-related soil protein in a mountain forest steppe and a desert steppe of Mongolia

Arbuscular mycorrhizal fungi (AMF) are important components of the grassland ecosystems in terms of plant phosphorus uptake and accumulation of glomalin-related soil protein (GRSP). Though Mongolian grasslands are seriously degraded by livestock grazing, the effects of grazing on soil AMF and GRSP remain unclear. Here, we examined community composition and diversity of AMF as well as amount of GRSP at three different grazing intensities: lightly grazed (LG), moderately grazed (MG), and heavily grazed (HG) under two different types of grassland, mountain forest steppe at Hustai and desert steppe at Mandalgobi. The diversity and biomass of AMF-host and non-AMF plants strongly affected the overall AMF community composition and its diversity. When we separately analyzed the factors affecting soil AMF diversity at Hustai and Mandalgobi, decrease in the shoot biomass of Poaceae plants at Hustai and decreases in the species number and shoot biomass of AMF-host plants at Mandalgobi were significantly correlated with AMF diversity. GRSP decreased with increasing grazing intensity, which was significantly correlated with soil pH and total root biomass at Hustai. The decrease in plant biomass caused by grazing thus led to GRSP reduction. Our results showed that change in soil AMF community caused by livestock grazing were associated with change in the biomass and diversity of functional vegetation groups such as Poaeceae, AMF-host and non-AMF plants, indicating the importance to focus on such functional vegetation groups to evaluate the effect of grazing on AMF.

     

Direct and indirect effects of grazing constrain shrub encroachment in semi‐arid Patagonian steppes

Question: What are the long‐term effects of grazing exclusion on the population structure and dynamics of, and interactions among, three dominant shrub species? Location: Grass‐shrub Patagonian steppe, Chubut, Argentina. Methods: Permanent plots were established in grazed paddocks and paddocks excluded from grazing in representative Patagonian rangelands. Shrub abundance, population size‐structure, short‐term (two 3‐yr periods) and long‐term (matrix models) population dynamics, and neighborhood interactions of three native and codominant shrub species (Mulinum spinosum, Senecio filaginoides and Adesmia volckmanni) were measured and analysed using different statistical approaches. Results: The total density of shrubs was 74% higher in paddocks excluded from grazing, owing mainly to increases in Mulinum (80%) and Senecio (68%) species. However, differences in size structure between ungrazed and grazed paddocks were only detected in Mulinum. Demographic rates differed between shrub species, time‐periods and grazing conditions. In particular, recruitment in the short term (especially in wet years) and population growth rate in the long term (λ) were higher in paddocks excluded from grazing only in Mulinum populations. Senecio populations showed a marginal increase in recruitment and mortality independent of the grazing condition in the wet and dry period. Grazing exclusion modified the balance of neighborhood interactions among the three shrub species. In grazing‐exclusion paddocks, there was a balance between positive and negative interspecific interactions, while in grazed paddocks there were more negative intraspecific and interspecific interactions, resulting in a net negative balance of neighborhood interactions. Conclusions: Our understanding of woody encroachment in arid rangelands can be informed through evaluation of direct and indirect effects of grazing exclusion on the abundance and demography of dominant woody species. In Patagonian arid steppes, the occurrence of woody encroachment in rangelands excluded from grazing can be explained by altered responses in plant‐animal and plant‐plant interactions among shrub species.     

Livestock exclusion increases the spatial heterogeneity of vegetation in Colorado shortgrass steppe

Abstract. Spatial heterogeneity, an important characteristic in semi‐arid grassland vegetation, may be altered through grazing by large herbivores. We used Moran's I, a measure of autocorrelation, to test the effect of livestock grazing on the fine scale spatial heterogeneity of dominant plant species in the shortgrass steppe of northeastern Colorado. Autocorrelation in ungrazed plots was significantly higher than in grazed plots for the cover of the dominant species Bouteloua gracilis, litter cover and density of other bunchgrasses. No species had higher autocorrelation in grazed compared to ungrazed sites. B. gracilis cover was significantly auto‐correlated in seven of eight 60‐yr ungrazed exclosures, four of six 8‐yr exclosures, and only three of eight grazed sites. Autocorrelograms showed that B. gracilis cover in ungrazed sites was frequently and positively spatially correlated at lag distances less than 5 m. B. gracilis cover was rarely autocorrelated at any sampled lag distance in grazed sites. The greater spatial heterogeneity in ungrazed sites appeared linked to patches characterized by uniformly low cover of B. gracilis and high cover of C₃ grasses. This interpretation was supported by simple simulations that modified data from grazed sites by reducing the cover of B. gracilis in patches of ca. 8 m diameter and produced patterns quite similar to those observed in ungrazed sites. In the one exclosure where we intensively sampled soil texture, autocorrelation coefficients for sand content and B. gracilis cover were similar at lag distances up to 12 m. We suggest that the negative effect of sand content on B. gracilis generates spatial heterogeneity, but only in the absence of grazing. An additional source of heterogeneity in ungrazed sites may be the negative interaction between livestock exclusion and B. gracilis recovery following patchy disturbance.     

Impact of Formica exsecta Nyl. on seed bank and vegetation patterns in a subalpine grassland ecosystem

The mound building ant Formica exsecta Nyl. is widely distributed in grassland ecosystems of the Central European Alps. We studied the impact of these ants on seed bank and vegetation patterns in a 11 ha subalpine grassland, where we counted over 700 active ant mounds. The mounds showed a distinct spatial distribution with most of them being located in tall‐grass, which was rarely visited by ungulates (red deer; Cervus elaphus L.). Heavily grazed short‐grass, in contrast, seemed to be completely avoided by ants as only few mounds were found in this vegetation type. The species composition of the ant mound and grassland seed banks was quite similar, i.e. from 15 common plant species 12 were found in both seed bank types. We found the same proportions of myrmecochorous seeds in ant mound and grassland soil samples. In contrast, the number of seeds was 15 times higher in mound compared with the grassland soil samples. Also, the vegetation growing on ant mounds significantly differed from the vegetation outside the mounds: graminoids dominated on ant mounds, herbaceous and myrmecochorous species in the grassland vegetation. We found significant continuous changes in vegetation composition on gradients from the ant mound centre to 1 m away from the mound edge. Overall, F. exsecta was found to have a considerable impact on seed bank and vegetation patterns in the grassland ecosystem studied. These insects not only altered grassland characteristics in the close surrounding of their mounds, but also seem to affect the entire ecosystem including, for example, the spatial use of the grassland by red deer.