High Sheep Density Research Articles

Unpacking multi-trophic herbivore-grass-endophyte interactions: feedbacks across different scales in vegetation responses to Soay sheep herbivory

Grazing can induce changes in both plant productivity and nutritional quality, which may subsequently influence herbivore carrying capacity. While research on Soay sheep (Ovis aries L.) dynamics on Hirta Island in the St. Kilda archipelago has elucidated the complexity of population drivers, including parasites, the role of herbivore-generated feedbacks as an intrinsic regulating factor remains unclear. The sheep lack large predators and every 3–9 years undergo population crashes (overcompensatory mortality). We investigated the effects of grazing on (1) sward productivity and (2) quality (toxicity) of the primary forage species, red fescue (Festuca rubra L.), which is highly infected by an alkaloid-synthesizing fungal endophyte. Grazing had a negative impact on both forage quantity and quality. At higher sheep densities, impacts on sward growth were magnified, resulting in a nonlinear relationship with plant productivity. Simultaneously, endophyte hyphal load (and by inference, toxicity) peaked close to the time of a crash. A greenhouse experiment showed that alkaloid concentration in F. rubra increased in response to artificial defoliation. We conclude that at high sheep densities, grazing-mediated reductions in productivity, together with sustained alkaloid production, are likely to influence sheep dynamics. Future research should consider the interactive effects of forage toxicity, quantity, and nutritional content.

The prevalence and distribution of sheep scab in Wales: a farmer questionnaire survey.

Outbreaks of ovine psoroptic mange in the U.K. have increased 100-fold since its deregulation in 1992, with the highest prevalence in Wales, a region of high sheep density. A cross-sectional, retrospective, questionnaire-based survey of 7500 members of the association of Welsh lamb and beef farmers [Welsh Lamb and Beef Producers Ltd (WLBP)] was used to investigate the prevalence and distribution of sheep scab in this region in 2015. The survey was completed by 14.0% (n = 972) of potential respondents. Scab outbreaks were reported on 15.8% (n = 154) of farms in 2015. However, 29.0% (n = 282) of farms reported at least one scab outbreak and 2.4% (n = 23) of farms had experienced between six and 10 outbreaks in the previous 10 years. Most outbreaks occurred during September-January (83.0%, n = 150), and were clustered around Brecon (mid-Wales) and Bangor (North Wales). Farmers who used common grazing were significantly more likely to report scab outbreaks in the previous 10 years than farmers who did not. No quarantine procedures for sheep bought in were used by 29.0% (n = 262) of farmers. Future research should be directed towards the development of localized management programmes, with a particular focus on areas of common grazing.

Population density of Dall’s sheep in Alaska: effects of predator harvest?

We measured population abundance and density of Dall’s sheep (Ovis dalli) before and during gray wolf (Canis lupus) and coyote (Canis latrans) harvest for 3 years (1998–2000) on two similar and adjacent study sites with treatment (canid harvests) and reference (no canid harvest) in interior Alaska, USA. Between 1998 and 1999, density of Dall’s sheep in the treatment area increased by 0.746 ± 0.163 sheep/km2. Between 1999 and 2000, after a winter with above-normal, crusted snow, density of sheep decreased by 2.1 ± 0.14 sheep/km2. Because of this large decline, sheep density on the treatment area decreased by 1.3 ± 0.08 sheep/km2 over the entire monitoring period. Sheep in the reference area showed no significant change in density between 1998 and 2000. We hypothesize that Dall’s sheep in the treatment area initially benefited from the harvest of wolves and coyotes via reduced predation and predation risk and that resultant high sheep densities coupled with severe winter weather caused a dramatic decline in Dall’s sheep between 1999 and 2000. Thus, this study illustrates the potential consequences of interactions between density-dependent and density-independent factors for the conservation of large mammals, especially those residing at high latitudes and altitudes. These results should serve as a cautionary tale for those wishing to increase ungulate numbers via predator control without regard to other ecological factors, such as the proximity of the prey population to ecological carrying capacity (K).

Effects of livestock on guanacoLama guanicoedensity, movements and habitat selection in a forest–grassland mosaic in Tierra del Fuego, Chile

AbstractLocally abundant ungulates often come into conflict with human activities. After a population collapse that reached its nadir in the 1970s, the guanacoLama guanicoepopulation in Tierra del Fuego, Chile, recovered and is now in conflict with sheep ranching and commercial logging. We studied the effects of livestock density and environmental factors on guanaco abundance and spatial ecology, using seasonal counts and radio-telemetry in a private protected area (Karukinka) and neighbouring ranches in a forest–grassland mosaic in Tierra del Fuego. Guanaco density was highest in low-elevation areas with more grassland cover and little snow accumulation in winter. In low-elevation areas, guanaco density decreased with increasing livestock density. Radio-tracked guanacos exhibited a partial migration pattern: two individuals migrated seasonally, selecting grasslands and avoiding forests mainly in summer, whereas six sedentary individuals used habitats according to their availability. Migratory guanacos spent the summer in Karukinka and winter on nearby ranches. High sheep densities and poor range condition on the ranches reduce key forage resources available to guanacos and may promote use of forests by guanacos, affecting forest regeneration and increasing conflict with logging. Current guanaco harvest by loggers may fail to reduce the impact of guanacos on logged-forest regeneration if guanaco spatial ecology and sheep management are not considered. Our results provide insight into the interactions among guanacos, forests and livestock ranching, and may be used to reduce conflicts and guide conservation in the Fuegian ecosystem.

A catch-22 conflict: Access to semi-domestic reindeer modulates Eurasian lynx depredation on domestic sheep

Conserving large carnivores in multi-use landscape is a global challenge. In northern Norway the presence of Eurasian lynx (Lynx lynx) conflicts greatly with the current ways of keeping free-grazing, unguarded livestock in large carnivore habitat. In contrast to most other places in Europe, livestock (sheep Ovis aries, reindeer Rangifer tarandus) are the only ungulate prey available for lynx in this area. The relative preference by lynx for these two domestic species will strongly influence depredation pressure for the respective species. We examined predation patterns on domestic sheep in summer from 17 GPS-collared lynx that had access to both free ranging sheep and reindeer. During 1115 lynx monitoring days, we documented 47 sheep and 274 reindeer killed by lynx. Most lynx individuals selected reindeer over sheep and the probability for a kill to be a sheep increased at low reindeer densities combined with high sheep densities. Kill rates on sheep were several times lower than for reindeer and were not related to density of livestock. General avoidance and low kill rates on sheep by most lynx generated low predation pressure on sheep within the reindeer husbandry, similar to what has been observed in areas of high roe deer densities. However, even a small shift of herd location within the designated summer grazing areas for reindeer could potentially cause a significant increase in depredation on sheep locally. This is one of several studies demonstrating that the density of alternative prey can modulate kill rates on livestock, which is important to include in spatially explicit risk models of large carnivore depredation on livestock.

Experimental Effects of Herbivore Density on Aboveground Plant Biomass in an Alpine Grassland Ecosystem

Herbivores may increase or decrease aboveground plant productivity depending on factors such as herbivore density and habitat productivity. The grazing optimization hypothesis predicts a peak in plant production at intermediate herbivore densities, but has rarely been tested experimentally in an alpine field setting. In an experimental design with three densities of sheep (high, low, and no sheep), we harvested aboveground plant biomass in alpine grasslands prior to treatment and after five years of grazing. Biomass of vascular plants decreased at high sheep density, and marginally increased at low sheep density. The ungrazed treatment was found to be intermediate. Companion studies conducted at the same site suggest, (1) that changes in soil N-mineralization and plant community patterns are contributing to the herbivore-induced effects on plant productivity in alpine grasslands, (2) that herbivore-driven changes in plant productivity feed into the future performance for the herbivore as the marginal increase in productivity at low density corresponds with a temporal increase in lamb growth. Our study provides experimental evidence for a nonlinear effect of increased grazing on plant productivity as predicted by the grazing optimization hypothesis. This has important repercussions for ecosystem function and management, as it demonstrates how herbivore density can either increase or decrease ecosystem productivity over time.

Long-Term Increase in Aboveground Carbon Stocks Following Exclusion of Grazers and Forest Establishment in an Alpine Ecosystem

Ecosystem stores of carbon are a key component in the global carbon cycle. Many studies have examined the impact of climate change on ecosystem carbon storage, but few have investigated the impact of land-use change and herbivory. However, land-use change is a major aspect of environmental change, and livestock grazing is the most extensive land use globally. In this study, we combine a grazing exclosure experiment and a natural experiment to test the impact of grazer exclusion on vegetation dynamics and ecosystem carbon stores in the short term (12-year exclosures), and the long term (islands inaccessible to livestock), in a heavily grazed mountain region in Norway. Following long-term absence of sheep, birch forest was present. The grazing-resistant grass Nardus stricta, dominated under long-term grazing, whilst the selected grass Deschampsia flexuosa and herb species dominated the vegetation layer in the long-term absence of sheep. The established birch forest led to vegetation carbon stocks being higher on the islands (0.56 kg C m−2 on the islands compared to 0.18 kg C m−2 where grazed) and no difference in soil carbon stocks. In the short-term exclusion of sheep, there were minor differences in carbon stocks reflecting the longer term changes. These results show that aboveground carbon stocks are higher in the long-term absence of sheep than in the continual presence of high sheep densities, associated with a vegetation state change between tundra and forest. The reduction of herbivore populations can facilitate forest establishment and increase aboveground carbon stocks, however, the sequestration rate is low.

Linking sheep density and grazing frequency to persistence of herb species in an alpine environment

AbstractLarge herbivores are important drivers in ecosystems worldwide. Changes in herbivore densities are predicted to especially affect herbs that are strongly preferred by herbivores. The persistence of herbs could be challenged by enhanced grazing, but also grazing cessation may affect persistence, especially for prostrate herbs, which might be out‐competed. To test how different herbivore densities (high, low, and no sheep) affect grazing frequency and plant responses (plant height, flowering frequency, and plant density) at the herb community and species level, we conducted a fully replicated, landscape‐scale experiment in an alpine environment. We found that none of the herb species changed their densities after 5 years with experimental changes in grazing pressure. Sheep density affected grazing and flowering frequency at the herb community level. Eight herb species were more grazed at high sheep density as compared to enclosures with no sheep. Herb height decreased at high sheep density as compared to no sheep for the two species with the highest grazing frequency. Increased height and flowering frequency were found for small herbs at high sheep density. Our experiment clearly shows that herbs do not constitute a homogeneous functional group and that, in particular, tall and small herbs are affected in contrasting ways but all species (n = 15) tolerated changes in grazing regimes as densities were maintained at both enhanced grazing and grazing cessation.

The Response of Alpine Salix Shrubs to Long-Term Browsing Varies with Elevation and Herbivore Density

The widespread expansion of shrubs into arctic and alpine regions has frequently been linked to climatic warming, but herbivory can play a role in addition to, or in interaction with, climate. Willow (Salix spp.) shrubs are important constituents of alpine ecosystems, influencing community structure and providing habitat and forage for many species. We investigate the impact of browsing by domestic sheep (Ovis aries), the dominant herbivore in Norwegian mountains, on Salix stem density, height, and radial growth. We used a field experiment, replicated along an elevational gradient, with manipulated densities of sheep (no sheep, low density, and high density at 0, 25, and 80 sheep km-2). We found that Salix shoot density and radial growth were greatest at high sheep density but only at low elevations, indicating that competition from field-layer vegetation at lower sheep densities reduced Salix performance. At higher elevations Salix shoot density and radial growth were lower at high sheep density than at low sheep density and in the absence of sheep. Thus at high elevations sheep browsing is likely to slow the expansion of Salix shrubs, whilst the removal of browsing is likely to constrain Salix expansion at lower elevations.

Elevational advance of alpine plant communities is buffered by herbivory

AbstractQuestionA warming climate has been linked to changes in the distribution of many species, yet the interactions between climate and other environmental drivers are relatively poorly understood. Mountain regions are expected to be particularly at risk in a warming climate, and in many upland regions land use has also changed dramatically over the past few decades, with densities of grazing livestock often decreasing. Does livestock grazing influence elevational shifts of plant communities?LocationSouthern Scandes, Norway.MethodsWe use a unique long‐term experimental study at the landscape scale along an altitudinal gradient to study elevational shifts of plant communities and species under high and low sheep densities and in ungrazed conditions.ResultsWe show that elevational advance of the whole alpine grassland plant community occurred in the absence of herbivores (with a 3 m elevational advance over 8 yr), but was suppressed at low sheep density. At high sheep density there was a downward shift in plant community composition of 4 m. The average change in median altitude across species was positive in the absence of herbivores, but not significantly different from zero under both grazing treatments. There was no convincing evidence that species from lower altitudes showed a greater elevational advance than species from higher altitudes.ConclusionsThis study presents evidence that grazing sheep suppressed an elevational advance of the plant community, suggesting that grazing can limit the altitude of the plant community. This implies that grazing management has the potential to buffer climate‐driven shifts in plant communities. Conversely, the widespread recent changes in land use, with reductions in large herbivore densities in alpine areas, could further increase the vulnerability of alpine communities to warming.

Effects of Sheep Grazing on Availability and Leaching of Soil Nitrogen in Low-Alpine Grasslands

Alpine ecosystems are generally nitrogen (N) limited with low rates of N mineralization. Herbivory may affect N cycling and N losses and thus long-term productivity of ecosystems.Using a controlled grazing experiment in a low-alpine region at Hol, southern Norway, with three density levels of sheep, we determined effects of grazing on in situ availability of inorganic N, potential N mineralization, and mobility of dissolved inorganic N (DIN) and dissolved organic N (DON) in soil water of O-horizons in grazing-preferred grassland habitats. In addition, we studied the within-season and spatial variation of these processes.The low alpine grasslands at Hol were characterized by small rates of N mineralization and relatively large plant demands for N. Significantly greater rates of potential N mineralization were found at sites with high sheep density compared to those with low density or no grazing. Effects of grazing on bioavailable N (as determined by buried PRS™ exchange resins) were greater at low as compared to high altitudes. At low altitudes, low sheep density reduced amounts of bioavailable N. Nitrogen concentration of plants as a proxy of N availability in soils revealed, however, no significant effects of grazing. There was a strong seasonal effect on inorganic N and DIN∶DON ratios of the soil water, with decreasing values in the course of the growing season, probably due to increasing nutrient demand of plants and/or microbes.We conclude that grazing may significantly stimulate N-cycling, but not sufficiently to release the system from its strong N deficiency, as we found no evidence for short-term increased risk in N loss via soil water due to herbivore activity. Nitrogen removal through grazing is small compared to the total soil N pool and at high sheep density is about half of the N deposition. This suggests that grazing in grassland habitats in this low alpine ecosystem is sustainable from a nutrient point of view.

Plant quality, seasonality and sheep grazing in an alpine ecosystem

Large herbivores are affecting a suite of plant traits in many ecosystems, including plant quality. At northern latitudes, the phenological development of plants over the growing season is also regarded crucial for plant quality. The relative role of grazing and seasonality for quality of different plant functional groups has not been quantified in northern, alpine ecosystems, but are assumed to form a key role in the ecosystems due to the feedback on herbivore behaviour and performance. In an experimental setting (3 sheep density treatments replicated 3 times) and based on 1518 samples from 6 species of plants belonging to 3 functional groups collected over the entire growing season, we tested the relative role of grazing and seasonality for plant quality as indexed by N- and P-content and C:N and C:P ratios. We examined sheep diet composition and quality in faeces. There was a marked seasonal decline in specific N- and P-content for all plant species. N-content was higher for grass tissue collected at high sheep density late in the season relative to those from controls without sheep, but not for forb and dwarf shrub tissue. P-content in tissue peaked at low sheep density for Avenella flexuosa (late season), forbs and Vaccinium myrtillus. C:N and C:P ratios mainly followed the patterns for N- and P-content, respectively. The grass A. flexuosa increased while forbs and Salix sp. declined in proportion in the diet over the growing season. A. flexuosa were more consumed at high sheep density, while forbs were more consumed at low density. N- and P-content in sheep faeces declined markedly over the grazing season, but was not related to treatment. We conclude that the endogenous seasonal changes in plant quality over the growing season are stronger than grazing effects, but that grazing extends the season of access to high quality grasses suggesting potential for grazing facilitation.

Effects of sheep grazing and temporal variability on population dynamics of the clonal herb Geranium sylvaticum in an alpine habitat

An improved understanding of population-level consequences of grazing on plants can be facilitated by an assessment of grazing effects on all stages in the life-cycle. In this study, 6 years of demographic data for three populations of the perennial herb Geranium sylvaticum were analysed. We examined the effects of sheep grazing (high sheep density, low sheep density and no sheep) and interannual climatic variability on vital rates and population growth rates (λ). Grazing did not affect survival or flowering rates, but reduced rates of growth and increased rates of clonal reproduction. At the population level, high contributions from retrogression and clonal reproduction buffered reduced rates of growth and stasis, and no consistent differences in λ between populations exposed to different sheep densities were found. Instead, large between-year variability in λ, independent of sheep density, was detected, related to variation in the local summer climate. The results indicated, however, that grazing effects on λ were more severe in unfavourable than in normal years. Our study highlights that increased clonal reproduction rates functioned as a tolerance mechanism towards grazing in this herb, which forms a mechanism to explain how moderate population responses to grazing in some herbs can arise.

Growth limitation of mountain birch caused by sheep browsing at the altitudinal treeline

Treelines are advancing on a global scale and encroaching upon alpine ecosystems. Browsing by vertebrate herbivores could affect treeline dynamics and forest expansion by limiting growth of trees. However, this has not been experimentally investigated, and due to a combination of herbivore behaviour and plant responses to herbivory, is unlikely to be a simple relationship. A long term (8 years of manipulation), large scale (0.3 km 2) sheep enclosure experiment, in the Scandes of Norway, was used to investigate the effect of three densities of sheep (no sheep, low and high densities with respectively 0, 25 and 80 sheep km −2) on tree growth within a mountain birch ( Betula pubescens tortuosa) treeline ecotone. Birch trees were most likely to be browsed at intermediate heights of around 100–130 cm and the likelihood of an individual being browsed did not differ between low and high sheep densities. However, when browsed, the intensity of browsing increased with sheep density. High densities of sheep reduced the net annual shoot growth of young mountain birch in comparison to growth in the absence of sheep. The long-term height growth of smaller mountain birch (stem diameter < 15 mm) was also reduced at high densities of sheep, but the height growth of larger birch was resistant to browsing, even though these trees were on average still within browsing height. This study shows that susceptibility to browsing in birch decreases with size, but by restraining the height growth of trees high densities of sheep can keep trees at heights at which they remain susceptible to browsing. Reductions in livestock density over a period as short as 8 years can thus facilitate altitudinal forest expansion.

Factors influencing guanaco distribution in southern Argentine Patagonia and implications for its sustainable use

The Guanaco (Lama guanicoe) has suffered a progressive decline in numbers because of unregulated hunting and poaching by an assumed competition with sheep. Inadequate livestock management, including keeping sheep numbers above carrying capacity, has led to a degradation of the Patagonian steppe. Recently, interest has grown towards a reduction in sheep density and diversification of extractive activities. Guanaco populations could be potentially amenable to a number of sustainable uses. Our aim was to investigate the factors that determine guanaco distribution in southern Argentine Patagonia and to generate a predictive cartography at the regional scale. We hypothesized that guanaco distribution could be determined by primary productivity, terrain ruggedness, human disturbance and poaching, and competition with livestock. Guanaco surveys were performed from vehicles using a road survey method. To analyze the relationship between guanaco occurrence and potential predictors we built Generalized Additive Models (GAMs) using a binomial error and a logistic link. We found that guanaco occurrence increased in the less productive and remote areas, far from cities and oil camps, and decreased in regions with high sheep density. These results suggest that guanacos tend to occur where human pressure is lower. One way to promote guanaco conservation would be to highlight the economic value of guanacos under the regulations imposed by a sustainable exploitation of their populations. The predictive models developed here could be a useful tool for the implementation of conservation and management programs at the regional scale.

Large herbivore grazing and invertebrates in an alpine ecosystem

The literature on how plants respond to grazing and other disturbance factors have advanced greatly in recent decades, but studies of invertebrates are comparably few. We here quantify the effects of 3 levels of sheep grazing on selected invertebrates in an alpine ecosystem in Norway. We tested the hypothesis that invertebrates are more sensitive to grazing than plants (responding mainly at high density), and that primary consumers (herbivorous beetles) are more sensitive than predatory species (beetles and spiders). We captured 1218 specimens belonging to 44 beetle species and 6672 specimens belonging to 66 species of spiders. The community was dominated by few species: 5 beetle and 3 spider species made up 53.0% and 37.4% of the catch, respectively. At the local (plot) scale, most negative responses were only recorded at high sheep density, and invertebrates were thus not more responsive than the plant community. Two dominant herbivorous beetles responded to grazing while 3 dominant species of predatory beetles did not (one marginally). Spider species richness and frequency of occurrence of 2 dominant species were negatively affected by sheep grazing, suggesting variation between different taxonomic groups of predators. Further functional details than simple classification like herbivorous, predatory and litter-dwelling invertebrates seem to be required before a framework to predict responses to disturbance are robust.

Interactions between local climate and grazing determine the population dynamics of the small herb Viola biflora

Plants of low stature may benefit from the presence of large herbivores through removal of tall competitive neighbours and increased light availability. Accordingly, removal of grazers has been predicted to disfavour small species. In addition to this indirect beneficial effect, the population dynamics of plants is strongly influenced by variation in external conditions such as temperature and precipitation. However, few studies have examined the interaction between large herbivores and inter-annual variation in climate for the population dynamics of small plant species not preferred by herbivores. We studied three populations of the perennial herb Viola biflora exposed to different sheep densities (high, low and zero) for 6 years in a field experiment. Plants were also impacted by invertebrate and small vertebrate herbivores (rodents). Rates of growth were marginally higher at high sheep densities, and during warm summers both survival and growth were higher when sheep were present. Thus, while the height of tall herbs was positively related to July temperature, it was less so in the treatments with sheep, suggesting that sheep reduce the negative effects of interspecific competition for this small herb. Life table response experiment analyses revealed that the population growth rate (lambda) was slightly lower in the absence of sheep, but between-year variation in lambda was larger than variation among sheep density treatments. lambda was negatively related to July temperature, with an additional negative effect of vertebrate grazing frequency (sheep or rodent grazing). The evidence from this 6-year study suggests that the population dynamics of Viola biflora is determined by a complex interplay between climate and grazing by both large and small herbivores.

Spatial distribution of the active surveillance of sheep scrapie in Great Britain: an exploratory analysis

BackgroundThis paper explores the spatial distribution of sampling within the active surveillance of sheep scrapie in Great Britain. We investigated the geographic distribution of the birth holdings of sheep sampled for scrapie during 2002 – 2005, including samples taken in abattoir surveys (c. 83,100) and from sheep that died in the field ("fallen stock", c. 14,600). We mapped the birth holdings by county and calculated the sampling rate, defined as the proportion of the holdings in each county sampled by the surveys. The Moran index was used to estimate the global spatial autocorrelation across Great Britain. The contributions of each county to the global Moran index were analysed by a local indicator of spatial autocorrelation (LISA).ResultsThe sampling rate differed among counties in both surveys, which affected the distribution of detected cases of scrapie. Within each survey, the county sampling rates in different years were positively correlated during 2002–2005, with the abattoir survey being more strongly autocorrelated through time than the fallen stock survey. In the abattoir survey, spatial indices indicated that sampling rates in neighbouring counties tended to be similar, with few significant contrasts. Sampling rates were strongly correlated with sheep density, being highest in Wales, Southwest England and Northern England. This relationship with sheep density accounted for over 80% of the variation in sampling rate among counties. In the fallen stock survey, sampling rates in neighbouring counties tended to be different, with more statistically significant contrasts. The fallen stock survey also included a larger proportion of holdings providing many samples.ConclusionSampling will continue to be uneven unless action is taken to make it more uniform, if more uniform sampling becomes a target. Alternatively, analyses of scrapie occurrence in these datasets can take account of the distribution of sampling. Combining the surveys only partially reduces uneven sampling. Adjusting the distribution of sampling between abattoirs to reduce the bias in favour of regions with high sheep densities could probably achieve more even sampling. However, any adjustment of sampling should take account of the current understanding of the distribution of scrapie cases, which will be improved by further analysis of this dataset.

Density dependent and temporal variability in habitat selection by a large herbivore; an experimental approach

Both density dependent and density independent processes such as climate affect population dynamics in large herbivores. Understanding herbivore foraging patterns is essential to identify the underlying mechanisms behind variation in vital rates. However, very little is known about how animals vary their selection of habitat temporally, alone or in interaction with density during summer. At the foraging scale, we tested using a fully replicated experiment whether domestic sheep Ovis aries stocked at high (80 per km2) and low (25 per km2) densities (spatial contrasts) varied their habitat selection temporally over a four year period. We predicted reduced selection of high productivity vegetation types with increasing density, and that seasonal and annual variation in climate would affect this density dependent selection pattern by increasing competition for high quality habitats in late grazing season and in years with poor vegetation development and over time related to vegetation responses to grazing. As predicted from the Ideal free distribution model, selection of high productivity habitat decreased at high density. There was also a marked temporal variation in habitat selection. Selection of the most productive vegetation types declined towards the end of each grazing season, but increased over years both at low and high sheep density. There was only weak evidence for interactions, as selection ratio of highly productive habitats tended to increase more over years at low density as compared to high density. Limited interactive effects of density and annual variation on habitat selection during summer may explain why similar interactions in vital rates have rarely been reported for summer seasons. Our results are consistent with the view that variation in habitat selection is a central mechanism for climate and density related variation in vital rates.

Large scale experimental effects of three levels of sheep densities on an alpine ecosystem

Plant–herbivore dynamics is a major topic in ecological research, but empirical knowledge on the ecological effects of different densities of large grazers from fully replicated experiments is rare. Previous studies have focused on grazing vs no grazing, and our understanding of the extent to which different levels of grazing alter vegetation composition, and how quickly such effects can be measured, is therefore limited. We performed a fully replicated, short‐term (four‐year) experiment using large enclosures (each ∼0.3 km2) with three different sheep densities (no grazing, low grazing and high grazing, respectively) in an alpine environment with summer grazing in southern Norway to address these issues. Sheep grazing mainly affected plant species at high densities of sheep as compared to no sheep after a four‐year treatment; few effects of low sheep densities were detectable. Highly selected herbs, herbs suggested vulnerable to trampling, and woody species decreased, while most graminoids, one ruderal, one prostrate species and two bryophyte taxa increased at high sheep densities. We found contrasting responses within main functional groups highlighting that fine details of plant life histories need to be known for responses to grazing to be successfully predicted. Vascular plant cover and bare soil responded to sheep density after two years of treatment, but only for one of the species was frequency change observed at this stage. Overall, plants in low grazing plots were found to be almost unaffected. Changes in abundance were mainly found at the no grazing and high grazing treatments. Plant species that decreased at high grazing generally increased at no grazing and vice versa, suggesting a response to both cessation of grazing and enhanced grazing respectively. Our study demonstrates, beyond a simple comparison of heavily grazed and non‐grazed sites, that herbivore effects on plants are typically non‐linearly related to herbivore density, and that the speed of plant responses will depend both on the plant property examined and the grazing pressure.