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Large 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.

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.

Plantherbivore 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 km 2 ) 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.

Domestic livestock drives ecosystem changes in many of the world’s mountain regions, and can be the dominant influenceon soil, habitat and wildlife dynamics. Grazing impacts on ecosystem services (ES) vary according to densities of sheep, butan ES framework accounting for these is lacking. We devised an experiment to evaluate synergies and trade-offs of ESs andcomponents of biodiversity affected by sheep density at the alpine landscape scale in southern Norway. We examined the effectsof increased (80 per km2), decreased (0 per km2) and maintained sheep densities (25 per km2) on ‘supporting’, ‘regulating’and ‘provisioning’ services and biodiversity (plants, invertebrates and birds). Overall, ESs and biodiversity were highest atmaintained sheep density. Regulating services, including carbon storage and habitat openness, were particularly favoured bymaintained densities of sheep. There was no overall decline in ESs from maintained to increased sheep densities, but severalservices, such as runoff water quality, plant productivity and carbon storage, declined when grazing increased. Our study providesexperimental evidence for a positive effect of grazing on ES, but only at maintained low sheep densities. By identifying ESand biodiversity components that are traded-off at decreased and increased grazing, our study also demonstrates some of thenegative impacts on ecosystems that can occur in mountain regions if management does not regulate herbivore densities. Herbivory; Ecosystem services; Livestock; Management; Optimal stocking levels; Overgrazing; Threshold Author postprint

Large herbivores directly affect plant communities in alpine ecosystems. In addition, they may compete with, or facilitate foraging by, small herbivores and also cause strong indirect effects on plants. We used an exclosure experiment to examine short-term (5-y) effects of cessation of sheep grazing on rodent grazing and plant communities in an oceanic alpine environment of low productivity with a long history of heavy sheep grazing. Exclusion of sheep significantly impacted plant communities. Vascular plant height increased, but Deschampsia flexuosa was the only vascular plant species that increased in sheep exclosures. Changes in the frequency of graminoids, herbs, and dwarf shrubs in exclosed plots were not related to cessation of grazing, but 6 bryophyte species significantly increased or decreased in response to exclusion of sheep. The absence of large grazers thus brought about a change in the species composition in favour of successional bryophytes and the preferred fodder plant. Neither vascular plant nor bryophyte species richness, nor the total cover of bryophytes and lichens, were affected. Cessation of sheep grazing reduced the level of rodent grazing. Rodent grazing correlated with changes in plant communities that led to reduced height and cover of vascular plants, reduced cryptogam cover, and reduced frequencies of 3 bryophyte species. A strong correlation between sheep fodder value index and rodent grazing indirectly indicated additive herbivory. In addition, some of the rodent effects were compensatory; e.g., Nardus stricta, which is not grazed by sheep, was significantly reduced by rodents. Our study points to a more central role of facilitation in structuring herbivore assemblages in the short term, with direct implications for the joint effects of large and small herbivores on the cover and frequency of graminoids and bryophytes.

A common management aim is to keep large herbivore densities at sustainable levels. However, measuring grazing pressure is difficult. We seek combinations of herb species and morphological traits that serve as indicators of grazing pressure in alpine areas of Scandinavia. In a fully replicated, landscape-scale experiment with three replicates of each of three levels of sheep density (zero, low, and high), we measured grazing frequency (numbers of plants eaten, as a proportion), flowering frequency, and plant height for 17 common herb species for two consecutive grazing seasons. For eight species, grazing frequency varied significantly between treatments, and for four of these species there was additional strong between-year variation. The estimate for total grazing frequency of herbs decreased when more species of decreasing preference were included, and it was significantly lower in the second grazing season. There were large between-year differences in plant height probably related to weather p...

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.