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PlantÁherbivore 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 Â0.3 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.Herbivores can strongly affect plant community patterns by favouring resistant and tolerant plants to the detriment of less tolerant, highly selected species (Crawley 1997, Hester et al. 2006. Although the outcome of direct and indirect herbivore effects in terms of (change in) overall species composition appears more or less predictable for the alpine tundra (Van Der Wal 2006), these are mostly qualitative patterns for main growth-form groups. Exactly how high herbivore intensity is required for different species in alpine communities to trigger abundance change over time has rarely been examined experimentally. The effect of herbivory has mainly been studied by small-scale exclosure experiments (Mulder 1999) or by use of spatial contrasts in herbivore density (rather than following grazing over time in one area) in quasi-experiments at coarse scales (Bråthen et al. 2007, Ims et al. 2007. Empirical knowledge of the shape of the density-effect function of large g...

derströ m, L. 2005. Changes in life-history traits in an expanding moss species: phenotypic plasticity or genetic differentiation? A reciprocal transplantation experiment with Pogonatum dentatum. Á/ Ecography 28: 71 Á/80.A transplant experiment was performed to investigate whether differences in lifehistory traits of the bryophyte Pogonatum dentatum that recently expanded its distribution range, were genetically or environmentally determined, or a combination of both. Plants were transplanted reciprocally between the original mountain area and a recently colonised lowland area. Vegetative biomass of annual segments and branches tended to be higher in the mountain area than in the lowland area. Reproductive investment was higher for plants transplanted to the lowland area, and lowland shoots tended to produce larger sporophytes than mountain shoots when placed in the same environment. Age of reproducing shoots showed no consistent pattern. Native shoots were often outperformed by non-native shoots transplanted into the same site. Non-native shoots grew larger and produced larger sporophytes than native shoots. Much of the observed variation was at the site level instead of between mountain and lowland areas, with both genetic origin and environmental effects contributing together. Range expansion of P. dentatum may have taken place by dispersal from populations with shoots whose growth is plastic. Such shoots grow larger and potentially produce more spores for dispersal.K. Hassel (kristian.hassel@bio.ntnu.no), B. Pedersen and L. Sö derströ m,

BackgroundRespiratory viruses often have been studied in children with respiratory tract infection (RTI), but less knowledge exists about viruses in asymptomatic children. We have studied the occurrence of a broad panel of respiratory viruses in apparently healthy children attending day care, taking into account the influence of possible confounding factors, such as age, clinical signs of respiratory tract infection (RTI), location (day-care section) and season.MethodsWe have studied 161 children in two day-care centers, each with separate sections for younger and older children, during four autumn and winter visits over a two-year period. A total of 355 clinical examinations were performed, and 343 nasopharyngeal samples (NPS) were analyzed by semi-quantitative, real-time, polymerase chain reaction (PCR) tests for 19 respiratory pathogens.ResultForty-three percent of all NPS were PCR-positive for ≥ 1 of 13 virus species, with high species variation during visits. Rhinovirus 26% (88/343 NPS), enterovirus 12% (40/343) and parechovirus 9% (30/343) were detected in every visit, and the rates varied in relation to age, day-care section and season. Ten other viruses were detected in ≤ 3% of the NPS. Generally, viruses occurred together in the NPS. In 24% (79/331) of the clinical examinations with available NPS, the children had clear signs of RTI, while in 41% (135/331) they had mild signs, and in 35% (117/331) the children had no signs of RTI. Moreover, viruses were found in 70% (55/79) of children with clear signs of RTI, in 41% (55/135) with mild signs and in 30% (35/117) without any signs of RTI (p < 0.001).ConclusionsPositive PCR tests for respiratory viruses, particularly picornaviruses, were frequently detected in apparently healthy children attending day care. Virus detection rates were related to age, presence of clinical signs of RTI, location in day care and season.

We investigated dietary variation during three winters in female Svalbard reindeer (Rangifer tarandus platyrhynchus) on the basis of rumen samples from animals culled in early and late winter during 2000-2002. The winter diet was compared to a late summer diet on the basis of samples from 1999, with variations in dietary composition in early and late winter among years also investigated. We conducted our assessment by identifying plant fragments in rumen samples using a point identification method. The effects of season and year on dietary composition of females were tested using non-parametric multifactorial MANOVA. During early winter, the diet seemed to be a function of both plant availability and plant quality, with the latter also highly important for the summer diet. During late winter, restricted forage availability due to snow cover seemed to have influenced dietary composition. We also found dietary variation among years both in early and late winter. We hypothesize that climatic effects previously found on the growth rate of Svalbard reindeer populations may be mediated through interannual variation in their diet where increased summer precipitation and winter rain may affect the dietary composition through decreased availability of forage biomass.