Herbivores in the tundra interact with vegetation through several mechanisms, especially defoliation, trampling and nutrient addition through urine and faeces. Through these mechanisms, herbivores drive shifts in plant species composition, richness and diversity. As reindeer effects on vegetation accumulate over time, they might cascade to other trophic levels, but how and when this happens is poorly understood. Since it is methodologically demanding to measure biodiversity across spatial gradients, an alternative approach is to assess it indirectly via biodiversity indices of vascular plants. Values from the Index of Biodiversity Relevance were coupled with vegetation data from a network of 96 fenced and paired grazed plots across Fennoscandia. We analysed the role herbivory has on plant richness and diversity, and on the number of organisms that depend on the vegetation according to the index values. We also explored how herbivores affect the competitive effects of shrubs on other plants since the dominance of a vegetation type links directly to biodiversity. Vegetation richness and diversity did not present any differences between treatments, yet reindeer had an increasing effect on plant diversity when testing the interaction between grazing and herbaceous vegetation. Three out of six biodiversity indexes were higher in fenced plots indicating a higher number of interactions between plants and organisms from other trophic levels. Finally, herb abundance was negatively related to shrubs in both treatments but with a faster decline in the absence of herbivores, suggesting that herbivory increases plant diversity and decreases the diversity of other taxa by reducing shrub abundance. This study highlights the importance of maintaining herbivore populations in the Arctic to prevent the expansion of climate-driven biodiversity into the tundra. The effect of herbivores on ecological communities is not merely a product of plant diversity but can be quantitatively and qualitatively different.

The effects of climate change in the Arctic are particularly pronounced since temperatures have risen nearly three to four times as fast as in the rest of the planet. Shifts in climatic patterns enable the expansion of temperature-limited vegetation at a global scale to higher latitudes and elevations. The purpose of this study is to predict the distribution of pine trees (Pinus sylvestris) across Fennoscandia by drawing from three distinctive datasets. I ask (i) How will the distribution of pine trees will respond to climate change in the next 50 years? (ii) Which method used to collect data is better at predicting the distribution of pine? Three datasets on pine presence together with environmental data were used to model pine distribution with Generalized Linear Models. The first dataset belongs to the Swedish National Forest Inventory and the second is from a Swedish online portal where citizens report their observations of species. The third was compiled by setting up a network of vegetation plots along an elevation gradient in Sweden and Norway. Current and future environmental data was sourced from the Coupled Model Intercomparison Project. The probability of pine presence in general increased with temperature and decreased with precipitation. Therefore, the model forecasts that pine will expand in distribution to areas of higher elevation. The citizen science dataset was superior in predicting pine distribution due to the large number and the wide spatial distribution of observations. The conservation of the tundra and the unique ecosystem process taking place in this area will be threatened by the encroachment of the evergreen treeline which is driven by climate, and citizen science holds unique importance for wide spatial and temporal ecological research.