Traditional grazing areas in Europe have declined substantially over the last century. Specifically, in northern Fennoscandia, the grazing land is disturbed by cumulative land-use pressures. Here we analysed the configuration of the grazing land for reindeer and sheep in northern Fennoscandia in relation to the concurrent land-use pressures from tourism, road and railway networks, forestry, industrial and wind energy facilities, together with predator presence and climate change. Our results show that 85% of the region is affected by at least one land-use pressure and 60% is affected by multiple land-use pressures, co-occurring with predator presence and rising temperatures. As such, a majority of the grazing land is exposed to cumulative pressures in northern Fennoscandia. We stress that, if the expansion of cumulative pressures leads to grazing abandonment of disturbed areas and grazing intensification in other areas, it could irreversibly change northern vegetation and the Fennoscandian mountain landscape.

Climate change is predicted to alter species interactions by exposing ecosystems to increasingly frequent and intense warm spells. In the mountain tundra, grazing by large herbivores, particularly reindeer, can limit shrub expansion and preserve Arctic plant diversity. However, the impact of rising temperatures on herbivores themselves remains understudied. 

Here, we combine long-term weather data with spatially-explicit behavioural data from 31 free-ranging reindeer from three Swedish herding districts equipped with GPS, temperature sensors, and tri-axial accelerometers over two consecutive summers to investigate how warming affects grazing. We hypothesise that both heat stress and insect harassment reduce grazing under warm conditions. 

First, we show that reindeer significantly reduce grazing beyond a body surface temperature (T_R) of 20.3°C, likely due to insect harassment. As reindeer speed sharply declines beyond 24°C T_R, our results suggest an onset of physiological heat stress, indicating that warm spells limit grazing through insect harassment, but also overheating. Second, warming also triggers a shift in habitat-use, as reindeer relocate their grazing activity outside their primary grazing land for less favourable high-elevation habitats, further reducing foraging efficiency. These behavioural and spatial shifts result in a net loss of foraging, with no evidence of compensatory grazing. Third, we find that warm spells - defined as 24-hour period with a maximum air temperature above 13°C - have become more frequent over the last 30 years, now occurring during over half the summer.

Overall, this study highlights how thermal discomfort can disrupt and relocate the foraging patterns of reindeer, a keystone herbivore in the tundra. Such reduced herbivory pressure could have severe cascading consequences by accelerating shrubification and contributing to local biodiversity loss. Hence, climate warming does not only alter abiotic conditions, but can also disrupt biotic processes that underpin the resilience of cold ecosystems.

Questions: The relative importance of abiotic versus biotic factors on structuring plant communities is debated, especially in the Arctic tundra where the harsh environment is limiting together with the effects of grazing by reindeer. To understand the relative and interactive effect of abiotic (bottom-up) and biotic (top-down) factors on vegetation in the Swedish mountain tundra, we ask how do bottom-up factors and their interaction affect reindeer grazing activity and vegetation composition?. Location: Summer pastures of Gran reindeer herding district, in Vindelfjällen mountain tundra (northern Sweden). Methods: We surveyed the composition of vascular plants, bryophytes, and lichens across 34 sites (17 north-facing and 17 south-facing) along a grazing duration gradient based on data collected from accelerometers collared on reindeer. Data on the bottom-up factors slope, soil wetness, soil depth, primary productivity and the top-down factor grazing duration were extracted for each of our sampled plots (n = 102). The additive and interactive relation between all factors and vegetation composition and species richness was analyzed using generalized linear models. Results: Reindeer grazed for a longer time in drier than wetter sites, indicating an important interaction between grazing and soil wetness. Bottom-up factors prevailed as the dominant driver of local vegetation patterns, while grazing duration had weak effects on the vegetation. Wetter sites with longer grazing duration had more graminoid species, whereas drier sites with shorter grazing duration had more shrub and lichen species. Conclusions: The study shows that species richness of vascular plants, lichens and bryophytes is related to soil wetness but is also influenced by reindeer grazing intensity. Based on these results, we stress the importance of further investigating the interaction between grazing and soil wetness in order to foresee changes in the tundra vegetation, especially as plant communities might change under altered grazing regimes and future hydrological conditions as an effect of predicted climate change.

1.     The long history of reindeer herding in the Fennoscandian mountains has shaped ecosystems and can help mitigate climate change effects on vegetation. Yet, outdoor tourism is discussed as a major pressure resulting in a changed grazing behaviour, potentially leading to shifts in the tundra plant communities. 

2.     We tagged reindeer with GPS and accelerometers in three reindeer herding districts in northern Sweden, and analysed their occurrence and grazing activity in relation to people, habitat and predators. 

3.     Overall, we found that reindeer presence and grazing activity was negatively associated with human presence, but where there was a positive association between reindeer and human presence, grazing activity remained mostly decreased. 

4.     In all districts, we detected interactive effects between predators and humans, suggesting a “human-shield” effect at play, i.e. reindeer use the proximity to specific types of human infrastructure as a refuge from predation.  

5.     Our study shows that human presence significantly influenced where reindeer chose to be and where they chose to graze, highlighting the need to recognize the human influence on herbivores, even when assessing vegetation change in the tundra. Our findings hence encourage to move beyond a single-driver perspective and to consider multiple, interacting pressures to better anticipate future ecological shifts under climate change.