Warming summers limit reindeer grazing, weakening herbivory pressure in the mountain tundra
Résumé
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 periods with a maximum air temperature above 13°C -have become more frequent over the last 30 years, now occurring for half of 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.
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