Seasonal diet composition of migrating red deer (Cervus elaphus) in temperate forests of the Western Carpathians

Wild ungulates play a pivotal role in shaping temperate Carpathian forests, with their feeding behaviour influencing forest community dynamics, regeneration processes and overall ecosystem biodiversity. Thus, understanding their diet is essential for assessing foraging impacts, improving management strategies and ensuring ecosystem sustainability, especially when facing challenges such as climate change and the continuous increase in deer populations across Europe over the last two decades, which has intensified pressure on forest ecosystems. The aim of our study was to investigate the spatio-temporal variation in red deer diet composition and to analyze key environmental factors driving observed dietary patterns across the Western Carpathians, Slovakia. We found that Carpathian red deer consumed a mixed diet, comprising 29.0% of graminoids and 70.4% of concentrate food items, characteristic of an intermediate feeder within the European landscape. As predicted, red deer primarily consumed concentrate dietary items year-round and did not switch to a grass-dominated diet in winter. We attribute this to the low availability of grasses during winter, which likely prevents red deer from shifting from summer browsing to winter grazing, despite strong selection for grasses in winter. We confirmed that differences in red deer diet composition between low and high elevations were driven by forage availability in these habitats. As forage availability is primarily determined by habitat composition, we concluded that the observed dietary differences resulted from variations in habitat structure rather than from changes in red deer feeding behaviour. Seasonal dietary patterns were influenced by human impact, followed by forage availability, deer density and wolf predation risk. Notably, human impact was the most significant factor shaping the summer diet at both low and high elevations and the winter diet at low elevations. Our findings demonstrate how red deer adjust their foraging behaviour flexibly in response to environmental constraints and provide important insights for management to reduce human impact in favourable habitats during critical life stages and recommends to regulate deer densities to prevent overgrazing of economically valuable regenerating forest stands. Our study further highlights the importance of implementing adaptive forest and wildlife management, which is essential for ensuring the long-term sustainability of forest ecosystems amid future challenges such as climate change and increasing wild ungulate populations.