Do donkeys grazing in woody-encroached Alpine pastures behave as mixed-feeders? A case study from Gran Paradiso National Park

Since the 1950s, land abandonment has led to extensive encroachment of woody species – mainly shrubs and trees – on alpine grasslands, resulting in a marked decline in both forage quality and quantity. In response, silvopastoral systems involving livestock browsing woody vegetation, such as donkeys, have emerged as a promising tool for grassland restoration. The research was guided by three key questions: (1) do donkeys maintain a mixed diet that includes both herbaceous and woody plant species? (2) Are certain plant taxa selectively preferred or avoided? (3) Does the relative consumption of woody species reflect their environmental abundance? Aiming to answer these questions, the study investigated the foraging behaviour of a herd of 18 donkeys grazing an 11-ha pasture in Gran Paradiso National Park (NW Italy) during summer 2023. A total of 6,472 direct observations were conducted at a 15-second recording interval followed by a 20-second pause, with feeding stations defined as the spatial volume extending 2 meters above ground level and a 0.5-meter radius around the animal mouth. Both available and consumed plant species were recorded at species level, unless for the broad category ‘herbaceous species’. Relationships between availability and consumption were modelled using Generalised Additive Models (GAMs) and further explored using cluster analysis, which delineated three groups of plants: preferred, indifferently consumed, and rejected. The results showed that donkeys consumed predominantly herbaceous forage – which accounted for 66 to 96% of their diet – although they were also able to browse woody species and ferns (0-22%). Particularly, they showed a marked preference for Sorbus aria and Rubus idaeus, while largely avoiding other species, e.g. Betula pendula. Furthermore, the consumption of several species – especially Alnus viridis and Athyrium filix-femina – was directly related to their availability in the environment and increased proportionally as these species became more abundant. These results suggest that integrating donkeys into grazing management strategies may help mitigate woody encroachment, support biodiversity, and improve forage quality.