Beyond density: Environmental and dietary drivers of endoparasite burden in overabundant red deer populations.

Red deer (Cervus elaphus) populations are increasing in both number and range, raising concerns that parasite abundance, such as gastrointestinal nematodes, may be influenced by host density. Research exploring this relationship in wildlife remains scarce, particularly in Mediterranean ecosystems. This field study aimed to investigate the impact of red deer overabundance on gastrointestinal nematode parasitism in two Mediterranean areas: Quintos de Mora (central Spain) and Muela de Cortes (eastern Spain). We used four enclosures, comprising two overabundance scenarios: high density (~ 45–58 deer/km²) and hyper density (~ 93–106 deer/km²). Parasite burdens were initially assessed using individual fecal samples, revealing low nematode loads across all individuals. Parasite dynamics were then monitored through pooled fecal samples over a 15–18-month period, while deer diet composition was analyzed to provide ecological context. Nematode egg excretion declined over time in all enclosures, reaching undetectable loads in three enclosures by 11 months after deer introduction, regardless of host density. Diet analyses indicated a clear dominance of woody vegetation, accounting for up to 86% of the diet in Quintos de Mora and up to 68% in Muela de Cortes, while the proportion of graminoids remained minimal. This depletion of herbaceous structure and the dominance of tannin-rich woody species containing high antiparasitic properties could contribute to reduced low parasite loads. Additional work testing the hypothesis would benefit from increased replication and higher initial worm loads. Clinical trial number: Not applicable.