Rewilding reshapes gut microbiomes and parasite exposure in European bison: a 17‑month release from Wilder Blean

Reintroductions can restore lost ecological processes, but managers require practical health indicators to track the acclimation of released animals. We longitudinally profiled the gut microbiomes of European bison ( European bison bonasus ) released to the Wilder Blean area (Kent, UK), sampling three adult females before and after release, and a post‑release male and calf. Using V3-V4 16S rRNA gene sequencing, we quantified alpha‑ and beta‑diversity, identified differentially abundant taxa, and screened faeces for Cryptosporidium , Enterocytozoon bieneusi and Blastocystis . Post‑release, adult microbiomes shifted significantly (PERMANOVA P = 0.001) and consistently across all examined animals. Calf microbiome profiles transitioned from early‑life communities to an adult‑like state concurrent with weaning. Parasite screening via separate PCR and qPCR showed that Cryptosporidium positivity declined in females from 36% pre‑release to 13% post‑release, whereas E. bieneusi emerged only after release (∼10% of samples), with multiple genotypes detected. These patterns are consistent with dietary and environmental turnover following release, and they highlight opportunities for using microbiome and parasite metrics as complementary, non‑invasive indicators of rewilding progress. We recommend reporting simple, management‑relevant indicators, archiving sequence data, and documenting soft‑release design and supplementary feeding info to aid interpretation. Integrating routine faecal microbiome and parasite monitoring into rewilding programmes can support adaptive management, inform supplementary feeding decisions, and strengthen biosecurity risk assessments.