Seasonal differences and downstream transport of terrestrial vertebrate eDNA: insights from integrated camera-trap survey and water eDNA metabarcoding

Effective biodiversity conservation requires efficient, comprehensive, and cost-effective monitoring tools. While environmental DNA (eDNA) metabarcoding is widely used for aquatic biodiversity assessments, its application for monitoring terrestrial biodiversity remains relatively underexplored. In particular, the effects of seasonal variation on eDNA sampling efficacy and the downstream transport distances of terrestrial vertebrate eDNA in river systems are not well understood. We conducted a year-long survey (April 2023–April 2024) that combined camera trapping and eDNA sampling in the southwestern mountains of China, a global biodiversity hotspot. Water eDNA samples were collected during low-rainfall (October–November 2023) and high-rainfall (April 2024) seasons. We compared seasonal effectiveness, evaluated costs, and developed an index to assess the relative importance of eDNA sampling sites. We also analyzed the elevational distributions of species detected by both methods and evaluated pairwise β-diversity patterns in relation to geographic distance to estimate the downstream transport distances. Our survey resulted in 143 camera trap deployments and 88 eDNA samples, yielding detections of 99 terrestrial vertebrate species. Across both seasons, eDNA and camera trapping exhibited high complementarity in species detection, with eDNA proving particularly cost-effective during the high-rainfall season. The relative importance of sampling sites varied between the low- and high-rainfall seasons. Terrestrial vertebrate DNA was transported downstream over distances of approximately 10–15 km. These findings provide valuable insights into sampling design and result interpretation in terrestrial vertebrate eDNA studies, underscoring the potential of eDNA metabarcoding as a reliable tool for monitoring terrestrial vertebrate biodiversity.