Combining environmental DNA data with oceanography, life history and ecology for detecting climate‐induced range shifts

Aim Tropicalisation and other climate‐induced range shifts are rapidly restructuring global biodiversity patterns. The detection of range shifts is often complex and requires big‐data approaches. Environmental DNA (eDNA) monitoring is emerging as a powerful method for assessing biodiversity changes at unprecedented spatial and temporal resolutions. While eDNA‐based methodologies continue to evolve, the impacts of species traits and eDNA dynamics are rarely measured, though they likely affect our eDNA data interpretation. Here we combine diverse methodologies to better understand processes affecting eDNA data and to elucidate how eDNA dispersal influences the interpretation of eDNA results in a tropicalisation context. Location Baja California Peninsula, Mexico. Methods We combined semi-quantitative field surveys with eDNA sampling, quantitative PCR assays of different amplicon sizes, assessment of spawning period, and oceanographic modelling. We used as a model system the range-retracting, marine gastropod Tegula gallina, which we sampled across a region that is experiencing tropicalisation. Results We detected eDNA of T. gallina across both its current range (i.e., occupied region) and > 250 km beyond the species' range limit (i.e., unoccupied regions). Shorter amplicons were detected more consistently than larger targeted fragments across the unoccupied regions. Tegula gallina was likely spawning at the time of eDNA collection, and oceanographic modelling revealed possible transport of eDNA (and early life-history stages) beyond the species' range limit. Main Conclusions Our study reveals that eDNA signals can be detected over substantial spatial scales, which can likely be explained by the interaction among spawning period, larval dispersal, and eDNA dispersal. The varying detection sensitivity of the different amplicon sizes may be due to eDNA decay during transport. Our results highlight the need for integrative approaches combining eDNA detection, life-history traits, field surveys of living organisms, and modelling to uncover the full potential of eDNA data, especially for ecological and conservation applications.