Bias in Discontinuous Elevational Transects for Tracking Species Range Shifts.

Climate change is compelling species to seek refuge at higher elevations and latitudes. While researchers commonly study these migrations using discontinuous elevational transects, this methodology may introduce significant biases into our understanding of species movement. These potential biases could lead to flawed biodiversity conservation policies if left unexamined. To address this concern, we utilized species distribution data from a novel continuous elevational transect to evaluate the accuracy of discontinuous transect methods. Our analysis focused on how quadrat spacing and survey time intervals affect bias in estimating species range shifts. The results were striking: the widely used settings for discontinuous transects failed to detect 7.2% of species, inaccurately estimated shift distances for 78% of species, and produced an overall error rate of 86%. Wider quadrat spacing increased these error rates, while longer survey intervals generally reduced them. Moreover, discontinuous transects consistently underestimated species shift distances, with this underestimation becoming more pronounced over longer survey periods. Our pioneering assessment of bias in discontinuous elevational transects demonstrates that a 50 m quadrat spacing combined with a 60-year survey interval optimizes monitoring species range shifts for conservation planning. This baseline protocol could be further strengthened through supplementary, frequent surveys targeting high-elevation species-a strategic approach that maximizes accuracy while maintaining cost-effectiveness.