Upstairs, downstairs: contrasting responses of lowland and highland amphibian communities to climate change and emerging infectious diseases

Climate warming is driving widespread upslope range shifts in mountain amphibians, yet it remains unclear whether these expansions reflect population growth or spatial redistribution from lower elevations. Long-term monitoring of a mountain amphibian community has documented the upslope expansion of thermophilic species, declines of cold-adapted species, and persistent chytridiomycosis following severe population crashes. However, whether the low-elevation populations potentially supplying these colonists have undergone comparable changes in abundance has remained unknown. We analysed ten years of repeated surveys from 22 breeding sites using a hierarchical Bayesian multispecies N-mixture model that accounted for imperfect detection while estimating abundance trends and testing the effects of environmental conditions and the estimated prevalence of Batrachochytrium dendrobatidis (Bd) and Ranavirus (Rv). Most species remained stable or showed uncertain temporal trends, providing no evidence of a community-wide increase in abundance within historically occupied lowlands. A composite habitat gradient associated with water-body permanence, type and substrate was the only environmental predictor with a community-level abundance effect. Although the best-supported model excluded pathogen prevalence, an otherwise identical model including Bd remained competitive, whereas support for Rv was substantially weaker. Our results indicate that recent upslope colonisation is more consistent with spatial redistribution than with widespread increasing abundance across the elevational gradient, highlighting that range expansion alone can provide a misleading picture of population change.