Warm-adapted encroachment outpaces cool-adapted retreat in a hotspot of trailing-edge population diversity

Many populations near low-latitude range margins are declining in response to climate change, but most studies of trailing-edge populations have focused on single species. Using ten years (2014–2023) of avian survey data from a trailing-edge population hotspot in the Appalachian Mountains, USA, we tested the hypothesis that high-elevation communities would experience turnover through thermophilisation, as warm-adapted species near the center of their geographic ranges expand into regions formerly dominated by peripheral populations of cool-adapted species. Three of the nine cool-adapted populations decreased in abundance whereas six species exhibited little change. For warm-adapted populations, one of 16 decreased in abundance, 11 increased, and four exhibited no change. Our results indicate that warm-adapted species are expanding their ranges faster than the rate at which ranges of cool-adapted species are contracting, suggesting that community turnover will lag behind encroachment by warm-adapted species.