High-elevation specialists increasingly disappear from European mountaintop floras

Re-survey and monitoring studies have demonstrated that species numbers on European mountaintops have been rising over the recent decades, and that this increase is most likely driven by the warming climate. However, to what extent rising species numbers are the result of a balance that also involves losers, and whether there are specific features that characterize these losers, remains elusive. Here, we use data from four consecutive surveys of the GLORIA Europe long-term monitoring project to analyse species’ local extinction rates in the period 2001 – 2022 at 900 1 m² plots distributed across 62 mountaintops situated in 16 different European mountain ranges that span a gradient from Mediterranean to boreal biomes. We find that, on average across all 900 plots, 21% of the species recorded on a plot in 2001 were not redetected in the same plot in 2022. Local extinctions have occurred in all three re-surveys (2008, 2015, 2022), but the frequency of these extinctions has increased over time, as well as with greater magnitude of climate warming. Simultaneously, the total number of species per plot and mountaintop as well as total vegetation cover have also increased over time. Species adapted to cooler conditions than the average other species found on a particular plot or summit in 2001 were more likely to disappear from a plot or summit in the subsequent resurveys. Local extinction from a plot in one survey was moreover significantly correlated with a decline in abundance in the preceding monitoring period, and even more so with a continuous decline in the two preceding monitoring periods, indicating that a steady decline is an early warning signal of eventual disappearance, despite considerable abundance fluctuations across all species and plots. These findings demonstrate that increasing total plant species richness and vegetation cover of high-elevation plant assemblages mask an accelerating but so far neglected loss of species which is biased towards high-mountain specialists.