European mountain summits may be gaining plant species as the climate warms, but that apparent boost in biodiversity is masking a growing wave of local extinctions, according to a two-decade survey of European mountain summit plant communities. The losses are greatest among species exposed to stronger warming and can be preceded by declines in abundance, suggesting that shrinking plant cover could provide an early warning of future alpine extinctions. Climate warming is causing mountain plants to shift their ranges. As a result, higher-altitude mountain regions have gained plant species in recent decades as plants from lower elevations have expanded upslope. Yet this apparent increase in richness may hide the decline or disappearance of specialized high-elevation resident species. Previous studies have indicated that these alpine communities contain both climate "winners" and "losers," with high-elevation species being particularly vulnerable to warming. However, the extent to which these declines in abundance foreshadow local or broader extinctions remains unclear.
Leveraging the GLORIA (Global Observation Research Initiative in Alpine Environments) network – the largest systematic monitoring program of climate effects on mountain summit floras – Johannes Wessley and colleagues studied plant communities on 62 European mountain summits across 16 mountain regions, using 4 repeated surveys recording the presence and abundance of plants in permanent plant plots between 2001 and 2022. Wessley et al. looked for species that disappeared between surveys and examined whether these local losses were associated with warming, earlier declines in plant abundance, or a species occurring near the upper edge of its normal elevation range. They found that local extinctions increased over time and were more common in mountain regions where warming was stronger. Species were particularly vulnerable near the lower edges of their elevation ranges and in communities experiencing greater thermophilization, or the increasing dominance of warm-adapted species. The authors argue that, although upward shifts increased local species richness on mountain summits, they came at the expense of loss of higher-elevation species, which may create extinction debts in the future. Importantly, Wessley et al. note that declines in species abundance preceded many local extinctions, suggesting that falling plant cover could provide an early warning of impending losses.
