Although alpine environments are considered to be particularly vulnerable to warming, extinctions at mountain summits have rarely been documented. This has been explained by a so-called “extinction debt”, due to demographic inertia as species drift slowly towards local extinction caused by increasingly unfavourable climatic conditions. Another possibility is that extinction is not driven by climate itself, but by the arrival of new species from lower elevations. To disentangle these possibilities, we used a whole-community transplant experiment to tease apart the effect of gradual warming and novel species on population growth (λ). Four alpine plant species were transplanted to three lower sites along an elevational gradient in the Swiss alps, to simulate gradual warming. At each site individuals were planted into whole-community turfs originating from low- and high elevation, representing novel and current ccommunities respectively. Over three seasons (2017-2019), survival, growth, flowering and reproduction was monitored for a total of 1400 individuals across the experiment. By using Integral Projection Models (IPMs) to project λ and Life Table Response Experiments (LTREs) to gain mechanistic insight into changes in λ, we asked how alterations in temperature and community identity change l and what vital rates underly this change.
Results/Conclusions
Climate warming decreased per capita population growth (λ) by 0.07/ °C across all species when interaction with current species. Novel species from lower elevations additionally suppressed λ with 0.11 on average across species and warming levels. Our study shows that warming and novel species contribute to extinction debt in alpine plants that are faced by warming. Moreover, under initial levels of warming, the relative contribution of novel species to extinction debt was larger in relation to climate warming. With warming levels above 2 °C, our results instead suggest that detrimental effects of climate will amplify extinction risk independently of novel species establishment. Given warming of 2.1 °C in Switzerland over the last 150 years our results suggest that lags in novel species establishment at high elevations might be the main cause of extinction debt in mountain plants up to now. Decles in λ was driven by decreased survival and recruitment success under both warming and with novel species.