Mountain ecosystems have so far been relatively shielded from the negative effects of biological invasions. Possible reasons for this include the fact that human activity and disturbances have been less intense at high elevation, and that most non-native species are introduced at low elevations, and so few species in the regional non-native species pool are able to thrive under conditions at higher elevations. This situation is expected to change with increasing human pressures at high elevation and climate warming. Until now, however, we have lacked the necessary data to assess how the abundance and richness of non-native species are changing in mountains, and how quickly.
We used a dataset collected by the MIREN Network (www.mountaininvasions.org) to ask whether the richness, abundance and elevational limits of non-native plants in mountains are increasing over time, and how these patterns vary across elevation gradients, between regions and between different functional groups of species. We implemented a standardized protocol in 10 regions to sample non-native plant richness and abundance along mountain roads, and into semi-natural vegetation. The first survey was conducted in 2007, and repeated in 2012 and again in 2017/2018 in 6 regions, or only two of these years for another 5 regions.
Results/Conclusions
The number of non-native plants in mountains has increased over 10 years of sampling in most of the regions included in our study. The elevational limits of individual species have also tended to increase, with changes being more pronounced at low elevation. This observation is consistent with a predominantly low-elevation introduction pathway for new species, followed by subsequent spread towards higher elevations. The magnitudes of elevational shifts were unrelated to species traits. Species reaching higher elevations appeared to be more in equilibrium with their climatic niche limits, even though population turnover was very high towards the upper elevational edge. Detailed population modelling of Solidago canadensis in Switzerland supported this view; despite a rapid increase in population abundance over 5-years, its elevation limits were stable and emerged from a dynamic balance of increasing extinction and decreasing colonization probability.
Our findings suggest that once introduced, non-native plants can rapidly expand their distributions to higher elevations, and that subsequent range-filling is ongoing. While plant invasions are currently still not widespread at the highest elevations, there is a need to monitor the continuing accumulation and upwards expansion of novel species at lower elevations, and where necessary intervene to protect high elevation mountain ecosystems.