Temporal variability in native plant composition clouds impact of increasing non-native richness along elevational gradients in Tenerife

Introductions of non-native plant species and their rates of expansion into novel environments are rapidly climbing, and their impact on recipient community composition is currently not well documented. Under the influence of rapidly intensifying human activity, pathways such as roads support such range expansions, especially in vulnerable mountain regions. Using species composition and abundance data collected in disturbed and natural habitats along three mountain roads covering almost 2500 m in elevation, we investigate how non-native plant species impact temporal change in community composition and spatial community dissimilarity on the island of Tenerife over 14 years. We found that, within communities, the number of both native and non-native species increased over time in disturbed habitats, while non-native species richness decreased in natural habitats. While species composition of communities changed over time, this change was not greater with or without nonnative species, though any signal of non-native species' influence was likely lost due to the surprisingly high variability in the native community. In disturbed roadside habitats, turnover of species over time played a larger role in temporal change in community composition than changes in species' abundances. Despite increases in richness and occurrences along the elevation gradient, non-native species did not spatially homogenize communities. Although impacts of non-native species on temporal changes in community dissimilarity were presently not found, increases in the number of non-native species and their occurrences illustrate the need for long-term monitoring of altitudinal spread in mountain plant communities, especially in anthropogenically disturbed habitats.