Alien plant species that invade high elevations are generalists: support for the directional ecological filtering hypothesis

Abstract

Questions: The richness of invasive alien plant species tends to decrease with increasing elevation. This pattern may be due to alien plant species requiring traits allowing survival at high elevations (the Abiotic Limitation Hypothesis; ALH). In contrast, the more recent Directional Ecological Filtering Hypothesis (DEFH) suggests that only species with broad environmental tolerances will successfully spread from lowlands (where most introductions occur) to high elevations. Here we test support for the DEFH and ALH along an elevational gradient by asking: (1) are alien species that occur at higher elevations generalists; and (2) do alien species occurring at higher elevations exhibit traits that distinguishes them fromlowland alien species? Location: Sani Pass, Maloti-Drakensberg Transfrontier Conservation Area, South Africa. Methods: A nestedness analysis was conducted to test whether alien species were nested along the elevational gradient, and ANOVA and Chi2 tests (supplemented by resampling procedures) were used to determine if functional traits differed between high- and low-elevation alien species. Results: Significant nestedness of the alien flora indicates that alien species occurring at high elevations are generalists, being widespread across the elevational gradient. Compared to low-elevation aliens, plant height was lower and cold toleranceweaker for high-elevation species. Conclusion: We found support for the DEFH, with the majority of high-elevation aliens being widespread generalists. Overall only two of the 11 functional traits differed between high- and low-elevation alien species, with only one trait supporting the ALH: shorter plant stature at higher elevations. Therefore, complementing nestedness analyses with trait data provides a more nuanced insight into the determinants of alien richness patterns along elevational gradients, and highlights how the two contemporary hypotheses might not be mutually exclusive.