Human activity facilitates altitudinal expansion of exotic plants along a road in montane grassland, South Africa

Abstract

Question: Do anthropogenic activities facilitate the distribution of exotic plants along steep altitudinal gradients? Location: Sani Pass road, Grassland biome, South Africa. Methods: On both sides of this road, presence and abundance of exotic plants was recorded in four 25-m long road-verge plots and in parallel 25 m × 2 m adjacent land plots, nested at five altitudinal levels: 1500, 1800, 2100, 2400 and 2700 m a.s.l. Exotic community structure was analyzed using Canonical Correspondence Analysis while a two-level nested Generalized Linear Model was fitted for richness and cover of exotics. We tested the upper altitudinal limits for all exotics along this road for spatial clustering around four potential propagule sources using a t-test. Results: Community structure, richness and abundance of exotics were negatively correlated with altitude. Greatest invasion by exotics was recorded for adjacent land at the 1500 m level. Of the 45 exotics, 16 were found at higher altitudes than expected and observations were spatially clustered around potential propagule sources. Conclusions: Spatial clustering of upper altitudinal limits around human inhabited areas suggests that exotics originate from these areas, while exceeding expected altitudinal limits suggests that distribution ranges of exotics are presently under estimated. Exotics are generally characterised by a high propagule pressure and/or persistent seedbanks, thus future tarring of the Sani Pass may result in an increase of exotic species richness and abundance. This would initially result from construction-related soil disturbance and subsequently from increased traffic, water run-off, and altered fire frequency. We suggest examples of management actions to prevent this.

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

“…All the alien plant species recorded on the Pass over 8 yr (Kalwij et al. , ) were considered to comprise the high‐elevation alien species pool. In the 2014 survey, the Pass was additionally divided into 19 elevational bands within which the presence or absence of all alien species was recorded for nestedness analysis.…”

“…The Pass spans a broad range of conditions (from 1307 to 2873 m a.s.l.) with more densely inhabited and heavily transformed lower elevations potentially acting as a source of invasive species propagules for higher elevations (Kalwij et al 2008b). The site floristically forms part of the Drakensberg Alpine Centre (DAC; Carbutt & Edwards 2003), and receives summer rainfall of ca.…”

“…The elevation of the three highest individuals of each alien species were recorded while both sides of the road were surveyed by observers walking down the Pass (see Kalwij et al 2015 for more details). All the alien plant species recorded on the Pass over 8 yr (Kalwij et al 2008b(Kalwij et al , 2015 were considered to comprise the high-elevation alien species pool. In the 2014 survey, the Pass was additionally divided into 19 elevational bands within which the presence or absence of all alien species was recorded for nestedness analysis.…”

Annual monitoring reveals rapid upward movement of exotic plants in a montane ecosystem

“…We used a paired t-test to compare our observed upper elevational limits with the expected, known upper elevational limits for each species (sensu Kalwij et al 2008). These expected elevational limits were based on the databases and checklists of the national herbarium, and are the most comprehensive and up-to-date datasets available for the study area.…”

The emerging invasive alien plants of the Drakensberg Alpine Centre, southern Africa

“…given the disturbance factor associated with the study area, the approach takes both disturbed and undisturbed habitats into account because invasive alien plants are able to dominate all stages of succession; early (~ suppression) and late (~ tolerance) successional strategies are contingent upon the specific competitive strategy employed and can shift in invaded ecosystems over time (see MacDougall & Turkington 2004). Furthermore, disturbed habitats such as mountain pass roads can extend the distribution of alien plants beyond reasonable altitudinal expectations (Kalwij et al 2008). A number of sites have been proposed for the fieldwork component ( Figure 1; Table 1).…”