Exotic plants of Sani Pass, Drakensberg. Insights from fourteen years of monitoring
Author: Jesse M Kalwij
Summary
Fourteen years of monitoring showed several interesting patterns: (i)
invasive and exotic species can occur at higher elevations and in larger
numbers than expected, (ii) road tarring has an impact on exotic species, but
this impact is species and location dependent, and (iii) initial habitat
restoration helps mitigate the impact of exotic species, but follow-up measures
are essential. Specifically, the upgrading of the Sani Pass road appears to
have mitigated the establishment of exotic species. Interestingly, some
small-sized populations have disappeared, probably due to local extinction,
while new exotic species such as Lespedeza cuneata (Sericea lespedeza;
poor man's lucerne) have established in the area. Adequate and persistent
management is needed to prevent a further spread of established invasive
species.
The uKhahlamba Drakensberg Park is a major
biodiversity hotspot harbouring many endemic or threatened flora and fauna
species. One of the most important roads in this area is the Sani Pass road.
This road is approximately 34 km long — starting from the outskirts of Himeville at an elevation of 1528 m, to
Sani Top at 2873 m — and
is the only connection between the eastern highlands of Lesotho and South
Africa. The road then continues across the Koti se Phola Pass at 3240 m to
Mokhotlong. Due to the ruggedness of the terrain, the 8km-long top section
between the South African border post, at 1952 m and Sani Top is open for 4×4 vehicles, bicycles, and pedestrians only (Figure 1).
The people in eastern Lesotho depend heavily on this road for access to
services and infrastructure in KwaZulu-Natal. At the same time, the scenic
route and unique biodiversity of the top section of the Sani Pass road attracts
tourists and nature enthusiasts, providing an importance source of income for
local tour operators and other businesses. The Sani Pass road is therefore
essential to various stakeholders.
 |
| Figure 1. View of the top section of the Sani Pass road during summer (January 2012). |
The Sani Pass road has a vibrant history; initially established as a donkey path to facilitate trade in 1913, and open to 4×4 vehicles since 1949 (Van Rensburg 1956). After WWII, the course of the road changed considerably. Traces of the road’s dynamic past such as abandoned buildings, overgrown paths and remnants of old bridges are still visible throughout the landscape (Figure 2). Back then, the road was in quite good shape, as illustrated in a magazine article that depicted a Land Rover Defender towing a Sprite caravan to the summit in 1964. Despite such improvements, the Sani Pass road continued to require constant maintenance.
 |
| Figure 2. The view from the north slope of the Mkomazana valley gives a good impression of the former track of the Sani Pass road. |
Nearly two decades ago, motivated by the need for
local socio-economic development, a major upgrade of the Sani Pass road was
announced (Department of Transport 2006).
A phased upgrade would result in a complete overhaul and tarring of the road,
all the way up to Mokhotlong in Lesotho. On the South African side, this
comprised the tarring from Himeville to the old trading post (phase 1), then to
the South African border post (phase 2), and finally to the Sani Top (phase 3).
Such improved connection between eastern Lesotho and KwaZulu-Natal would
benefit various stakeholders and provide the area with a much-needed economic
boost. However, the project was not positively received by all stakeholders (GIBB 2012). For example, tour operators and
environmentalists feared that tarring the Sani Pass road would diminish its
scenic appeal, and that road construction work would be detrimental to local
biodiversity. After public consultation, phase 2 of the Sani Pass upgrade
received approval in May 2014. The start and finish dates for phase 3 have not
yet been confirmed.
The lowest section of the road was largely
straightened and tarred between 2007 and 2012, but the indigenous vegetation of
the verges was not restored. On the Lesotho side, the section between Sani Top
and Mokhotlong was tarred between 2014 and 2016. On the South African side, phase
2 took long to complete due to the necessity of constructing numerous culverts
and bridges first (Figure 3). By 2022, the tarring and verge rejuvenation of
this section was largely finished. To date, the steepest and most challenging
8km section between the South African border post and Sani Top remains
unchanged, except for the construction of gabion walls along the most
erosion-prone sections.
 |
| Figure 3. The crossing at the foot of the Fountain of Youth, shown here in a picture from 2009, has been replaced by a bridge, but remains aesthetically appealing and easily accessible for visitors. |
Exotics are species that have reached an area with the
help of humans, either intentionally or accidentally, and that are able to
sustain their population without human assistance. Invasive species are exotics
too and have a severe negative environmental or socio-economic impact
(Richardson et al. 2000). Typically, improving a road will lead to more traffic
and more visitors, which in turn results in the introduction and establishment
of more invasive and exotic species. On the other hand, road tarring reduces
erosion and verge disturbance, which means fewer opportunities for ruderal
species — plant species that colonize disturbed habitats — to establish (Kalwij et al. 2008a). Since many exotic plants
are short-lived species with a ruderal life strategy, road tarring could thus
actually result in fewer exotic species occurring along the road, despite a
short-term increase in disturbance. Interestingly, no long-term ecological
research exists on the impact of mountainous roads before and after tarring.
Therefore, the tarring of the Sani Pass road provided a unique opportunity to
monitor its impact on biodiversity, especially on invasive and exotic plant
species.
 |
| Figure 4. Road verge maintenance disturbs the soil, facilitating the establishment of ruderals as illustrated by the abundance of Phytolacca octandra (forest inkberry) seedlings here. |
Inspired by the potential impact of the road tarring
on the ecosystem, I started monitoring the invasive and exotic plant species in
the verges of Sani Pass road in close collaboration with Mark P Robertson and Berndt Janse van Rensburg.
This monitoring programme has been running since 2007. Each January, we walk
down the Sani Pass road with a group of experienced plant ecologists to record
the highest elevation occurrence of each exotic plant species. Occasionally, we
also monitor the exotics of the lowest section of Sani Pass road, and of the
section between Sani Top and the Koti se Phola Pass. The advantage of such a
monitoring programme is that it gives us detailed insight in the annual
variation in exotic plant species colonisation and disappearance. After all,
some species may not be easily visible each year, while others may only be
present for a limited time before disappearing again. Trends in exotic species
number and elevation help us to determine the rate at which exotics colonise
the area.
 |
| Figure 5. Each year, a team of experienced plant ecologists walks down Sani Pass road and records the highest elevation for each exotic plant species. Photo credit: Mark Robertson, 2010. |
To our surprise, we found many more invasive and
exotic plant species along the Sani Pass road than expected: between 2007 and
2022, we recorded some 170 exotic species. Many of these exotic species were
found at a higher elevation than previously known. For
example, we observed the invasive alien Cirsium vulgare (spear thistle) at an elevation of 2687 m, which was substantially higher than the
expected upper elevation limit of 2134 m (Kalwij et al 2008b). Moreover, exotics were ascending the Sani Pass at a
much faster rate than expected: 24.5 m per year for annuals, and 9.7 m per year
for perennials (Kalwij et al. 2015). To
our concern, we also found small populations of potentially invasive species that had not been observed
in this area before such as Solidago gigantea (giant or late goldenrod; Kalwij et al. 2014) and Parthenium
hysterophorus (famine weed; Turner et al.
2021). Both populations were successfully removed by us. Most exotics were found in areas that were disturbed by humans,
for example near dwellings or access gates (Steyn et al. 2017). Indeed, the lowest section of the
Sani Pass road is mainly characterized by invasive and exotic plants. On the
other hand, relatively few exotics were found on the Lesotho side because of
the harsh environmental conditions of the highland plateau (Figure 6).
 |
| Figure 6. We found that nearly half of the exotic species in Sani Pass occurred at a higher elevation than expected. For example, we observed Tagetes minuta (khaki weed) at an elevation of 3111 m, which was much higher than the previously recorded 2425 m in southern Africa (Kalwij et al. 2008b). |
The tarring of the road has reduced the habitat for
invasive and exotic species. In places where the road verge was sufficiently
restored with indigenous plants, the number and abundance of exotics was low.
This is especially noticeable for the section of road between the Mkomazana Mountain cottages and the South African border post, where the abundance of exotics in
the verges is very low.
Regular follow-up of habitat restoration measures is
essential, especially in the early stage of this process. A lack of adequate
follow-up measures gives perennial invasive species an opportunity to
re-establish, undoing initial costly habitat restoration. For example, the road
verge between Himeville and the ruins of the old trading post was not restored
with indigenous vegetation, providing habitats for emerging undesired species.
These include emerging exotics, such as Lespedeza cuneata — which has recently
been observed as far up the road as the ruins of the Goodhope trading store —
while invasive Acacia dealbata (silver wattle), Cotoneaster spp.
(cotoneasters), Ligustrum spp. (privets) and Rubus cuneifolius (American bramble) remain
locally dominating as far up as the South African border post. The presence of
invasive and exotic species in high elevation verges is cause for concern,
because these species may invade the pristine grasslands of the uKhahlamba
Drakensberg Park.
 |
| Figure 7. The Sani Pass road verges are locally dominated by invasive species such as Acacia dealbata, Cotoneaster spp., Ligustrum spp., and Rubus cuneifolius as illustrated in this photo from 2016. |
The tarring of the Sani Pass road did not result in a
dramatic increase of invasive and exotic species. Extensive habitat and
vegetation restoration efforts have given the indigenous vegetation a major
advantage to re-establish before exotics did. This is especially the case for
phase 2 — the section between the old trading post and the South African border
post — where only few exotics remain. This demonstrates that adequate and
persistent restoration can minimise the impact of road construction on the adjacent
ecosystem, if planned and implemented by ecological professionals.
Acknowledgements
This study would not
have been possible without the support of Mark P. Robertson (University of
Pretoria), Berndt Janse van Rensburg (The University of Queensland), and Peter
C. le Roux (University of Pretoria).
Lisa-Maricia Schwarz
and Peter le Roux kindly provided feedback on an earlier version of this
article.
References
Department of Transport. 2006. Keynote address by Minister of Transport J.T. Radebe, MP, at the joining hands to develop a road that brings development, Sani Pass road upgrade and Border Relocation Joint Project launch. Ministry of Transport.
GIBB. 2012. Environmental Impact Assessment for the Proposed Upgrade of the Sani Pass Road (P318): Phase 2. Final Environmental Impact Assessment Report. DEA Ref. No.: 12/12/20/1184, Department of Transport, Durban.
Kalwij, J. M., S. J. Milton, and M. A. McGeoch. 2008a. Road verges as invasion corridors? A spatial hierarchical test in an arid ecosystem. Landscape Ecology 23:439–451. https://doi.org/10.1007/s10980-008-9201-3
Kalwij, J. M., M. P. Robertson, and B. J. Van Rensburg. 2008b. Human activity facilitates altitudinal expansion of exotic plants along a road in montane grassland, South Africa. Applied Vegetation Science 11:491–498. https://doi.org/10.3170/2008-7-18555
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Kalwij, J. M., C. Steyn, and P. C. le Roux. 2014. Repeated monitoring as an effective early detection means: first records of naturalised Solidago gigantea Aiton (Asteraceae) in southern Africa. South African Journal of Botany 93:204–206. https://doi.org/10.1016/j.sajb.2014.04.013
Richardson, D. M., P. Pyšek, M. Rejmánek, M. G. Barbour, F. D. Panetta, and C. J. West. 2000. Naturalization and invasion of alien plants: concepts and definitions. Diversity and Distributions 6:93–107. https://doi.org/10.1046/j.1472-4642.2000.00083.x
Steyn, C., M. Greve, M. P. Robertson, J. M. Kalwij, and P. C. Le Roux. 2017. Alien plant species that invade high elevations are generalists: support for the directional ecological filtering hypothesis. Journal of Vegetation Science 28:337–346. https://doi.org/10.1111/jvs.12477
Turner, S. C., K. J. Esler, and J. M. Kalwij. 2021. Road verges facilitate exotic species expansion into undisturbed natural montane grasslands. Applied Vegetation Science 24:e12615. https://doi.org/10.1111/avsc.12615
Van Rensburg, B. 1956. Kewertjie was in '56 eerste uit teen Sanipas. Beeld.
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