4. the StatuS of aLieN SPeCieS
5.5. impact of invasions
5.5.1. impacts on surface water runoff and groundwater by primary catchment
The adverse impacts of alien plant invasions on water flows have provided a strong argument for the control of invasive plants (le maitre et al., 1996; van Wilgen, cowling & burgers 1996). le maitre, Versfeld & Chapman (2000) estimated that the total reduction in runoff due to invading alien plants was about 3 300 million m3 per year, or about 7% of the country’s mean annual runoff. about a third of this estimated water use, by volume, was accounted for by invasive plants in the Western cape, followed by KwaZulu-natal (17%), the eastern cape (17%) and mpumalanga (14%). This section summarises current estimates of the impacts on water flows for primary catchments and biomes and highlights the species with the greatest impacts. The reductions take the form of changes in runoff from invaded dryland areas due to increased evaporation, and evaporation of groundwater from invaded river floodplains (riparian invasions) and from invaded areas with groundwater in aquifers accessible to root systems (groundwater). The total reduction is expressed as a proportion of mean annual runoff because all these reductions ultimately result in a reduction in surface water runoff as measured in rivers.
le maitre et al. (2016) subsequently used new information on the distribution of invasive alien plants, and improved flow reduction models, to put forward a revised estimate of 1 444 million m3 per year, or 2.9% of the naturalised mean annual runoff (less than half of the 3 300 million m3 per year estimated in 2000). Two main factors accounted for the difference between the estimates of le maitre, Versfeld & Chapman (2000) and those of le maitre et al. (2016). The first was a decrease in the estimated unit-area flow reduction to 970 m3 per ha per year compared with 1 900 m3 per ha per year estimated in 1998, largely due to refinements of the models. The second was the use of a smaller estimated invaded area (alien plants that covered 1 million ha compared to the 1.76 million ha used in the 2000 assessment). This was due to the use of a new alien plant distribution data set (Kotzé et al., 2010) that excluded south africa’s arid interior and thus the entire nama Karoo, almost all of the succulent Karoo and desert biomes, about a third of the savanna, and half of the grassland biome. This meant that the 2016 figure for water use by invasive plants was an under-estimate.
in addition, the revised estimate was also considered to be an underestimate by le maitre et al. (2016) because the extent and impacts of riparian invasions had been underestimated. The estimate of le maitre et al. (2016) of 1 444 million m3 per year was based on the mapped data showing that only 4–6% of invasions of some of the major contributing taxa (Acacia mearnsii, black wattle, Eucalyptus, gum trees, Populus, poplar trees, and Salix, willows) were found in riparian zones (where water use is higher). however, the actual proportion of these taxa in riparian zones is probably much higher, and this could increase the estimate by as much as 70%, from 1 444 to 2 444 million m3 per year.
ChaPter 5IThe sTaTus of invaded areas
Impacts on surface water runoff by primary catchment: The largest reductions (over 5% of mean annual runoff) were in the Western cape (catchments g, h and K), the eastern cape (catchments K, m and r), and KwaZulu-natal (catchment u) (Table 5.3). only about 5% of the orange river system (catchment d) was mapped, as was only about 33% of the vaal river system, so the total reductions in these catchments were significantly underestimated.
The main difference from the 2000 estimate is the much greater estimated reductions in catchments in the eastern cape (where alien plant invasions were inadequately accounted for in the 2000 estimate).
tABle 5.3 The estimated extent of reductions in surface water runoff due to invasive alien plants in South Africa’s primary catchments. Table adapted from le Maitre et al. (2016). condensed ha is the equivalent area occupied at a canopy cover of 100%
(i.e. 50% cover on 10 ha = 5 condensed ha). See figure 5.3 for the location of primary catchments.
priMAry
CAtCHMent river SySteMS eStiMAted invASion
level (CondenSed HA) eStiMAted reduCtion (MillionS oF m3)
eStiMAted reduCtion (% oF MeAn AnnuAl runoFF)
A crocodile-limpopo 86 510 24.44 1.06
B olifants-letaba 123 328 61.79 2.13
C vaal 138 557 64.25 1.53
d orange 54 383 31.57 0.46
e olifants-doring 4 825 3.65 0.31
F namaqualand coast 795 0.00 0.02
g berg-agulhas 92 970 111.36 6.04
H breede-goukou 45 164 126.21 6.11
J gouritz 25 438 11.69 1.86
k hartenbos-Kromme 60 951 102.51 8.43
l gamtoos 24 228 10.86 2.09
M swartkops 23 662 11.64 6.46
n sundays 39 906 0.89 0.34
p bushmans 12 432 3.31 1.99
Q great fish 30 385 4.83 0.90
r Keiskamma-nahoon 45 414 42.92 7.41
S great Kei 59 130 46.58 4.49
t umbashe-umzimvubu 220 942 321.96 4.51
u umzimkulu-umvoti 111 698 154.35 5.03
v Thukela 81 139 100.87 2.60
w umfolozi-Pongola 154 984 148.66 2.31
x incomati 58 025 59.19 1.90
total 1 494 867 1443.56 2.88
Tus of biological invasions and Their managemenT in souTh africa2017
Figure 5.3 estimates of the reductions in mean annual runoff (MAR) due to invasive alien plants in the quaternary catchments of South Africa. capital letters refer to primary catchments. The quaternary catchments that were completely excluded are shown in grey; many others were only partially mapped; the kruger National Park was also excluded. Map: D le Maitre unpublished data.
Impacts on surface water runoff by biome: although some biomes were excluded or only partially mapped by Kotzé et al. (2010), the data show that while the grassland biome and the wetter areas of the savanna biome (i.e. excluding the Kalahari) have the most extensive invasions, the most heavily invaded ones are the indian ocean coastal belt and fynbos. The invasions and impacts for the forest biome (Table 5.4) are overestimated, due to mapping scale mismatches. This means that the greatest percentage reductions are found in the indian ocean coastal belt and in the fynbos biome. however, the volume of the reduction for the grassland is of particular concern because the surface runoff from this biome is critical for water supplies to gauteng, the eThikweni region and mangaung, as well as for power generation and much of the irrigated agriculture in south africa.
ChaPter 5IThe sTaTus of invaded areas
tABle 5.4 The estimated impacts on the annual surface water runoff of all invasions in the biomes included in the landscape mapping for the RSA by kotzé et al. (2010). MAR = mean annual runoff.
StAtiStiC
BioMe AlBAny
tHiCket ForeSt FynBoS grASSlAnd indiAn oCeAn CoAStAl Belt
SAvAnnA (wetter areas only)
total reduction (million m3/yr) 23 12 365 621 113 309
MAr (million m3/yr) 659 66 5 213 16 709 1 509 7 726
reduction (% MAr) 3.48 18.36 6.99 3.72 7.52 4.00
The available estimates of the impact of invasive plants on surface water runoff from catchments therefore are underestimates and, at best, coarse approximations, due to the issues regarding the accuracy of the mapping and the number of assumptions and extrapolations that had to be made. further research is needed to provide better estimates of the impacts.
in the 2016 estimate, the taxon with the greatest estimated impact was wattles (Acacia mearnsii, black wattle, A. dealbata, silver wattle, and A. decurrens, green wattle) which accounted for 34% of the reductions, followed by Pinus species (pine trees) (19.3%) and Eucalyptus species (gum trees) (15.8%) (Table 5.5). nearly 70% of the wattle invasions, 60% of gum trees, 40% of pines and most of the poplar and willow invasions are in the grassland biome and explain why estimated reductions in this biome are so hig. Prosopis (mesquite) invasions in the northern cape were mapped in 2007 (van den berg 2010) and this information was used to estimate a reduction of about 9 million m3/yr, most of this being in the orange river catchment (le maitre et al., 2013).
tABle 5.5 A comparison of the estimated extent and impact of invasions by different taxa on the mean annual surface water runoff in South Africa (Middleton & Bailey 2008) based on the landscape mapping for the RSA by kotzé et al. (2010). condensed ha is the equivalent area at a canopy cover of 100% (i.e. 50% cover on 10 ha = 5 condensed ha)
eStiMAted CondenSed AreA
(HA)
eStiMAted reduCtion (Million M3)
eStiMAted reduCtion (MM rAinFAll
eQuivAlent)
Acacia cyclops (rooikrans) 54 679 28.95 53
Acacia mearnsii (black wattle) 474 489 483.23 102
Acacia melanoxylon (Australian blackwood) 2 796 18.07 646
Acacia saligna (port Jackson willow) 50 052 11.66 23
Agave spp. (Century plants) 11 341 0.89 8
Arundo donax (giant reed) 3 202 1.59 50
Atriplex nummularia (old man saltbush) 5 862 0.94 16
Caesalpinia decapetala (Mauritius thorn) 8 830 10.95 124
Tus of biological invasions and Their managemenT in souTh africa2017
eStiMAted CondenSed AreA
(HA)
eStiMAted reduCtion (Million M3)
eStiMAted reduCtion (MM rAinFAll
eQuivAlent)
Cereus jamacaru (queen of the night) 10 948 0.13 1
Cestrum spp. (cestrums) 7 217 19.27 267
Chromolaena odorata (triffid weed) 101 992 100.29 98
Eucalyptus spp. (gum trees) 273 573 217.37 79
Hakea spp. (hakeas) 36 344 72.20 199
Jacaranda mimosifolia (jacaranda) 4 200 1.76 42
Lantana camara (lantana) 32 328 40.29 125
Melia azedarach (seringa) 14 224 7.34 52
Opuntia spp. (cacti) 95 010 7.70 8
Pinus spp. (pine trees) 132 937 272.31 205
Populus spp. (poplars) 58 082 26.89 46
Prosopis spp. (mesquite) 5 232 1.95 37
Psidium guajava (guava) 6 354 7.16 113
Rosa rubiginosa (eglantine) 11 801 8.75 74
Salix babylonica (weeping willow) 37 555 22.48 60
Senna didymobotrya (peanut butter cassia) 11 586 13.84 119
Sesbania punicea (red sesbania) 1 683 2.22 132
Solanum mauritianum (bugweed) 40 413 58.20 144
Tamarix chinensis (Chinese tamarisk) 2 137 7.13 334
total 1 494 867 1 443.56 97
Projected invasions: The initial estimates of the costs of control and the impacts of invasions were based on an increase of 5% per year (e.g. le maitre et al., 2002) but a synthesis of the information on spread suggests a rate closer to 10% (van Wilgen & le maitre 2013). Projections of the impacts based on increases in invasions in the area of the catchments under natural vegetation show that the impacts are likely to become substantially greater.
at an expansion rate of 5%, and densification of 1%, the total reduction would increase to 2 589 million m3/yr (5.2% of mar) in 25 years (i.e. in about 2032). at 10% the projected reductions in 25 years will be about 3 153 million m3/yr (6.3% of mar). The increases in the percentage reductions occur throughout the mapped area but are greatest in the higher rainfall parts of the eastern cape, Kwa-Zulu-natal and the Western cape. The simple spread model did not allow invasions in a catchment to spread to adjacent ones that were not initially invaded so the estimated impacts probably are conservative.
These findings have significant implications for water security within and downstream of these invaded areas, highlighting the need to focus investment in areas where it will yield the greatest long-term benefits.
ChaPter 5IThe sTaTus of invaded areas
Figure 5.4 Projected reductions in the mean annual runoff (MAR) in 2032, at different rates of spread of invasive alien plants (assumed to be 5% in upper map and 10% in lower map). Map: D le Maitre unpublished data.
Tus of biological invasions and Their managemenT in souTh africa2017