Eastern Cape by the early 1900s that prevented livestock access to forage and
ultimately seriously perturbed the pastoral economy of the region (van Sittert 2002).
Physical damage to livestock by well-armed alien plants has been recorded in
East Africa for Opuntia stricta (Australian Pest Pear) (Shackleton et al. 2017)
and Prosopis spp. (Bekele et al. 2018), most commonly by thorns damaging
hooves.
Animal products are the most important ecosystem service provided by
rangelands. De Lange and van Wilgen (2010) estimated the economic impact of
alien plant invasions on this service, using van Wilgen et al.’s (2008) estimates of
livestock reduction due to invasive alien plants, and assuming a value of ZAR 2500
per large livestock unit. The estimated annual value of livestock production from
rangelands in South Africa would have been ZAR 33 billion (2010 values) had they
not been invaded. Current levels of invasion have reduced this by an estimated ZAR
340 million annually. These authors also attempted to estimate what the reduction
would have been in the absence of historical control efforts, and concluded that
current losses could have amounted to ZAR 12.7 billion had there not been historical
control. However, because this estimate was based on an approach that required
large assumptions, there is a low level of confidence in estimates of avoided cost
(van Wilgen and Wilson 2018).
Rangelands are also critical for the provision of water services, and they provide
native medicines, fuel wood, wildlife-based tourism, pollination services, and maintain the biodiversity which underpins these. Water resources provided by the higherrainfall Grassland Biome, for example, were estimated to be worth ZAR 50 billion
per year under a scenario of no invasion, and this had been reduced to ZAR
48.9 billion per year due to invasions in 2010 (De Lange and van Wilgen 2010).
Biodiversity can provide economic value from harvested products and non-use
sources (such as tourism). The annual value of biodiversity-based services from
all terrestrial biomes was estimated at ZAR 22.1 billion per year (with no invasions)
and ZAR 21.7 billion per year at current levels of invasion (De Lange and van
Wilgen 2010). Again, these reductions would almost certainly have been far more
severe had there been no attempt at control in the past, but the magnitude of the
economic value of historical control cannot be estimated with high confidence due to
a lack of data.
A cost-benefit threshold is expected to apply in that alien plants may offer
considerable value when at low abundance but their detrimental impact far outweighs advantages as their abundance increases (Shackleton et al. 2007; van Wilgen
and Richardson 2014). For example, Prosopis trees delivered positive benefits in the
form of edible pods when first introduced to South Africa, but as they spread the
negative impacts grew, eventually exceeding the benefits and resulting in net
negative impacts (Richardson et al. 2000; Wise et al. 2012; Fig. 16.3).
In the following paragraphs we illustrate some consequences of invasion by
individual alien species on rangeland quality and condition.
476
T. G. O’Connor and B. W. van Wilgen
ultimately seriously perturbed the pastoral economy of the region (van Sittert 2002).
Physical damage to livestock by well-armed alien plants has been recorded in
East Africa for Opuntia stricta (Australian Pest Pear) (Shackleton et al. 2017)
and Prosopis spp. (Bekele et al. 2018), most commonly by thorns damaging
hooves.
Animal products are the most important ecosystem service provided by
rangelands. De Lange and van Wilgen (2010) estimated the economic impact of
alien plant invasions on this service, using van Wilgen et al.’s (2008) estimates of
livestock reduction due to invasive alien plants, and assuming a value of ZAR 2500
per large livestock unit. The estimated annual value of livestock production from
rangelands in South Africa would have been ZAR 33 billion (2010 values) had they
not been invaded. Current levels of invasion have reduced this by an estimated ZAR
340 million annually. These authors also attempted to estimate what the reduction
would have been in the absence of historical control efforts, and concluded that
current losses could have amounted to ZAR 12.7 billion had there not been historical
control. However, because this estimate was based on an approach that required
large assumptions, there is a low level of confidence in estimates of avoided cost
(van Wilgen and Wilson 2018).
Rangelands are also critical for the provision of water services, and they provide
native medicines, fuel wood, wildlife-based tourism, pollination services, and maintain the biodiversity which underpins these. Water resources provided by the higherrainfall Grassland Biome, for example, were estimated to be worth ZAR 50 billion
per year under a scenario of no invasion, and this had been reduced to ZAR
48.9 billion per year due to invasions in 2010 (De Lange and van Wilgen 2010).
Biodiversity can provide economic value from harvested products and non-use
sources (such as tourism). The annual value of biodiversity-based services from
all terrestrial biomes was estimated at ZAR 22.1 billion per year (with no invasions)
and ZAR 21.7 billion per year at current levels of invasion (De Lange and van
Wilgen 2010). Again, these reductions would almost certainly have been far more
severe had there been no attempt at control in the past, but the magnitude of the
economic value of historical control cannot be estimated with high confidence due to
a lack of data.
A cost-benefit threshold is expected to apply in that alien plants may offer
considerable value when at low abundance but their detrimental impact far outweighs advantages as their abundance increases (Shackleton et al. 2007; van Wilgen
and Richardson 2014). For example, Prosopis trees delivered positive benefits in the
form of edible pods when first introduced to South Africa, but as they spread the
negative impacts grew, eventually exceeding the benefits and resulting in net
negative impacts (Richardson et al. 2000; Wise et al. 2012; Fig. 16.3).
In the following paragraphs we illustrate some consequences of invasion by
individual alien species on rangeland quality and condition.
476
T. G. O’Connor and B. W. van Wilgen