Why does it matter?

Over 100 invasive species are believed to have major negative impacts on ecosystem services, including on water resources, rangeland productivity and biodiversity (see section 4.6)

surprisingly, there have been very few studies that formally document evidence on the impacts of invasive species, and consequently the level of confidence in estimates of the magnitude of these impacts is low, but it is clear that the invasive species that have major negative impacts are many and varied. some examples are given in Table 9.1. almost all of the estimated impacts of invasions in monetary terms (~Zar6.5 billion per year) is due to these hundred or so invasive species that are believed to have major negative impacts. country-level species-specific management strategies have only been developed for a very small number of invasive species, and none have been formally implemented.

tABle 9.1. examples of invasive species that are believed to have major or severe negative impacts in South Africa.

group (nuMBer oF SpeCieS witH MAJor or

Severe negAtive iMpACtS) exAMple oF SpeCieS And tHe iMpACtS tHey CAuSe

plants (80) north american mesquite trees (genus Prosopis) reduce grazing potential; deplete groundwater resources; and negatively impact on biodiversity.

australian wattle trees (genus Acacia) reduce grazing potential and surface water runoff; and negatively impact on biodiversity.

north american and european pine trees (genus Pinus) reduce surface water runoff; negatively impact on biodiversity; and increase the fire intensity and damage done by wildfires.

herbaceous and succulent species (triffid weed - Chromolaena odorata; famine weed - Parthenium hysterophorus; pompom weed -Campuloclinium macrocephalum; and many cactus species) severely reduce rangeland productivity and thus the livelihoods of rural people.

Mammals (8) feral domestic cats (Felis catus) and house mice (Mus musculus) are serious threats to breeding marine birds on offshore islands.

Freshwater fish (5) north american smallmouth bass (Micropterus dolomieu) decimate indigenous and endemic fish and invertebrates in streams, rivers and dams.

terrestrial invertebrates (5) argentine ant (Linepithema humile) disrupts ant-plant mutualisms that are responsible for the seed dispersal of indigenous plants, and thus pose serious threats to indigenous vegetation survival.

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Figure 9.5 Pine trees invading a fynbos mountain catchment area. Several species of invasive pine trees have major impacts by reducing water resources, displacing globally unique biodiversity, and increasing fire hazard. Photograph: B. van Wilgen.

Invasive trees and shrubs reduce surface water resources by between 3 and 5%, and threaten up to 30% of the water supply of cities like Cape Town and Port Elizabeth (see section 5.5.1)

invasive alien plants, particularly trees and shrubs, use more water than the indigenous plant species that they replace, because they are larger and deeper-rooted, and have different physiologies. at a national scale, the combined impacts of invasive alien plants on surface water runoff have been estimated at between 1 450 to 2 450 million m³ per year (between 3 and 5% of the mean annual runoff for the country). Primary catchments most affected are in the Western and eastern cape, and KwaZulu-natal, where reductions in mean annual runoff are greater than 5%. if no remedial action is taken, reductions in water resources could rise to between 2 600 and 3 200 million m³ per year; and if fully invaded, catchments in the Western and eastern cape Provinces will deliver 30% less water to the cities of cape Town, mossel bay, george, Knysna, Plettenberg bay and Port elizabeth. This severely constrains the prospects for economic growth, threatening the ongoing creation of new employment opportunities to millions of south africans. deep-rooted invasive species such as mesquite (Prosopis species) that invade arid areas also deplete groundwater resources, and lower the water table.

reducing the extent and abundance of water-consumptive invasive alien plants through efficient management can make a valuable contribution to water security and sustainable agriculture in south africa.

The reductions in water resources if no remedial action is taken are estimated to be between

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2600 ^&

3200 million m

per year

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Figure 9.6 Invasive mesquite trees (Prosopis species) in the arid Northern cape can substantially deplete groundwater reserves. Research has shown that savings of up to 70 m³/month could be achieved in spring for each hectare of Prosopis cleared.

Photograph: R. Shackleton.

Invasive alien plants reduce the capacity of natural rangelands to support livestock production by over 100 000 large livestock units, thereby threatening rural livelihoods and food security (see section 5.5.2) invasive plants such as cacti, and several herbaceous weeds such as pompom weed (Campuloclinium macrocephalum), famine weed (Parthenium hysterophorus), and triffid weed (Chromolaena odorata), invade grassland, savanna and Karoo vegetation, where they displace palatable indigenous plants, and consequently these areas cannot support as many livestock as uninvaded areas. invasive alien plant infestations reduce the amount of livestock that can be supported in south africa by around 115 000 large stock units. This is just over 1% of the potential number of livestock that could be supported. however, these impacts could increase dramatically, more than halving the livestock production potential, if infestations of invasive plants spread into all suitable habitats.

Figure 9.7 Invasion of Highveld grasslands by alien plants such as pompom weed (Campuloclinium macrocephalum) reduces the capacity of rangelands to support livestock by displacing palatable grasses and shrubs.

Photograph: L. Henderson.

Figure 9.8 The boxing glove cactus (Cylindropuntia fulgida var. mamillata) near Upington in the Northern cape Province. The species is extremely damaging to rangelands, but fortunately can be controlled using biological control.

Photograph: T. Xivuri.

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Biological invasions are the third-largest threat to South Africa’s terrestrial biodiversity (after cultivation and land degradation), and currently account for 25% of all biodiversity loss (see section 5.5.3)

south africa is one of the most biodiverse countries in the world, and this diversity underpins large parts of its economy, including fisheries, livestock production, harvesting of natural products, national and international tourism, and recreation. invasive species have been identified as a significant threat to biodiversity throughout the country. such losses of biodiversity have large negative knock-on effects on the economy and food security, among others. note: biodiversity here refers to the variety of genes, species and their interactions, and ecosystems in a given area. areas of high biodiversity are characterised by many species, and diverse ecosystems (such as forests, thickets, grasslands, wetlands, estuaries), and biodiversity is measured using standard international metrics.

Figure 9.9 Species-rich fynbos vegetation is transformed into species-poor monocultures through invasion by alien trees such as pines (Pinus species), with substantial loss of biodiversity. Photographs: B. van Wilgen.

The South African Department of Environmental Affairs (DEA) currently invests over ZAR 1.5 billion a year on managing biological invasions. The expenditure from other government agencies and the private sector is large but has not been precisely determined (see sections 6.4.1 and 6.4.2)

The dea’s Working for Water programme spent a total of Zar 5.65 billion between 1995 and 2016, mainly on invasive plant control projects across the country (figure unadjusted for inflation). annual expenditure has risen in real terms, from an initial investment of Zar 27 million in 1995 to Zar 1.55 billion in 2016. all figures that follow are expenditures over the duration of the project, adjusted for inflation and expressed in 2015 rands. Protected areas in the cape floristic region comprise provincial nature reserves and national parks covering approximately 750 000 ha. here, Zar 564 million has been spent on the control of invasive alien trees and shrubs in the genera Acacia (australian wattles), Pinus (north american and european pine trees) and Hakea (australian shrubs). in the 8 000 ha catchment of the berg river (Western cape), Zar 50 million has been spent on the removal of pine plantations, and of invasive pines and wattles in the upper catchment. in the 2 million ha Kruger national Park, Zar 350 million has been spent, mainly on the control of the invasive shrubs Lantana camara (lantana) and Chromolaena odorata (triffid weed), as well as several species of annual herbaceous weeds.

many other government departments and agencies at all levels of government

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Invading alien plants are the largest threat to South Africa’s unique biodiversity,

3 ^rd 1207

indigenous and endemic plant species being threatened by alien plant invasions

WITH

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(national, provincial and local) spend a significant amount of money on controlling biological invasions, including the daff on preventing the introduction of pests and diseases and managing organisms after introduction, and the department of health on human diseases and their vectors. non-governmental and civil society organisations and private landowners have also invested in control measures. one documented example comes from the 3  200 ha vergelegen estate in the Western cape, where anglo american have spent Zar 43 million on management of invasive alien plants. These costs need to be quantified if the totals spend by south africa on managing biological invasions are to be estimated.