4.1 Introduction
Aquatic ecosystems in South Africa have been prone to invasion by alien macrophytes, since the first introductions in the early 1900s. These alien freshwater plant
species have become invasive in many rivers, man-made impoundments, lakes and
wetlands in South Africa (Hill 2003), due to anthropogenic dissemination, combined
with increasing urbanisation, industry and agriculture, which have resulted in nutrient enrichment and ultimately eutrophication. Aquatic macrophytes have a number
of key traits that increase their invasiveness, such as rapid vegetative and sexual
reproduction leading to fast population build-up, the ability to regenerate from
fragments, high phenotypic plasticity and efficient dispersal mechanisms (Hill and
Coetzee 2017). If the impacts of these invasive macrophytes are to be alleviated, then
reductions in agricultural, industrial and urban runoff that are high in nitrates,
ammonium, and phosphates will be needed (Cook 2004; Chambers et al. 2008).
This chapter reviews the factors that contribute to the invasiveness of alien
freshwater macrophytes in South Africa, discusses their impacts, and assesses the
control programmes implemented against these aquatic invaders.
4.2 Invasive Macrophytes
The most important invasive freshwater macrophyte in South Africa remains Water
Hyacinth, which was first recorded as naturalised in KwaZulu-Natal in 1910. Four
other species have also been extremely problematic, but are currently under successful biological control and together with Water Hyacinth, were referred to as the
‘Big Bad Five’ (Table 4.1). The presence of new invasive aquatic plant species,
which are still in their early stages of invasion but targets for biological control, have
been recorded recently in South Africa. These include submerged, rooted emergent,
free-floating and rooted floating macrophyte species (Table 4.1). Additional species
that are widespread invaders elsewhere in the world, but are not yet present in
South Africa, pose a major threat should they be introduced (Table 4.1).
4.3 Pathways of Introduction
Invasive macrophyte species have been introduced and spread by means of numerous pathways, including the horticultural and aquarium trade, unintentional movement of propagules (i.e., hitchhikers) via boating enthusiasts and anglers, and,
increasingly, via the unregulated internet trade that supplies aquatic plant enthusiasts
(Cohen et al. 2007; Maki and Galatowitsch 2004; Padilla and Williams 2004; Martin
and Coetzee 2011). For example, the horticultural and aquarium trade is the primary
introduction pathway of submerged plants, such as E. densa and H. verticillata into
98
M. P. Hill et al.
Aquatic ecosystems in South Africa have been prone to invasion by alien macrophytes, since the first introductions in the early 1900s. These alien freshwater plant
species have become invasive in many rivers, man-made impoundments, lakes and
wetlands in South Africa (Hill 2003), due to anthropogenic dissemination, combined
with increasing urbanisation, industry and agriculture, which have resulted in nutrient enrichment and ultimately eutrophication. Aquatic macrophytes have a number
of key traits that increase their invasiveness, such as rapid vegetative and sexual
reproduction leading to fast population build-up, the ability to regenerate from
fragments, high phenotypic plasticity and efficient dispersal mechanisms (Hill and
Coetzee 2017). If the impacts of these invasive macrophytes are to be alleviated, then
reductions in agricultural, industrial and urban runoff that are high in nitrates,
ammonium, and phosphates will be needed (Cook 2004; Chambers et al. 2008).
This chapter reviews the factors that contribute to the invasiveness of alien
freshwater macrophytes in South Africa, discusses their impacts, and assesses the
control programmes implemented against these aquatic invaders.
4.2 Invasive Macrophytes
The most important invasive freshwater macrophyte in South Africa remains Water
Hyacinth, which was first recorded as naturalised in KwaZulu-Natal in 1910. Four
other species have also been extremely problematic, but are currently under successful biological control and together with Water Hyacinth, were referred to as the
‘Big Bad Five’ (Table 4.1). The presence of new invasive aquatic plant species,
which are still in their early stages of invasion but targets for biological control, have
been recorded recently in South Africa. These include submerged, rooted emergent,
free-floating and rooted floating macrophyte species (Table 4.1). Additional species
that are widespread invaders elsewhere in the world, but are not yet present in
South Africa, pose a major threat should they be introduced (Table 4.1).
4.3 Pathways of Introduction
Invasive macrophyte species have been introduced and spread by means of numerous pathways, including the horticultural and aquarium trade, unintentional movement of propagules (i.e., hitchhikers) via boating enthusiasts and anglers, and,
increasingly, via the unregulated internet trade that supplies aquatic plant enthusiasts
(Cohen et al. 2007; Maki and Galatowitsch 2004; Padilla and Williams 2004; Martin
and Coetzee 2011). For example, the horticultural and aquarium trade is the primary
introduction pathway of submerged plants, such as E. densa and H. verticillata into
98
M. P. Hill et al.