Impacts of invasive alien plants on water quality, with particular emphasis on South Africa

J Chamier,Dc Le Maitre,Pj Ashton,Bw Van Wilgen,K Schachtschneider

doi:10.4314/wsa.v38i2.19

Abstract

We review the current state of knowledge of quantified impacts of invasive alien plants on water quality, with a focus on South Africa. In South Africa, over 200 introduced plant species are regarded as invasive. Many of these species are particularly prominent in riparian ecosystems and their spread results in native species loss, increased biomass and fire intensity and consequent erosion, as well as decreased river flows. Research on the impact of invasive alien plants on water resources has historically focused on water quantity. However, although invasive alien plants also affect the quality of water, this aspect has not been well documented. Alien invasive plants increase evaporation rates, and reduce stream flow and dilution capacity. The biomass inputs of alien invasive plants, especially nitrogen fixers such as Acacia spp., alter nutrient cycles and can elevate nutrient concentrations in groundwater. Alien plant invasions alter the fire regimes in invaded areas by changing the size, distribution and plant chemistry of the biomass. More intense fires increase soil erosion and thereby decrease water quality. In contrast to riparian invasions, aquatic invasive plants have been more extensively studied in South Africa and their impacts on water quality have been relatively well monitored. Water quality in South Africa is rapidly deteriorating, and all factors that influence this deterioration need to be taken into account when formulating actions to address the problem. The changes in water quality brought about by alien plant invasions can exacerbate the already serious water quality problems.

Highlights

Invasive alien plants are a significant environmental problem in South Africa’s terrestrial and freshwater ecosystems (Richardson et al, 1997)
The increased biomass that accompanies plant invasions results in more intense fires that damage the vegetation and soil and lead to excessive erosion; in addition, invasive alien plants reduce the capacity of rangelands to support livestock and wildlife, and significantly reduce biodiversity (Richardson and Van Wilgen, 2004)
These areas are especially sensitive to invasions as most of South Africa’s surface water originates from the Drakensberg mountain grasslands, and from the Cape mountains which are dominated by fynbos vegetation (Turpie et al, 2008)

Summary

Introduction

Invasive alien plants are a significant environmental problem in South Africa’s terrestrial and freshwater ecosystems (Richardson et al, 1997). The reduction in surface water runoff as a result of current invasions was estimated to be 3 300 Mm3 (about 7% of the national total, Le Maitre et al, 2000), most of which is from the fynbos (shrubland) and grassland biomes (Van Wilgen et al, 2008). These areas are especially sensitive to invasions as most of South Africa’s surface water originates from the Drakensberg mountain grasslands, and from the Cape mountains which are dominated by fynbos vegetation (Turpie et al, 2008). The Cape fynbos has been invaded by alien plants that have escaped from commercial plantations and woodlots on farms (i.e. Pinus, Acacia and Eucalyptus species), and grasslands’ ecological integrity has been compromised by poor land use management practices, which include overgrazing and burning regimes, and damming and reclamation of wetlands (Turpie et al, 2008)

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