Impacts of invasive plant species on river systems in South Africa and their implications

Impacts of invasive plant species on river systems in South Africa and their implications
David Le Maitre
CSIR Natural Resources and the Environment

This paper focuses on the impacts of invasive alien plants on the water flows in South Africa based on riparian invasions. Water-use by riparian trees is higher than in the adjacent dryland areas and has significant impacts on river functioning. Wattles and eucalypts are mainly invaders of riparian zones where they frequently become dominant and completely exclude native species. Riparian invaders also alter river sediment dynamics, nutrient fluxes and assimilatory capacity, with significant impacts on ecosystem services. However, we currently do not have adequate information on the extent and impacts of riparian invasions, nor have priorities been set for riparian invasions. Invasive alien plants have invaded most of our river systems, on average about 30%, particularly many of those where water demands are high and water quality is an issue. Modelling of the impacts of invasions suggests a reduction in river flows of about 3%. For example, the landscape mapping suggests that only about 4-5% of invasions by Acacia mearnsii, Eucalyptus, Populus and Salix species are riparian. A more reasonable split for these taxa would be about 20%, 50%, 80% and 80% respectively. If the riparian invasions are adjusted accordingly, riparian reductions would increase by about 1 000 million m3/yr and the total reductions by nearly 70%. Invasions have many other impacts including altering river ecosystem structure and functioning, aquatic-terrestrial interactions, and ecosystem service delivery. More studies of the extent and impacts of riparian invasions are needed so that appropriate actions can be taken, including defining riparian zones.