Alien Invasive Plants

Posted by: Saving Water SA (Cape Town, South Africa) – partnered with Water Rhapsody conservation systems – 28 July 2010



Invasive alien plants (IAP's) now infest 20-million hectares of South Africa – an area twice as large as previously estimated.  Wattles have taken over more than 1.6-million hectares of South Africa.  The shock finding comes from an Agricultural Research Council (ARC) report commissioned by Water Affairs.  “The previous figure was 10 million hectares. We knew this was an under-estimate, but we didn’t think it was this big. It’s come as quite a shock,” the department’s natural resource management programme operations head, Christo Marais, told Sapa.

The ARC had briefed the department on the new estimate at a Working for Water (WfW) implementation meeting earlier this month.  Marais said it had long been obvious there was an under-estimation of the scale of the problem, particularly in the Eastern Cape and KwaZulu-Natal. Invasive alien vegetation, including various species of wattle, pine, poplar,weeping willow, gum trees, hakea and prickly pear, among others, pose a serious threat to South Africa’s water supply, as well as the country’sagricultural potential and biodiversity.

If the 20-million hectares of alien invasive vegetation across the country could be condensed into a single area, it would form a dense, impenetrable thicket about twice the size of the Kruger National Park.  Marais said that 15 years ago, government had established WfW to tackle the problem of invasive aliens, while at the same time provide skills training and employment for thousands of poor, jobless citizens, particularly in rural areas.  In the current financial year, the project had been allocated “a more than R635 million budget”.  Asked how long it would take to clear 20-million hectares of alien vegetation, and what this would cost, Marais said a “conservative” estimate was R34 billion over the next 25 years.  Left untouched, the alien vegetation would spread at an average rate of one percent a year, threatening water and food security.  “This is actually one bit of good news. We initially estimated it was spreading at five percent a year, but the figure now appears to be one percent,” he said.

According to the ARC report, over 600 000 hectares (condensed area) of the Eastern Cape are infested with black, green and silver wattles, as are more than 300 000 hectares in KwaZulu-Natal.  The Eastern Cape has also lost over 200 000 hectares to prickly pear, and the same area again to invasive Australian gums.  A further 250 000 hectares of KwaZulu-Natal have also been taken over by the North American invader, Chromolaena odorata, more commonly referred to as “paraffin” bush because of the fire hazard it poses.  Poplars infest more than 150 000 hectares in the Free State, while prosopsis, better known as mesquite, takes up 350 000 hectares of the Northern Cape.  On a national scale, black, green and silver wattles have taken over more than 1.6-million hectares of South Africa; gums occupy 1.4-million hectares; and a million hectares are under invasive pines and poplars (500 000 hectares each).

Other significant alien invasives listed in the report include lantana, syringa, queen of the night cactus, agave, guava, Spanish reed and sesbania.  Marais said the department was looking at ways of off-setting the cost of clearing invasive plants, including biological control.  “Biological control could reduce the clearing costs by between 10 and 20 percent, assuming such control can reduce the plants’ seed set and expansion.  Another possibility is utilising dense stands of alien vegetation for bulk fibre production, or as an energy source. This could further offset costs.”

Land owners also needed to take responsibility for invasive plants on their properties.  “According to current environment legislation, the land user is primarily responsible for management of their land, therefore land managers must take responsibility. The [cost] burden cannot be placed on government alone. Land managers must come to the party.”  Marais said the department was also examining the cost benefits of clearing land, particularly water catchments.  

Surveys showed that clearing invasive alien plants from a watershed could increase the water yield significantly. This extra water could then be sold and the money used to offset the clearing costs.