A new recruit holds a chainsaw to the base of a pine tree, making careful cuts around its perimeter, while about a dozen men and one woman watch from a distance. Standing just behind him, trainer Sakhumzi Sidukwana gives the signal. Everyone stands back, and with a final cut the 40-foot tree crashes to the ground.
Mr. Sidukwana eyes the fallen tree, lying next to a growing pile of others, with satisfaction. This new group is making progress, he says.
All of these trainees were unemployed a few weeks ago. Now, as recruits in a government-sponsored program called Working for Water, they will be getting paid to wage war against the pine trees and other invasive plants that threaten the biodiversity and water resources of this region.
The program, which employs long-term jobless people to clear hillsides and riverbanks of destructive plants, represents one of the most comprehensive efforts worldwide to tackle the growing threat posed by organisms known as invasive alien species, experts say.
Every year, countless plants, animals, and insects travel back and forth across continents, often with disastrous consequences. Invasive plants cost the United States economy an estimated $7 billion a year.
Some species are introduced as pets and ornamental plants, while others, like the Argentine ant and the Chinese mitten crab, hitch transcontinental rides on imported produce or cross the ocean in the ballast of cargo ships.
In South Africa, trees such as the American pine and the Australian wattle are among the most destructive plants, particularly in the southwest part of the country, a region of extraordinary biodiversity that is dominated by the unique fynbos, a shrub that largely comprises the Cape floral kingdom. It once blanketed the hillsides of this picturesque wine-growing valley, but alien trees - originally imported for commercial forestry - have colonized vast swaths of land, threatening both biodiversity and water resources. Invasive alien species now occupy about 8 percent of South Africa's land.
Thirsty invaders, like the Australian blue gum tree, which consume far more water than plants native to this drought-prone region, are sucking rivers and streams dry in some areas. Their presence also increases the fuel load in fire-prone regions, and contributed to a devastating fire on the slopes of Cape Town's Table Mountain in 2000.
"It's an issue on the same scale as global warming, probably," says Guy Preston, head of Working for Water and also chair of the Global Invasive Species Project (GISP), an international partnership established in 1997 with headquarters in Cape Town.
"Invasive species have significant impacts on water security, fire, biological diversity, productive use of land," he says. "These impacts are all very negative, and ultimately unaffordable."
Programs like Working for Water offer a model for combating invasive species around the world, Mr. Preston says. In a new initiative funded by the World Bank, the GISP will this month begin efforts to coordinate a global response to the problem.
Established in 1995, Working for Water was conceived both as a measure against destructive species and as a New Deal-style social program. The government's largest job-creation scheme, it employs more than 21,000 people, including the disabled and former prisoners. More than half of the participants are women, as mandated by the government.
But the wages are low, and the work is challenging and sometimes dangerous, Mr. Preston says. In some cases, teams are dropped by helicopter on mountaintops to remove unwanted plants. Others rappel down cliff faces to attack the most inaccessible pockets of vegetation. At this site, the recruits will be clearing a large area surrounding the Berg River, where a dam will be built to supply water to Cape Town.
Trainees initially make around $5 a day. On average, participants in the program earn about $150 per month, which is barely enough to meet a family's basic needs.
"Ultimately one can say people must earn more, but obviously the more individuals earn, the fewer numbers of participants in the program," Preston says. "It's very much what the communities want themselves."
Sidukwana, the trainer who supervises the new recruits as they cut down the trees, says he enjoys the work, and gains satisfaction from knowing that he is helping people to gain job skills to support themselves.
Florah Manundu, who joined the program three years ago after being unemployed for a year and a half, now supervises a team of 11. At first, she says, she was happy to find work, but couldn't understand why she was being hired to cut down trees with a chainsaw.
"Now I think I am saving the world," she says with a smile. "I am trying to prevent the aliens from drinking a lot of water."
So far, invasive plants have been cleared from more than a million hectares (2.5 million acres) of land in South Africa. But this is still only a small fraction of the land occupied by alien vegetation.
"It's made a huge difference, but the trees are still growing faster than we're cutting them down," Preston says.