Weeding out alien invaders

Haru Mutasa

Haru Mutasa reports on how geographic information technologies are being used in South Africa’s efforts to eradicate invasive plants in South Africa.

Jonathon Pryor wades through a dense mass of overgrown vegetation – weeds, clumps of grass and thorn bushes – and pulls out his hand-held global positioning system (GPS) device. This little baby has made my life so much easier’, he says.

‘Clearing out these bushes is a cumbersome process as it is. This tool allows me to calculate how many people I need to hire to remove the weeds, how many seeds are needed for replanting, and how long the process should take’.

Jonathon is a member of the South African Working for Water (WfW) programme, which was launched in 1995 as part of a major effort to tackle the invasive alien species problem. This initiative, involving several government departments, is now implementing 300 projects nationwide focusing on improving water security, restoring land for agriculture, and promoting the sustainable use of natural resources.

Invasive alien plants (IAPs) have become established on over 10 million hectares, and are now threatening South Africa´s native plants and animals. Since the first European settlers arrived, about 8000 herbaceous and 750 tree species have been introduced from all over the world, 200 of which are classed as invasives. Among the worst offenders are the kangaroo thorn (Acacia paradoxa) from Australia, pampas grass (Cortaderia selloana) from South America, kahili ginger (Hedychium gardnerianum) from Asia, and the Barbados gooseberry (Pereskia aculeate) from the West Indies.

One of the problems is that these plants consume an estimated 3300 million cubic metres of water, or about 7% of annual runoff. Even though many of the country’s native plants have a low biomass and require little water, the invaders use so much water that very little remains and local plants then die off. Once established, they make it very difficult to farm the land they occupy, they increase the risk of flooding and fires, increase soil erosion and dry up streams and rivers – all of which could result in the extinction of many indigenous plants and animals.
‘These plants were brought in as ornamentals for gardens and parks’, Pryor explains, as he logs the GPS coordinates of an area covered with fresh shoots of kangaroo thorn, a shrub that grows up to 3.5 meters. ‘But they have now become pests that destroy everything in their path. They have to go’.

In December 2004 the Western Cape region was devastated by intense bush fires caused by extremely high summer temperatures. The area was clogged with alien vegetation, which had formed a highly combustible mass that burned 10 times faster than the native plants. ‘What is most irritating is that the seeds of the alien species also survived and are now sprouting faster than local plants’, says Pryor. ‘In just a few months the area would have been completely overgrown’.

The cost of controlling IAPs in South Africa is estimated at R600 million per year over 20 years. If the aliens are left to spread un-checked, they will be much more difficult to remove in the future. The approach adopted by WfW to control the weeds com-bines the use of sophisticated information technology with the difficult manual work of clearing the invaded areas, which is usually outsourced to a contractor.

Prior to drawing up a contract, a WfW programme manager equipped with a GPS device undertakes a field survey of the areas earmarked for clearance, and records the exact location, the species found, and their density, age and growth stage. Other features of the area are also recorded, including the slope and the type of habitat (e.g. a riverbank or open grassland).

For each invaded area, the attribute data from the field survey are entered into an MS Access 2000 database program, and linked to the spatial data in the GIS-based Working for Water Information Management System (WIMS), running on ArcView 3.2. Based on the field survey report, a GIS and mapping officer then generates a map for the contractor showing the exact areas of land that need to be cleared, and the degree of infestation. On the basis of the map, the officer also calculates how long it should take to complete the job.

Alternatively, the programme manager can also use the GPS device directly to calculate the size of an invaded area and to estimate the amount of time and resources needed to remove the alien vegetation.

The actual clearance is done by workers who spend days cutting, slashing and uprooting the alien plants. It is hard, back-breaking work, and the pay is minimal, but it is enough to put food on the table. ‘Over the last couple of years we have cleared a lot of land for agriculture’, said Pryor, ‘Farmers in the area are now growing crops on land that previously couldn’t be used for agriculture. More land to work means more food for the people – what could be better than that?’

Haru Mutasa ( haru@ameinfo.com ) is a journalist for the Highway Africa News Agency (HANA), which specializes in ICT events and trends in Africa.

For more information, please visit www.dwaf.gov.za/wfw/

25 February 2005

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