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BRIDGE OVER TROUBLED LANDSCAPES

Invasive alien grasses from around the world are a common sight in many southern African ecosystems, especially in disturbed sites, riparian zones and along roads. In some cases, rivers and roads can act as corridors along which invasive alien species gain access to natural ecosystems. Several research papers have considered the role of roads and rivers as ecological ‘corridors’ for the dispersal of alien invasive grasses, although most of these treated rivers and roads separately, and without looking at how these features can hasten the invasion process.

In a recent paper in the journal Weed Research former C·I·B PhD student, Sebataolo Rahlao and C·I·B core team members Professors Karen Esler and Sue Milton, and Dr. Phoebe Barnard (SANBI), considered the effects of the interchanges between road bridges and rivers in the invasion success of the grass Pennisetum setaceum (fountain grass). This grass is classified as a Category-1 invader in South Africa. Its successful spread is attributable to its drought resistance, unpalatability to animals, rapid growth and prolific seed production.

To determine the effects of river-road interchanges on the performance and abundance of fountain grass, the researchers set out to record and map the distribution of the species along 5112km of national roads in South Africa. They found the grass at 10% of the total road length sampled, especially in fynbos (41%). Although the occurrence of the grass did not depend on the presence of river-road interchanges, its growth and reproduction performance were more vigorous at these interchanges than at areas away from these features.

Several things could make grass grow and reproduce better at river-road interchanges. Management activities, such as mowing and ditch excavation create disturbances which make the soil more suitable for colonization. A second consideration is the condition of the materials used in roadside construction. Soils transported from elsewhere may be unsuitable for indigenous species, leaving the area open for invasion by this grass and other weeds. Moisture and perhaps nutrients are more continuously available at these interchanges. In addition, river-road interchanges are often just outside the roadside management area, so that grasses which successfully establish there can continue to grow for years.

The authors suggest that the presence of this weed along roadsides poses a major problem by facilitating invasion of natural areas. Since small populations of P. setaceum at river-road interchanges may lead to the spread through these corridors over the landscape, their early detection and control is important. To prevent the spread of this invasive alien grass, management should consider road-river interchanges as important targets for regional prevention and control efforts.

Dr Sebataolo J. Rahlao is now with the Energy Research Centre (www.erc.uct.ac.za) at the University of Cape Town and can be contacted at sebataolo.rahlao@uct.ac.za, tel: 021 650 2831; for further details.

Read the paper:

Rahlao, S.J., Milton, S.J., Esler, K.J. & Barnard, P. (2010) The distribution of invasive Pennisetum setaceum along roadsides in western South Africa: the role of corridor interchanges. Weed Research 50, 537 – 543.

Measuring fountain grass near Piketberg

Sebataolo Rahlao measuring Pennisetum setaceum (fountain grass) near Piketberg, Western Cape.