The distribution of invasive Pennisetum setaceum along roadsides in western South Africa: the role of corridor interchanges

Roads and rivers may be dispersal corridors for invasive alien grass seeds that fly and float. These two systems interact at bridge interchanges that are also disturbed artificial habitats. The invasive grass Pennisetum setaceum (perennial fountain grass) establishes on roadsides and river banks and benefits from habitat conditions prevailing at these interchanges. The distribution of the grass across biomes and vegetation types and the influence of environmental variables were assessed. A road survey method was used to record and map the distribution of P. setaceum along 1 km roadside transects at 10 km intervals and at every road-river corridor interchange for 5112 km of South African national roads in the arid and semi-arid parts of the country. Pennisetum setaceum populations occurred in 10% of the total length sampled, including the interchanges. Fynbos Swartland Shale Renosterveld was the most significantly invaded amongst the vegetation types surveyed. Our results indicate that, although P. setaceum performs better on the interchanges, it does not preferentially colonise them over other parts of the landscape. The presence of P. setaceum was, however, closely associated with the presence of water bodies and disturbances away from the roads. Corridor interchanges should be considered important targets of both local and regional efforts to prevent and control P. setaceum invasions.Centre of Excellence for Invasion Biolog