The City of Cape Town has urged residents to be on the lookout for aliens which are wiping out vast tracts of valuable grazing land. The city launched its "Invasive Species Week" yesterday.Ulrike Irlich, invasive species monitoring and evaluation programme manager, listed Port Jackson, fountain grass, Australian bluebell creeper, yellow water lily, and the water hyacinth as the top five invasive plants in the city.South Africa had a "very big" problem and was losing an estimated R6.5-billion annually as a result of alien plants, said Professor Brian van Wilgen of Stellenbosch University's Centre for Invasion Biology."We have estimated that 10million hectares of South Africa have been invaded by 180 alien species," said Van Wilgen."South Africa's ecosystems deliver services - water from catchments, grazing for animals, and other harvested products - that are worth at least R152-billion annually. Of this value, we currently lose about R6.5-billion every year due to invading alien plants. If we had not conducted control operations in the past, this would have amounted to an estimated additional R 41.7-billion."He said several million rands had been spent researching the problem.Alien species displace indigenous ones, often drive rare species towards extinction and reduce the capacity of rangelands to support livestock, he said. Many deplete water sources and increase the risk of fires.In the past, plants were moved from country to country to provide food and as a "reminder of home". But many arrive by accident these days - for example as seeds that stick to the clothing of travellers."We have many beautiful indigenous alternative plants that can be planted in gardens, so it is advised to not buy plants that are not from your area," said Irlich.The City of Cape Town has a specialised emerging weeds and target weeds programme and can assist with the responsible removal of certain invasive species. (www.capetowninvasives.org.za)Top six to nixPort Jackson (Acacia saligna). This species outcompetes indigenous vegetation, becomes a hide-out for criminal activities and fuels fires.Water hyacinth (Eichhornia crassipes). Clogging waterways and causing flooding, this species outcompetes and shades out indigenous plants.Bluebell creeper (Billardiera heterophylla). Stimulated by fire, this emerging weed could smother and displace indigenous fynbos.Fountain grass (Pennisetum setaceum). This grass has jumped the garden fence and is invading Cape Town's road verges, resulting in increased fuel loads and a higher risk of fires.Yellow water lily (Nymphaea mexicana). Covering water with its large, flat leaves, this species forms infestations which can reduce water flow, displace other submerged plants and deprive water of oxygen.European paper wasp (Polistes dominula). The northern suburbs are particularly badly affected by this species. It is a nuisance and a health hazard to humans and kills bees.Source: CITY OF CAPE TOWN..