Potential spread of the invasive plant Hydrilla verticillata in South Africa based on anthropogenic spread and climate suitability

Hydrilla verticillata (L.f.) Royle is a submerged aquatic plant native to Asia and Australia that is highly invasive in the USA and was first recorded in South Africa in 2006. It is only known from one locality, Pongolapoort Dam in KwaZulu-Natal Province, but there are fears that it might spread to other sites. The primary vector of spread in the USA is recreational boaters and anglers. A survey at a fishing competition on Pongolapoort Dam showed that anglers travel considerable distances around South Africa (73% of water bodies were >200 km, visited by 68% of the respondents). A Threat Index for freshwater bodies throughout South Africa visited by participants of the competition was calculated showing that dams in the vicinity of the infestation were more at risk from invasion. Further, the potential distribution of the weed based on climatic matching with the region of origin showed that most of the country was suitable for establishment, with the exception of the high-lying interior of the country. Recommendations for reducing the potential spread of hydrilla in South Africa are presented.