First record of Spartina alterniflora in southern Africa indicates adaptive potential of this saline grass

Abstract

Spartina alterniflora was recorded in 2004 in the Great Brak Estuary, a system along the southern coast of South Africa that closes to the sea. This is alarming as this is a species with a known history as an aggressive invasive plant which has now been found 8000 km from its nearest known location and furthermore it is spreading under atypical conditions of submergence. This first recorded population in Africa indicates the adaptive potential of this invasive grass which survives inundation and non-tidal conditions for months at a time. Spartina alterniflora spread from 2566 m2 in 2006 to a maximum area covered of 10,221 m2 in 2011. There was an increase in silt, sediment organic matter and a significant reduction in sediment redox potential at sites invaded by S. alterniflora. When the estuary closes to the sea the water level rises and S. alterniflora is flooded, limiting opportunities for mechanical and chemical control. Application of a glyphosate-based herbicide in 2012 showed that chemical control was more effective in reducing the stands than mechanical removal. The additional use of imazapyr in 2014 significantly reduced stem density and the proportion of live stems. Spread of this invasive plant to the intertidal marshes in adjacent estuaries is a potential biodiversity threat although, fortunately, this population does not seem to produce viable seed. There is also the concern that hybridization with the resident S. maritima may occur. Important research and management questions remain i.e. how quickly will the natural marsh re-establish following eradication and how can we prevent movement of the grass to other estuaries?