Several Prosopis species have been introduced into South Africa in the last century many of which have become invasive. Here we determine the extent to which increased mortality of Acacia erioloba, a keystone species in the Kalahari, can be attributed to competition for water with Prosopis. We do this through a determination of canopy vitality, plant water stress and plant water source. We use a visual estimate to determine plant vitality. Plant water stress we determine through a combination of stable carbon isotope ratios and xylem pressure potentials and plant water source we determine using stable hydrogen and oxygen isotope ratios. Our results show that Prosopis abundance increases in the riparian zone and that there is a good correlation between increased Prosopis abundance and Acacia erioloba mortality. We show that both species are reliant on the same water resource in the riparian zone but that Acacia erioloba uses a deeper water source away from the river. We conclude that the decline in Acacia erioloba vitality at the river is related to competition for water with Prosopis. Our study gives strong support for the eradication of Prosopis from rivers in arid parts of Southern Africa.

Keywords: Kalahari Desert, water access, alien invasive, keystone species, Acacia erioloba, Prosopis.