Quantifying the ecological impacts of the woody invasive, Prosopis juliflora (mesquite) in Gantsi District, Botswana

Abstract

Humans have introduced plants into new areas for centuries. These plants can provide food, shelter and other positive benefits. However, introduced plants become invasive plants when they spread uncontrollably and become destructive by transforming the environment of their invaded areas. This is the case with Prosopis juliflora, a woody invasive plant of Central American origin, introduced into the Kgalagadi Desert in the early 1980s. This introduction has tremendously transformed Kgalagadi Desert, improving both its soil and species richness but it has now invaded the adjacent Gantsi District, a place of high economic and ecological value to Botswana. This study assessed the impacts associated with P. juliflora invasion on soil properties and native plants along Hanahai Valley, in Gantsi District. Species richness, density and composition were evaluated in 20 invaded and 20 uninvaded plots. Soil samples were also collected for a range of chemical analyses. A total of 165 plant species were recorded during this study; 158 species in the uninvaded plots and 132 species in the invaded plots, a difference of 26 species. Analysis of P. juliflora invasion association with species richness and abundance in Gantsi District showed that P. juliflora is associated with significantly reduced evenness, richness and density and an altered community composition of associated plants. The presence of P. juliflora was associated with significant increase in values of soil variables including pH, calcium, nitrogen, potassium, sodium, phosphorus, magnesium and cation exchange capacity. Moverover, P. juliflora invasion was found to be associated with an increase in invasion index and a decline in dominant native species heights. It remains unclear whether the measured differences between invaded and uninvaded plots were a result of P. juliflora invasion or P. juliflora preferentially invading sites with certain soil and vegetation characteristics. These results provide a baseline on the probable impacts associated with the presence of P. juliflora in Gantsi District, and suggest that management and control measures should be put in place before it spreads further and exerts more pressure on the ecosystem.