| Summary: | This study investigated the impact of exotic <i>Tamarix</i> species on vascular plant biodiversity in riparian ecosystems in the Western Cape Province, South Africa. Vegetation was sampled, using 5 m wide belt transects, along the Leeu, Swart, and Olifants riparian areas, which had varying invasion intensities. Each transect was split into three zones (Zone 1: 0–15 m; Zone 2: 15–35, and Zone 3: >35 m), which were identified at each site based on species composition across each riparian zone. Woody plant species were identified, counted, and their heights measured within the transects that were laid out from the waterpoint (Zone 1) outwards (Zone 2 and 3). Herbaceous aerial cover (HAC) was determined subjectively and objectified using the Walker aerial cover scale. Leeu River had the highest species richness (Dmg = 2.79), diversity (H′ = 2.17; −lnλ = 1.91; N1 = 8.76 and α = 4.13), and evenness (J′= 0.80). The Swart River had the lowest species richness, which declined from Dmg = 1.96 (Zone 1) to Dmg = 1.82 (Zone 3). Exotic <i>Tamarix</i> species ranked in the top three most abundant woody vascular plant species along the Swart and Olifants rivers, where they ranked first and third, respectively. The Jaccard’s and Sorenson’s coefficients of similarity indicated that species differed greatly between the different sites, <inline-formula><math display="inline"><semantics><mover accent="true"><mi>x</mi><mo>¯</mo></mover></semantics></math></inline-formula> < 27% for both indices. The indices also indicated that the Swart River had the lowest level of species distinctness between zones (<inline-formula><math display="inline"><semantics><mover accent="true"><mi>x</mi><mo>¯</mo></mover></semantics></math></inline-formula> > 80%) while the Leeu River had the highest level of species distinctness (<inline-formula><math display="inline"><semantics><mover accent="true"><mi>x</mi><mo>¯</mo></mover></semantics></math></inline-formula> < 50%) between the different zones. These findings suggest a possible displacement of herbaceous and woody tree species by exotic <i>Tamarix</i> invasion, inter alia, a decrease in ecosystem functions and services associated with the loss in biodiversity, as well as significant bearings on the agricultural ecosystem by reducing the faunal diversity such as crop pollinators, inter alia.
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