Abstract
Many Prosopis species have been introduced to South Africa; some taxa and their hybrids have naturalised and become widespread invasive trees. These invasions have detrimental effects on biodiversity, ecosystem services and human livelihoods. Although several studies have documented these impacts, the studies have been limited to single sites or restricted areas. This study assessed the Prosopis population across the full invasive range of the genus in South Africa, and quantified the effects of invasions on native woody and herbaceous species. Basal areas of invasive Prosopis stands reached 9m2/ha, and were on average higher along perennial rivers than along ephemeral rivers (mean basal areas of 3.2 vs. 1.4m2/ha). Native woody species density, basal area, richness and diversity all decreased significantly as the basal area of Prosopis stands increased. For example, up to eight native woody species occurred at basal area of <2m2/ha, this decreased to three native species or fewer at basal areas of >4m2/ha. The cover of native perennial grasses and herbaceous plants declined from 15–20% where the basal area of Prosopis was <2m2/ha to zero where the basal area of Prosopis was >4.5m2/ha. The results highlight the widespread nature of the impacts across all invaded biomes. Current control of Prosopis has had limited success, and alternative, potentially more effective, options are controversial. In the light of the widespread impacts, we recommend that a thorough assessment of the problem be undertaken to inform policy.
Highlights
Many ‘multi-purpose’ trees have been transported around the world and several have subsequently become naturalised and invasive (Rejmánek and Richardson, 2013)
We investigated the degree of variation in the basal area of invasive Prosopis trees at 11 sites across three biomes (Nama Karoo, Savanna and Succulent Karoo, see Mucina and Rutherford, 2006) and three river classes
Prosopis density and basal area across different environments We investigated whether the basal area of Prosopis invasions differed between the river categories
Summary
Many ‘multi-purpose’ trees have been transported around the world and several have subsequently become naturalised and invasive (Rejmánek and Richardson, 2013). Negative impacts of Prosopis invasions on a wide range of native organisms have been documented in many parts of the world These include reductions in plant species richness, density and diversity in Hawaii, India, Kenya and the United Arab Emirates (El-Keblawy and Al-Rawai, 2007; Kaur et al, 2012; Muturi et al, 2013), increased native tree mortality in Brazil and South Africa (Schachtschneider and February, 2013; de Souza Nascimento et al, 2014; Shackelton et al, 2015), negative impacts on bird and insect community composition in South Africa (Steenkamp and Chown, 1996; Dean et al, 2002) and reductions in turtle and bird recruitment on Atlantic islands (Belton, 2008). Ecosystem services such as soil quality, grazing and water supply are affected by Prosopis invasions, leading to a range of negative consequences for local human communities (Geesing et al, 2004; Mwangi and Swallow, 2005; Ndhlovu et al, 2011; Wise et al, 2012; Dzikiti et al, 2013; Ayanu et al, 2014; Shackleton et al, 2014)
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