The invasion potential of selected Berberis species in South Africa

English: Invasive alien species are a serious threat to global biodiversity. They have considerable negative effects on the economy and the environment, using up valuable natural and monetary resources. Every year new species are introduced into South Africa and with that the list of potentially new invasive species grows. These emergent invasive species should receive a very high priority in terms of assessing their invasion potential and possible impacts. No formal study has been conducted regarding the invasion potential of any species in the genus Berberis within South Africa, even though it has been a popular horticultural genus for many decades and is known to have invasive species. The current study focussed on four key points, namely: 1) the elucidation of all Berberis spp. that have been historically and are currently cultivated within South Africa, 2) assessing the size, extent and reproductive age of any naturalized/invasive Berberis populations, 3) determining potential habitable areas of naturalized/invasive Berberis species and 4) determining whether any Berberis species should be officially listed under the National EnvironmentalManagement Biodiversity Act. A total of 30 Berberis species/cultivars/hybrids were found to have been cultivated in South Africa in the past and present (11 species, 11 cultivars, 8 hybrids). Three of these species are widespread invasives in other parts of the world, namely B. darwinii, B. thunbergii and B. vulgaris. The KwaZulu-Natal Province has the highest diversity of species/cultivars/hybrids stocked by nurseries/private/wholesale growers, while B. thunbergii and B. thunbergii var. atropurpurea were found to be the most widely stocked species. Two invasive Berberis populations were found. The first population was from the species Berberis aristata and occurs in the Woodbush State Forest (Limpopo Province; 23.8192°S 29.9608°E). This population has an extent of occurrence of 115 ha and an area of occupancy of 1.58 ha. A total of 5 725 individuals were geotagged and the population was found to be highly reproductively active, with more than 40% of the population able to flower and set seed. The second population was from the species B. julianae and occurs at the Glen Reenen rest camp in the Golden Gate Highlands National Park (Free State Province; 28.5049°S 28.6187°E). This population has an extent of occurrence of 0.42 ha and an area of occupancy of 0.02 ha. It was also found to be highly reproductively active with 38% of the population able to flower and set seed. A few minor occurrences of this species were found at the Alma Ranger Station, also in the Golden Gate Highlands National Park. A total of 473 individuals of this species were geotagged. Bioclimatic modelling revealed that substantial parts of South Africa are suitable for both Berberis aristata and B. julianae. The former species was found to be more suitable to the mountainous regions while the latter has a higher suitability to the central parts of the country. The situation regarding future climate scenarios (2020) was the same, although a range contraction was found for both species. Weed risk assessments revealed that both B. aristata and B. julianae would have failed a pre-border screening (score of 27 and 22, respectively, according to the Australian Weed Risk Assessment system), indicating that both species pose an environmental risk. Herbicide trial clearing revealed that three chemicals can be used for the successful control of Berberis spp., namely metsulfuron-methyl, triclopyr and glyphosate. The final outcome of the study is the following recommendations: Berberis aristata should be listed as a category 1a invasive species in the National Environmental Management Biodiversity Act, while B. julianae should be listed as category 1b in non-urban areas and not listed in urban areas.