Assessment of the invasive potential of Grevillea robusta in South Africa

Marikhele, Rabelani
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Grevillea robusta is classified as a Category 3 invader by the National Environmental Management: Biodiversity Act (NEMBA) in South Africa. The species has recently been observed invading riparian areas from old cultivated trees in Limpopo, Mpumalanga and Gauteng provinces. Since the early 1980’s Grevillea robusta A.Cunn populations have increased from 15 observations in 1980─1985 to 39 in 1996─2000 and 47 between 2016 and 2017 in South Africa. The species is currently distributed in a total of 188 localities in 7 South African provinces. The aim of this study was to assess the invasiveness of G. robusta in South Africa and determine its appropriate Category status by assessing its invasive capacity by conducting a population census and assessing its transformative capacity at Moederlief farm, South Africa. Grevillea robusta is widespread in South Africa, however, there are no recorded observations in the Free State and Northern Cape provinces. The results from iSpot, CLIMEX, Southern African Plant Invaders Atlas (SAPIA) and field observations in figures 13, 14, 15 and 16 shows 52 observations in Limpopo and 37 between KwaZulu-Natal and Mpumalanga provinces. A climatic match index by CLIMEX was employed, and results indicate that G. robusta has the potential to grow well in Limpopo, Mpumalanga and Gauteng provinces and along the coastal regions. Furthermore, healthy populations of G. robusta were observed at Moederlief farm, where the biggest recorded tree in the field was estimated to be 45 to 52 years old. Grevillea robusta produces viable seed which may be dispersed 364 m away from parent trees. Looking at the population structure, age of reproduction, seed viability and ease of dispersal, the populations of G. robusta at Moederlief farm will continue to grow and invade even in other areas that are yet to be considered risk areas. Grevillea robusta is an invader that needs further evaluation of all the localities in South Africa. The species should be placed in Category 1b of invasive species and immediate control is required in all riparian zones.
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, in partial fulfilment of the requirements of the degree of Master of Science, Johannesburg, 2018
Marikhele, Rabelani (2018) Assessment of the invasive potential of Grevillea robusta in South Africa, University of the Witwatersrand, Johannesburg,