Global environmental and socio-economic impacts of selected alien grasses... 19 Global environmental and socio-economic impacts of selected alien grasses as a basis for ranking threats to South Africa Khensani V. Nkuna 1,2 , Vernon Visser 3,4 , John R.U. Wilson 1,2 , Sabrina Kumschick 1,2 1 South African National Biodiversity Institute, Kirstenbosch Research Centre, Cape Town, South Africa 2 Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Matieland, 7602, South Africa 3 SEEC – Statistics in Ecology, Environment and Conservation, Department of Statistical Scien- ces, University of Cape Town, Rondebosch, 7701 South Africa 4 African Climate and Development Initiative, University of Cape Town, Rondebosch, 7701, South Africa Corresponding author: Sabrina Kumschick (sabrina.kumschick@gmail.com) Academic editor: C. Daehler  |  Received 14 May 2018  |  Accepted 14 November 2018  |  Published 21 December 2018 Citation: Nkuna KV, Visser V, Wilson JRU, Kumschick S (2018) Global environmental and socio-economic impacts of selected alien grasses as a basis for ranking threats to South Africa. NeoBiota 41: 19–65. https://doi.org/10.3897/ neobiota.41.26599 Abstract Decisions to allocate management resources should be underpinned by estimates of the impacts of bio- logical invasions that are comparable across species and locations. For the same reason, it is important to assess what type of impacts are likely to occur where, and if such patterns can be generalised. In this paper, we aim to understand factors shaping patterns in the type and magnitude of impacts of a subset of alien grasses. We used the Generic Impact Scoring System (GISS) to review and quantify published impact records of 58 grass species that are alien to South Africa and to at least one other biogeographical realm. Based on the GISS scores, we investigated how impact magnitudes varied across habitats, regions and impact mechanisms using multiple regression. We found impact records for 48 species. Cortaderia selloana had the highest overall impact score, although in contrast to fve other species (Glyceria maxima, Nassella trichotoma, Phalaris aquatica, Polypogon monspeliensis, and Sorghum halepense) it did not score the highest possible impact score for any specifc impact mechanism. Consistent with other studies, we found that the most frequent environmental impact was through competition with native plant species (with 75% of cases). Socio-economic impacts were recorded more often and tended to be greater in magnitude than environmental impacts, with impacts recorded particularly often on agricultural and animal production (57% and 51% of cases respectively). Tere was variation across diferent regions and habitats in impact magnitude, but the diferences were not statistically signifcant. In conclusion, alien grasses present in South Africa have caused a wide range of negative impacts across most habitats and Copyright Khensani V. Nkuna et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. NeoBiota 41: 19–65 (2018) doi: 10.3897/neobiota.41.26599 http://neobiota.pensoft.net RESEARCH ARTICLE Advancing research on alien species and biological invasions A peer-reviewed open-access journal NeoBiota