Where are the invaders hiding? Insights on the compilation of species and occurrence data for alien plants in small urban areas: Examples from Riebeek Kasteel, Western Cape, South Africa

Where are the invaders hiding? Insights on the compilation of species and occurrence data for alien plants in small urban areas: Examples from Riebeek Kasteel, Western Cape, South Africa
Phil McLEAN1,2, David M. Richardson1, John R. Wilson1,2, Mirijam Gaertner1,3
1DST-NRF Centre of Excellence for Invasion Biology, Department of Botany & Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa
2Invasive Species Programme, South African National Biodiversity Institute, Kirstenbosch National Botanical Gardens, Private Bag X7, Claremont 7735, South Africa
3Invasive Species Unit (ISU), Environmental Resource Management Department (ERMD), Westlake Conservation Office, Ou Kaapse Weg, Postal address: Invasive Species Unit, Postnet Suite 80, Private Bag X26Tokai, 7966

More than half the world’s population live in towns or cities. Among urban environments, small urban centres (towns) that harbour invasive species have a large edge-effect that exposes the surrounding natural environment to elevated levels of invasive propagule pressure. Despite this threat, there are very few studies globally that investigate any patterns and processes evident in alien invasions within urban environments. For this study, I undertook a detailed analysis of the invasive species load (type, number and location) within the small urban centre of Riebeek Kasteel in the Berg River Catchment, Western Cape, South Africa. Total numbers of all alien woody and succulent plant species were recorded at 10m intervals on every road surface throughout the town and further categorised by land-use type. The status of invasion Riebeek Kasteel is comprehensively mapped and analysis of the data reveals hypotheses around the density, location and possible spread of alien plant taxa within this town. Lessons learned allow the search methodology to be refined and these hypotheses tested on other small urban centres in the catchment to determine whether trends are uniform across towns or particular to them. This has application for determining how and where to search for similar species or growth forms in other small towns as well as how much search effort is required to sufficiently determine a given town’s invasive species ‘load’.