Drivers of compositional turnover of the non-native urban flora in the Western Cape, South Africa

Abstract

Abstract Interest in understanding the drivers of biological invasions in urban centres is increasing globally. Previous research focused primarily on the drivers of non-native species richness in urban centres (i.e. why some cities harbour more species than others) and there has been much less focus on compositional turnover (i.e. why different cities harbour different species). Exploring compositional turnover between urban centres can differentiate drivers and barriers of non-native species at different stages along the introduction-naturalisation-invasion continuum and help to inform effective management strategies. This paper explores the drivers of compositional turnover of wild-growing non-native plants across urban centres of the Western Cape province in South Africa, ranging from the city of Cape Town (821 km 2 ) to the small town of Bot River (1.7 km 2 ). We assess the role of temperature, precipitation, urbanisation intensity, urban area, travel time, year of establishment, and population density in mediating richness-independent species turnover between a range of urban centres. Multi-site generalised dissimilarity modelling (MSGDM) of zeta diversity (number of shared species among multiple urban centres), and variance partitioning are applied to analyse inventories compiled from cleaned iNaturalist occurrence records. The importance of environmental conditions increased toward the later stages of the invasion continuum while variables related to introductions decreased. The study concludes that, despite the major changes to habitats caused by urbanisation and anthropogenic activities, climate is the primary driver of turnover of urban non-native plant species within the Western Cape. Management initiatives in the Western Cape should be guided by regional biogeographic context, with strategies adapted to local species pools and climatic constraints rather than relying on uniform, across-city approaches.