A preliminary checklist of alien and invasive plant species within protected areas of the City of Johannesburg, Gauteng, South Africa

Background: Urbanisation has significantly transformed natural landscapes, posing substantial challenges to biodiversity, especially in protected areas managed by municipalities. In South Africa, limited data exist on the diversity and distribution of alien plant species within urban protected areas. Objectives: This study assesses alien plant species’ distribution, abundance and invasion dynamics in protected areas managed by the Johannesburg City Parks and Zoo, Gauteng. Methods: We systematically surveyed alien plants across 10 protected areas and documented their abundance, life form, pathways of invasion, legal status, degrees of establishment and origins. Results: The alien flora in the surveyed protected areas comprised 175 species from 125 genera and 56 families. Of these species, 82 were herbs, 43 trees and 27 shrubs, with most species forming established populations (48.2%) and few are invasive (10%). Species richness was highest at Albert Farm (n = 72) and Beaulieu (n = 69), with the former also having the highest species abundance. Most alien plant species originate from South (36%) and North America (34%) and were likely introduced to these protected areas as ornamental and horticulture plant escapes. Most species (n = 103) are not listed in the South African Alien and Invasive Species Regulations, while those listed were predominantly Category 1b species (n = 51). Conclusion: The study highlights that protected areas in urban areas often have alien plant species, likely because most of the species are introduced by humans as ornamentals and through horticulture. These areas can act as foci for new invasions, and it is therefore important to compile lists of alien species in these areas that can be used to develop appropriate management interventions.