Urban green spaces as reservoirs of exotic plant species with invasion risk: A case study on the ornamental flora of Nairobi City, Kenya

Abstract

Human-driven global redistribution of flora for various economic purposes has contributed to the naturalization of several thousand species of plants outside their native ranges. A significant proportion of naturalized species have become invasive whereby they significantly alter biodiversity, disrupt ecosystems, and cause socioeconomic losses. Urban ornamental horticulture is a major pathway for the introduction of exotic plants, as these species are widely cultivated for their aesthetic appeal and adaptability. However, inventories of exotic plants grown in urban areas are lacking in many African countries. This study surveyed ornamental plants in 100 urban green spaces (parks, gardens, urban forests, cemeteries, playgrounds and squares) in Nairobi City, Kenya. The objective was to assess the prevalence of exotic species, their geographic origins, economic uses, and naturalization/invasion status. We identified 638 plant species, 11 of which are native to Kenya. Trees were the most common, followed by shrubs and perennial herbs. Most exotic species originated from Africa, South America, and tropical Asia, while fewer came from North America, Europe, and Oceania. Of the exotic species, 53 % were casual (not yet established in the wild), 30 % were invasive somewhere in the world and 17 % were naturalized. In addition to their ornamental use, all species served various purposes: medicinal (48.43 %), construction (29.31 %), environmental conservation (17.40 %), human food (2.98 %), multiple uses (0.94 %), fodder (0.63 %), and biofuels (0.31 %). The finding that 30 % of exotic ornamental plant species grown in Nairobi City had a history of being invasive in different parts of the world emphasizes the need for proactive measures to prevent their potential invasion in Kenya and other regions with similar climates.