Limited and socially unequal contributions of rain gardens to native biodiversity in a tropical megacity

Rain gardens are nature-based solutions designed to promote stormwater infiltration and filtration through the activity of plants and microorganisms. Although they are recognized for supporting biodiversity, data substantiating this co-benefit remain limited. In this study, we evaluated the contribution of rain gardens to plant biodiversity in São Paulo, Brazil. Using a social-ecological and spatially explicit approach, we characterized species composition in 30 rain gardens and analyzed environmental, socioeconomic, and management factors that could explain alpha diversity within and outside the gardens, as well as beta diversity between rain gardens and adjacent areas. Our results indicate that rain gardens enhance urban plant communities by an average of 9.3 species, including 4.8 exotic and 4.5 native species. However, they remain strongly influenced by management practices, with 64 % of ornamental species being exotic, including some invasive. Herbaceous and shrub cover had a significant negative effect on all species groups (native, exotic, and entire community) and was the most important variable explaining alpha diversity in gardens. Species dissimilarity between rain gardens and adjacent areas exceeded 50 % citywide, although no explanatory variable accounted for beta diversity. Additionally, per capita income positively influenced native species richness in adjacent areas and negatively affected exotic species richness in rain gardens. These findings highlight a trend of better management in wealthier areas and expose social disparities in green space quality across the city. Rain garden biodiversity reflects a complex interplay of environmental, management, and socioeconomic factors, and while these systems contribute to urban plant diversity, their potential is limited by the dominance of exotic ornamental species. • The conservation potential of rain gardens is constrained by the frequent use of ornamental exotic and sometimes invasive species; • Socioeconomic variables modulate the environmental effects of rain gardens on plant diversity; • Rain garden species composition reflects social inequality patterns.