The direct and indirect effects of road verges and urban greening on butterflies in a tropical city-state

Road verges have considerable potential to benefit wildlife, but in highly urbanised areas management often limits their value for biodiversity. Evaluating how the management of road verges affects wildlife, both directly and indirectly, provides opportunities to integrate biodiversity into urban planning, design, and management. We studied butterfly pollinators next to main roads across Singapore, a highly urbanised tropical city-state that envisions itself as ‘A City in Nature’. Using structural equation models we quantified how road verge habitat quality (nectar-floral diversity, structural complexity, size, and plant richness) and surrounding landscapes (traffic density and greenness as a ratio of green to concreted areas) directly and indirectly affected butterflies. We found direct positive effects of nectar-floral diversity and structural complexity within road verges on butterfly diversity (abundance and richness). While road verge size and plant richness had no direct effects on butterfly diversity, both had indirect positive effects by increasing nectar-floral diversity and structural complexity. Greenness at a landscape (≥ 500 m radius) rather than local (≤ 250 m radius) scale positively affected butterfly diversity. Traffic density had a direct negative effect on butterfly diversity likely though increased mortality due to collisions. Our findings offer valuable insights for city planners and policymakers, and suggest that simple management decisions, such as improving resource quality within verges, can have positive benefits for biodiversity in highly urbanised areas. As cities around the world develop policy mechanisms to create greener environments, our results highlight opportunities to improve road verges to benefit butterflies, a commonly used flagship taxon for biodiversity.