City road verges rich in flowers and cut less often improve butterfly diversity
Butterflies are important pollinators in both natural and urban ecosystems. Yet, their populations are declining globally, particularly in urban settings like cities.
New research led by Nanyang Technological University, Singapore (NTU Singapore) reveals that simple and space-efficient ways to manage road verges—strips of vegetation planted along roads—can significantly improve butterfly numbers and diversity in urban environments.
Importantly, these interventions do not require additional land. Instead, small changes to existing road verges, such as increasing the variety of flowering plants and adjusting how and when vegetation is trimmed, can create more butterfly-friendly habitats.
The findings, published in Landscape and Urban Planning, are based on field surveys of 101 road verges at least 30 m long across Singapore’s dense urban landscape. These verges are artificially constructed and mainly planted with non-native shrubs, and located along major roads with speed limits of between 50 and 70 km/h.
The research offers four key recommendations for planners and policymakers aiming to enhance urban biodiversity.
1. Increase flowering plant diversity
The study found that road verges with a greater variety of flowering plant species directly supported higher numbers of butterflies and a wider range of butterfly species.
Additionally, larger road verges with greater overall plant diversity tend to host more flowering plants, indirectly benefiting butterflies by providing continuous sources of nectar throughout the year. This consistent floral availability is especially important for butterflies that depend on multiple nectar sources.
2. Adopt flexible vegetation management
Butterfly biodiversity also increased in road verges where vegetation was allowed to grow to different heights, either by selectively cutting plants or pruning them less frequently.
When the structure of plants in a verge is uneven, it creates a range of microclimates—variations in temperature, light, and humidity—which suit different butterfly species. Taller plants may also act as windbreaks and offer protection from predators like birds, making the verge a more hospitable habitat.
However, these adjustments should also be balanced with safety needs, since tall verge plants could obstruct motorists’ view at certain parts of a road.
3. Integrate verge design with urban green networks
Butterfly abundance and species richness was highest in verges located near larger green spaces such as parks, gardens, or natural greenery within a 500 m to 1 km radius. This indicates that road verges can function as ecological corridors, especially when they are connected to broader urban green networks. Promoting connectivity between small, long habitats like road verges and nearby larger green spaces helps butterflies move across the landscape, access nectar, and find suitable breeding sites.
4. Reduce adverse traffic impacts with verge design
Heavy traffic was found to negatively impact butterfly populations. Fast-moving vehicles generate wind turbulence and increase collision risk with butterflies, while vehicle pollution may affect both plant health and floral scent cues that butterflies rely on to find flowers.
However, these effects can be partially mitigated. On roads with moderate traffic, where speed limits hit 70 km/h or lower, verges with diverse and structurally complex vegetation still support healthy butterfly communities. Combining slower traffic speeds in residential areas with nature-sensitive verge designs can help both biodiversity and pedestrian safety.
But when roads have higher speed limits, the adverse effect of dense traffic could be too strong for verges to compensate.
Singapore’s green vision: Where nature meets the city
Previous research on how road verges affect butterflies were focused on temperate regions and naturally occurring roadside plants in less urbanised areas. The impact in tropical countries like Singapore was not well understood.
But the latest study explores the effect of cultivated vegetation along major roads in a highly urbanised tropical city-state.
Its findings complement Singapore’s long-standing commitment to urban greening, which began in the 1960s.
Under the nation’s current “City in Nature” vision, more roads are being transformed into “nature ways”—green corridors lined with trees and shrubs that mimic the layered structure of natural forests. These corridors are designed to support urban biodiversity by helping wildlife, including butterflies, move between parks and other green spaces.
