Even narrow strips of flowering plants along roadsides can be surprisingly effective in maintaining high butterfly diversity in urban environments, according to recent research conducted in Singapore.

A study published at the end of 2023 saw researchers survey 101 roadside green spaces – narrow, elongated green areas along roads – across the tropical city-state. They recorded a total of 1,320 observations of 56 butterfly species feeding on nectar from 96 different flowering plant species.

"Perhaps the most surprising finding was how effective even these narrow, managed, and somewhat artificial roadside green spaces can be at supporting butterfly communities," stated Tharaka S. Priyadarshana and Eleanor M. Slade of Nanyang Technological University (NTU) in Singapore, co-authors of the study, in a joint email.

The research revealed that a significant majority – 90 out of the 96 plant species visited by butterflies – were non-native. The researchers suggest this indicates that even introduced plant species can provide "meaningful habitat" for butterflies in tropical cities where flowering and butterfly activity occur year-round.

The study also highlighted the common practice in many cities of regularly mowing roadside vegetation for "aesthetic reasons" and to ensure driver visibility. However, the findings showed that roadside green spaces with higher flower diversity, lower mowing frequency, and vegetation of varying heights supported greater butterfly diversity compared to areas with "uniform, flat vegetation."

In an interview with Mongabay, the researchers emphasized that flower-rich roadside strips with reduced mowing offer a "cost-effective and space-efficient way to support urban pollinators without the need for large green spaces."

Unsurprisingly, the researchers observed a decline in butterfly diversity in roadside green spaces with increasing traffic density and vehicle speed. Priyadarshana and Slade suggested that while "reducing speed limits may not always be feasible, promoting low-speed driving in residential areas and incorporating biodiversity-friendly roadside green space designs could help mitigate traffic impacts while also improving pedestrian safety."

Furthermore, the study found that the presence of larger green spaces surrounding roadside strips positively influenced butterfly diversity.

"Connecting small linear habitats like roadside green spaces to larger green spaces such as parks, gardens, or natural remnants at a landscape scale is crucial," explained Priyadarshana and Slade. They added that this "broader green network" provides essential nectar and host plants for butterfly larvae and "creates favorable microclimates for butterflies by buffering against harsh weather conditions."

The researchers noted that while most previous studies on roadside habitats have been conducted in temperate regions, their research fills a gap by focusing on a rapidly urbanizing tropical city. Singapore, having lost most of its primary forests by the early 20th century, has since seen about 50% of its land area covered by urban parks and secondary forests due to greening efforts since the 1960s. The researchers believe that "Singapore, with its vision of becoming a ‘city in nature’ by integrating green infrastructure into urban planning, can serve as a valuable model for other tropical cities."

The findings of this study carry significant implications for urban ecology and conservation biology. While urban environments are often associated with negative impacts on biodiversity, this research demonstrates that well-managed urban green spaces can serve as important habitats. The ability to enhance biodiversity even in narrow spaces has crucial implications for land-efficient urban planning.

Butterflies, as vital pollinators, play an essential role in maintaining ecosystem functions. A decline in butterfly diversity can lead to reduced plant reproductive success and overall ecosystem instability. Therefore, conserving and enhancing butterfly habitats within cities is a critical task for ensuring urban ecosystem health.

Singapore's proactive greening policies provided the backdrop for this research. The "Garden City" initiative, launched in the 1960s, has significantly contributed to securing green spaces and planting diverse vegetation throughout the city, transforming it into a green metropolis. These efforts have positively impacted not only aesthetics but also the resilience of the urban ecosystem. Singapore's example highlights the importance of considering biodiversity conservation from the initial stages of urban planning for other cities.

This study offers important insights into roadside green space management practices. Frequent mowing and uniform vegetation management reduce their value as butterfly habitats, while planting diverse flowering plants, maintaining natural vegetation heights, and reducing mowing frequency are effective in enhancing butterfly diversity. This implies that urban authorities should recognize and manage roadside green spaces not just as areas for aesthetic maintenance but as crucial components of the urban ecosystem.

Furthermore, the finding that traffic volume and vehicle speed negatively impact butterfly diversity underscores the need to seek harmonious solutions between urban transportation policies and ecological conservation. Efforts to mitigate ecological fragmentation and damage caused by traffic, such as easing speed limits in certain areas, promoting eco-friendly transportation, and creating ecological corridors, are necessary.

Moreover, the importance of connectivity between roadside green spaces and surrounding green areas highlights the need to build a city-wide green network. Organically linking various types of green spaces, such as parks, gardens, and riverside greenery, is crucial for enhancing the health of the urban ecosystem and securing wildlife corridors. This emphasizes the importance of establishing long-term visions and plans for urban biodiversity conservation.

The Singapore study scientifically proves that narrow roadside flower strips can play an effective role in maintaining urban butterfly diversity. This presents a new approach to urban ecosystem conservation and offers significant implications for urban planning and management policies. Other cities should also look to Singapore's example and actively utilize and manage green spaces within their urban environments to enhance biodiversity and create sustainable urban environments.

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