Evaluating the biodiversity of green roofs in Hong Kong by benchmarking natural sites

As urbanization intensifies, maintaining and enhancing biodiversity within limited urban spaces becomes increasingly challenging and critical. Urban green roofs have emerged as an important opportunity for expanding green spaces and conserving biodiversity. This study evaluated the biodiversity of green roofs in Hong Kong by benchmarking comparable natural sites. Over the course of a year, we monitored species composition at eight green roofs and eight natural sites, focusing on four biological groups: plants, butterflies, birds, and bees/wasps. The findings indicate that while green roofs can contribute to urban biodiversity, they often fall short compared to natural habitats, particularly in supporting native plant species and butterflies. At the natural sites, about 2/3 of plants are native, whereas those on the green roofs are mainly alien species. Overall, green roofs achieved 66 % of the plant species richness and 27 % of the butterfly species richness observed at natural sites. The natural sites and green roofs also differ greatly in the distributions of main plant families and life forms. The findings reveal strong and significant correlations between butterfly richness and butterfly diversity and a variety of plant traits. This study highlights the necessity for strategic planning of urban green roofs, including identification of the key species for conservation, prioritization of native plant species, and enhancement of habitat complexity. The findings offer insights for urban planners and policymakers seeking to foster resilient and diverse ecosystems within compact cities.