Green Roof Performance Towards Good Habitat for Butterflies in the Compact City

Urban ecology is threatened by habitat loss and fragmentation due to increasing urbanization. Green roofs may act as habitats to compensate for loss of green space at the ground level. Here, we assessed greening variables of 11 green roofs for butterflies in Taipei City. Butterfly number, species, and richness on green roofs were lower than parks, but some less common species were observed on green roofs. The nectar plant area, number of nectar plant species and age of green roof were the main positive effectors of butterfly number. The height above ground of green roof had not impact on butterfly survival; supposed high rise buildings are spreading out over city that would be a potential good habitat for butterflies on skyscape. However, since the scale of green roofs was small in Taipei, we adopted a habitat suitability index (HSI) method to determine optimal value of selected greening variables to attract more butterfly number on green roofs. HSI curves' findings suggested that achieving a nectar plant area of more than 25 m2 and not less than 10 nectar plant species would greatly benefit butterfly number; meanwhile, the age of green roof was higher than 38 months, butterfly number is expected to increase rapidly. We confirmed that carefully design green roof could play a good habitat for butterflies in Taipei city.