Building biodiversity: drivers of bird and butterfly diversity on tropical urban roof gardens

Conservation of faunal diversity in highly urbanized landscapes is facilitated through the integration of anthropogenic and natural elements in urban green spaces. Roof gardens have the potential to provide resources for urban wildlife populations, yet they typically offer marginal ecological conditions relative to ground-level habitat so it is necessary to identify the factors associated with focal taxa diversity and abundance to better inform their planning, design, and management. We conducted 20 monthly surveys of diurnal bird and butterfly visitors to 30 urban roof gardens in the tropical city-state of Singapore. Sixty-five site variables were evaluated for their relative importance to four community diversity metrics (species richness, Simpson's diversity, abundance, and functional dispersion) using a conceptual framework relevant to key stakeholders involved in roof garden planning, design, and management. Surveys from a total of 80 observation hours per site recorded more than 23,000 independent bird and butterfly encounters, comprising 53 bird and 57 butterfly species, which represents 13% and 18% of the avifauna and butterfly fauna in Singapore, respectively. Twenty-four species (12 birds, 12 butterflies) were considered uncommon or rare. Reproductive behaviors (courtship, mating, or nesting) were noted in 35 species (20 birds, 15 butterflies). Understory birds were under-represented, whereas understory butterflies were over-represented compared to the overall Singapore fauna. Early morning noise showed strong negative linear correlations with diversity of both taxa, but this variable was also negatively associated with planted area, perhaps indicating proxy effects. Faunal diversity decreased asymptotically with height. Both managed and spontaneous floral diversity had clear positive effects on bird and butterfly diversity. Our results provide clear indication that tropical urban roof gardens can support a diverse subset of the bird and butterfly species assemblages. Site height, site area, and plant selection are suggested to play dominant roles in determining the attractiveness of roof gardens for wildlife. To a first approximation, roof gardens should be built lower than 50 m in height and contain planted areas larger than 1100 m2, as well as shrubs for birds and both managed and spontaneous nectar plant species for butterflies.