As cities expand, green spaces are disappearing – gobbled up by new buildings and infrastructure. This leaves less and less habitat for urban wildlife. However, a new study shows that adding plants to existing rooftops can help bring nature back into the concrete jungle.

Katherine Berthon and colleagues recently monitored how insects and birds colonised a new “green roof” installed in the heart of the city. Green roofs are vegetated spaces created on top of existing buildings, essentially turning boring old rooftops into elevated parks. The researchers wanted to see how quickly animals would find and start using these manmade habitat islands in the sky. Would insect pollinators like bees pay a visit to the new floral resources? And would birds stop by to nest or feed?

By observing the brand-new green roof over time and comparing it to nearby sites, the study provides a unique look into the process of colonisation on an urban rooftop.

Observing Colonisation in Action

The researchers used a BACI (before-after-control-impact) study design to monitor the green roof over time.

First, they observed the rooftop before it was “greened” to get baseline data. Then, the green roof was installed, and the team tracked how biodiversity changed in the months after construction.

They also monitored two control roofs nearby, as well as a ground-level urban park. The park acted as a comparison to the rooftop habitats, while the control roofs provided insight into normal fluctuations in local wildlife.

By watching the green roof transition from barren to vegetated and comparing it to the controls, the researchers could pinpoint patterns in how insects and birds colonised and used the new space.

Rooftop Oases in the Melbourne CBD

The new green roof was installed atop an 8-story car park in central Melbourne as part of a community retrofit project. The impact site was designed as an edible oasis with a mix of flower gardens and vegetable plots.