Green roofs are a peculiar kind of designed habitat. In already highly urbanised areas, where there is large pressure on efficient land use, green roof retrofits are a key strategy to bring back nature without losing building capital.

While green roofs are known to have many economic and social benefits, such as stormwater retention and thermal buffering, their usefulness as wildlife habitat remains an open question. Multiple studies record insects, birds, spiders and other animals existing on green roofs, but there is little known as to how (and if) they are subsisting, or indeed what they are doing up there.

This is the focus of a new study by ICON Science researchers Georgia Garrard and Katherine Berthon, and research assistant Jess Baumann, who are setting out to document how animals (particularly birds and insects) are using green roofs in the City of Melbourne by monitoring two new green roof retrofit developments. This project is funded under the City of Melbourne ‘Green Our Rooftop’ initiative, and uses a large new green roof retrofit that is set to be constructed on 1 Treasury Place in Fitzroy, as well as the innovative Melbourne Skyfarm that will replace the top level of the Siddeley Street Carpark in Docklands.

Monitoring a green roof retrofit before and after its construction is an ideal way to answer some of the tricky questions about the secret life of rooftop animals. For example, we don’t yet fully understand the value of a single roof, the primary pathways by which animals come to live or forage on a roof, and whether there is any movement of animals between rooftops.

It is often not acknowledged that some invertebrates (like spiders and flies) and some birds (like the Peregrine Falcons nesting on 367 Collins St) are capable of using bare roof spaces. Knowing this baseline of buzzing activity helps us know what animals have been attracted after a green roof is installed. This allows us to quantify the added value of a new green roof in the landscape.

It is likely, however, that the construction of a green roof is so disturbing and prolonged for a site that it might wipe the slate clean. This is where monitoring the site immediately after construction is important – it tells us the first-comers, and potentially highlights stowaways that have been transported onto the roof with the plants or in the soil. Perhaps surprisingly, snails have been found in large quantities on some green roofs, many stories high – likely as a result of hitch-hiking on plant material during roof installation.

We will also monitor nearby ground sites that might act as sources of animals that appear on the new green roofs. This may also show whether anything that is transported onto the roof might spill over into adjacent environments. Often soils and plants used in green roof construction come from far away, and can create assemblages of species that are atypical of the regional area. Finally, monitoring adjacent roof sites helps answer whether there is any spill over effects that might generate synergistic or additive effects of multiple green roofs popping up in the landscape.

During our study we will also be recording specific plant-insect interactions so we can not only know what insects are up on the roof, but what they are using it for. Pre-covid lockdown we had just finished collecting the baseline data to show what’s happening before the roofs have been constructed. Not surprisingly there wasn’t much happening on the bare roofs except for a few spiders and flies, whereas ground sites were still bustling with summer insect life.

At the end of our study we hope to know a little more about what makes a green roof good for biodiversity. Importantly, we want to avoid making roofs that act as “ecological traps” by enticing animals to live there without adequately fulfilling their life-cycle needs. Already we know that some ground nesting birds, whose nestlings are left to fend for themselves after hatching, find low reproductive success on green roofs. Solitary bees also struggle to produce viable offspring on roofs above 5 stories high. So, how do we create green roofs that provide the right resources, especially for breeding? To stay tuned follow the ICON blog or follow me on Twitter @CityKat75

References:

Baumann, N. (2006) Ground-nesting birds on green roofs in Switzerland: Preliminary observations. Urban Habitats 4 (1), 37-50.

MacIvor, J.S. (2015) Building height matters: nesting activity of bees and wasps on vegetated roofs. Israel Journal of Ecology & Evolution (ahead-of-print), 1-9.

Madre, F. et al. (2013) A comparison of 3 types of green roof as habitats for arthropods. Ecological Engineering 57, 109-117.

Williams, N.S. et al. (2014) Do green roofs help urban biodiversity conservation? Journal of Applied Ecology 51 (6), 1643-1649.

Shafique, M. et al. (2018) Green roof benefits, opportunities and challenges – A review. Renewable and Sustainable Energy Reviews 90, 757-773.