Don't lose your lid! (the green roof argument)
The novel ecosystem sitting on our local school is composed of Sedum and Sempervivum, grass and sedge. It saves heat energy and conserves some water within the roof, but does it encourage the insect, bird, worm or any other aspect of aerial biodiversity. It isn't a tree, but lichens and epiphytes can use it. There are no fruits, but seed-eaters might find some nutrition within its plant community. Of course, we can make a "heavy" green roof of around 12.8cm minimum depth, but they require a lot of maintenance and don't therefor suit very high situations.
Nicholas S.G. Williams, Jeremy Lundholm and J. Scott MacIvor of St Mary's University, and York University in Canada and the University of Melbourne in Australia have conducted a worldwide survey of green roofs from South Africa to Europe and North America to Australia. Of the 6 theories about possible biodiversity derived from the conservation of certain habitats within these roofs, some seem to have legs! Their paper will soon appear in the Journal of Applied Ecology as - Do green roofs help urban biodiversity conservation?.
We started a series of sustainability articles earlier this year with Energy to people. Now a sustainable green roof seems likely to become common, even for great civic buildings. It seems this research, when it is published, won't establish certain proof that conservation claims are correct. The communities are not equivalent to ground level conserved areas, as many species are lacking. The growing need for ecological research in conjunction with the building industries' architects and technicians will possibly produce the first "ecological engineers."
The small animals need to be considered if full benefit is to be realised. Urban bees such as Lasioglossum and Halictis spp. can certainly feed within the roofs, fungus gnats are among the flies that can survive in such a shallow ecosystem and the beetle fraternity can work their ways into any available woody plant, its flowers and seeds. But the insects are only part of the puzzle. The only vertebrate community could be birds, or probably some agile lizards, but as prey populations build, the other vertebrates could join these early colonists! What about communities of a small monkey feeding on plant material? If your mind isn't boggling, then there's something wrong with you!
To keep our feet and imagination on the ground, it would perhaps be wiser to concentrate on encouraging species that aid pollination or provide agricultural benefits in other ways, then grow crops such as tomato, pepper and, as they have done in Toronto. Germany, especially Berlin, produced the first green roofs for cities in the 19th century and now has about 10% "greening," but ancient sods have been used on roofs since (human) time began. They act even in cities to ameliorate the climate (potentially reducing "heat island effects" and lowering temperatures by 4 degrees Celsius) not only of the immediate area, but the whole urban area Among many beneficial effects, cooling can be reduced by from 50-90% and stormwater run-off that plagues cities is reduced according to the area of the green roofs.
The jury then are still out for the final verdict, but the clincher could be that your own house could gain in value by 7%, if it were given a green roof. The added expense of installation is offset by the benefits such as the length of time it remains in place, as well as the energy benefits we describe.