Green roofs: new ecosystems to defend species diversity

Many contributions to the symposium seek to expand the role of green roofs in the conservation of biodiversity. Indeed, if green roofs can be harnessed for biodiversity, they will add area to that now available to nature. That would have the mass effect of increasing the sustainable number of species in simple conformity with the species--area relationship. Because all green roofs are novel ecosystems, all represent instances of reconciliation ecology, i.e., re-engineering human uses to permit simultaneous beneficial use by people and nature. Green roofs can provide a large number of experiments that might teach us how to improve their design. But those experiments, like any in science, must be overtly designed so that their hypotheses are clear and explicit, their methods repeatable, and their data appropriate for rigorous analysis. I present an embryonic example using native plant species growing at ground level in the urban environments of Tucson, AZ, USA. Steps include: (1) formulating a hypothesis; (2) developing a database of species' attributes to allow intelligent selection for hypothesis testing; (3) developing software to allow winnowing the list of species to sets with a good chance, according to the hypothesis, of growing together; (4) installing the sets of plants and measuring the results; (5) defining a continuous measure of conformity with the hypothesis; and (6) comparing results to hypothesis. If ecologists can successfully design reconciled ecosystems in urban settings – green roofs included – city people will be able to re-establish their everyday connection to nature.