Your 'Science and the city' special issue (Nature 467, issue 7318; 2010) overlooks the potential of urban areas as habitats for native species other than pigeons, rats and cockroaches.
Scientists and planners have traditionally treated human-dominated landscapes as incompatible with nature conservation. But the cost of city development need not include a complete loss of ecological function.
For instance, Norwegian-style green roofs, built with native plant species, would provide energy and allow reduction or collection of water run-off. Other ecological features, such as nesting cavities, could be incorporated into new buildings. Public spaces could host plants for native pollinators and provide habitat links within cities and with surrounding non-urban areas. Urban boundaries can offer resources to edge species that live in these transition areas, as well as to humans.