Designing the biodiversity-friendly city of the future: An avian community perspective on land sharing and land sparing

The continuing increase in the global urban human population will exert profound pressures on biodiversity and ecosystem services. Urban planners face the challenge of accommodating such growth while minimising its impact on biodiversity. This could be achieved through urban land sparing (developing currently low density urban areas without expanding into adjacent semi-natural habitats) or land sharing (creating relatively low density areas in place of semi-natural habitats). We assessed support for these two strategies by examining how urban breeding bird diversity and associated ecosystem services (bird aesthetic attractiveness) and disservices (occurrence of species prone to generate human-wildlife conflicts) responded to a gradient of human population density measured across six Italian cities. As human population density increased, there was a decline in avian taxonomic and functional diversity and a bird-mediated ecosystem service, but an increase in bird biomass and a bird-related ecosystem disservice. Land sparing was supported in forest species and cavity nesters. Other relationships were linear, with no support for either strategy. To enhance bird diversity and related ecosystem services in our study region, we recommend increasing human population densities within already urbanised areas, while conserving existing bird diversity by preserving semi-natural habitats, particularly at the urban fringe. Among these, forest and urban green spaces significantly enhanced bird diversity, in contrast to the simplified assemblages in peri-urban farmlands. To design more bird-friendly cities, we recommend to: 1) improve the quality of existing urban forest remnants, 2) prioritise urban reforestation on farmland at the urban–rural fringe, and 3) expand urban green infrastructures to support forest-associated bird-diversity and to reduce social costs of densely populated cities.