Urban development, land sharing and land sparing: the importance of considering restoration.
At present, there is limited knowledge of how best to reconcile urban development with biodiversity conservation, and in particular whether populations of wild species would be greater under low-density housing (with larger gardens), or high-density housing (allowing more area to be left as undeveloped green spaces). The land sharing/sparing framework - originally developed in the context of farming - can be applied to address this question. We sampled the abundance of trees in the city of Cambridge, UK, along a gradient of human density. We designed different scenarios of urban growth to accommodate the human population predicted in 2031. For each scenario, we projected the future city-wide tree population size and quantified its carbon sequestration potential. We also considered, for the first time in an urban sharing-sparing context, the implications of habitat restoration on degraded urban green space. We found that the density of most native and non-native tree species is presently highest in areas of low human density, compared to both higher density areas and green space (which is largely maintained with few trees). However, restoring woodland in green spaces would lead to far greater densities of native trees than on any existing land use. Hence, provided >2% of green space is restored, native tree population sizes would be larger if urban growth followed a land-sparing approach. Likewise, carbon sequestration would be maximised under land sparing coupled with restoration, but even so only a maximum of 2.5% of the city's annual greenhouse gas emissions could be offset. Although both tree populations and carbon storage thus appear to benefit from land-sparing development, the risk that this might widen the existing disconnect between people and nature must also be addressed - perhaps through a combination of adding housing in low-density areas while ensuring these are in close proximity to high-quality green space. Synthesis and applications. In regions which have already been cleared of intact habitat, a combination of land-sparing urban development with the restoration of green space could accommodate urban population growth whilst dramatically improving the existing status of local tree populations. Where cities are expanding into intact habitat, the merits of urban development by land sparing may be even more pronounced. Studies in such regions are urgently needed.