Compact cities or sprawling suburbs? optimising the distribution of people in cities to maximise species diversity

1. Conservation of biodiversity in urban environments depends on the responses of species to the intensity of urban development. ‘Land sharing’ and ‘land sparing’ represent alternate ends of a gradient that conceptualises a trade-off between the human population and biodiversity. We used a linear optimisation procedure to 1) identify the optimal allocation of land for people and nature, 2) assess whether the optimal allocation is more similar to land sparing or land sharing, and 3) examine how this might change under scenarios of human population growth. 2. We surveyed birds in 28 landscapes, each 25 ha in size, along a gradient of human population density (zero to ~1600 persons/25 ha) in the Greater Melbourne region, Australia. Species’ responses to population density were estimated using generalised additive models (GAMs). These relationships were then used to determine the optimal allocation of land among different categories of population density based on maximising a community index, the geometric mean of relative abundance (G) of bird species. 3. Human population density was an important driver of the reporting rate for 28 species. Response curves differed among ‘urban avoider’, ‘urban adapter’ and ‘urban exploiter’ species. For the current human population in the study area, optimal allocation of land included elements of both land sharing and land sparing. However, for scenarios of increased population size, optimal allocation converged upon a land sparing design. 5. Synthesis and applications. Urban areas represent a mosaic of land uses that offer habitats of differing quality. Land sharing, based on sustaining biota amongst residential areas, performed poorly under all scenarios due to its inability to support species that depend on natural or semi-natural habitat. To sustain more than a homogenised avifauna in urban regions, large tracts of natural vegetation are needed within, or adjacent to, the urban environment. Protecting natural areas on urban fringes will be critical to the safeguarding of nature in the future as urban populations and land-use inevitably expand.

1. 城市环境中的生物多样性保护，取决于物种对城市开发强度的响应。“土地共享（land sharing）”与“土地预留（land sparing）”是构成权衡人类人口与生物多样性关系的梯度两端的两种理念。本研究采用线性优化流程，旨在达成三个目标：其一，确定服务于人类与自然的土地最优配置方案；其二，评估该最优配置更趋近于土地共享还是土地预留模式；其三，探究在人口增长情景下，该最优配置可能发生的变化。
2. 研究团队在澳大利亚大墨尔本地区，沿人类人口密度梯度（0至约1600人/25公顷），对28处面积均为25公顷的景观开展了鸟类调查。通过广义加性模型（generalised additive models, GAMs）估算物种对人口密度的响应特征，随后基于最大化鸟类群落相对丰度几何均值（G）这一社区指数，确定不同人口密度类别间的土地最优分配方案。
3. 人类人口密度是影响28个鸟类物种观测记录率的重要驱动因子。“避城种（urban avoider）”、“城市适应种（urban adapter）”与“城市利用种（urban exploiter）”三类物种的响应曲线存在显著差异。针对研究区域当前的人口规模，土地最优配置方案同时包含土地共享与土地预留的要素。但在人口规模增长的情景下，最优配置将逐渐趋近于土地预留模式。
5. 综合与应用。城市区域是由不同质量栖息地构成的土地利用镶嵌体。基于在居住区中维持生物群落的土地共享模式，由于无法支撑依赖自然或半自然生境的物种，在所有情景下的表现均欠佳。若要在城市区域维持均质化鸟类群落之外的生物多样性，必须在城市环境内部或邻近区域保留大片自然植被。随着城市人口与土地利用规模不可避免地扩张，保护城市边缘的自然区域，将成为未来守护自然的关键举措。