自然生境是指为生物提供生存空间的自然环境，是维持生物多样性的基础和促进区域可持续发展的重要保障。为了实现与自然和谐相处的愿景，2021年10月联合国生物多样性公约第十五次缔约方大会通过了2020年后全球生物多样性框架。该框架倡导各国政府和全社会采取行动保护自然生境，以实现2050年生物多样性保护目标。城市扩展过程会通过直接或者间接的方式对自然生境产生影响。近年来，全球经历了快速的城市扩展过程。未来，全球城市扩展过程仍将继续。因此，准确评估全球城市扩展过程对自然生境的影响具有重要意义。然而，已有研究主要从直接影响或者间接影响单一角度进行评估，没有同时定量评估城市扩展过程对自然生境的直接和间接影响。此外，已有研究尚缺乏城市扩展过程对自然生境影响与2050年生物多样性保护目标之间的关系定量研究。

为此，本论文围绕“如何准确评估全球城市扩展对自然生境的影响”这一核心科学问题，基于“过程-影响-可持续”的总体研究思路，综合利用情景分析和模型模拟等方法开展相关研究。具体而言，首先在理解全球1992-2016年城市扩展过程的基础上，结合城市扩展模型和共享社会经济路径情景框架模拟并分析了全球2016-2050年城市扩展过程。其次，结合土地利用/覆盖数据和城市土地数据，多尺度评估了全球1992-2016年和2016-2050年城市扩展过程对生境面积的影响，并讨论了城市扩展过程影响生境面积与土地利用/覆盖的关系及城市扩展过程对自然生境景观格局的影响。然后，改进生境质量模型，从直接影响和间接影响角度定量评估全球1992-2016年和2016-2050年城市扩展过程对生境质量的影响，讨论城市扩展过程对生境质量直接和间接影响的差异。最后，耦合城市扩展过程影响的自然生境范围和物种适宜生境范围，定量评估全球1992-2016年和2016-2050年城市扩展过程对濒危物种的影响，并分析了其与2050年生物多样性保护目标的关系，进而为全球生物多样性保护提供政策建议。

论文的主要方法创新是基于生境质量指数，从直接影响和间接影响角度多尺度定量评估了全球城市扩展过程对自然生境的影响。现有研究主要通过计算评估指标与物种丰富度的相关性，进而检验该指标用以评估城市扩展过程对自然生境影响的有效性。研究发现，生境质量指数能够有效反映城市扩展过程对自然生境的影响。全球各生态区的生境质量指数与物种丰富度呈显著正相关关系，相关系数为0.81（P<0.01）。相较而言，已有研究中常用的生境面积指数和城市面积指数与物种丰富度没有显著相关关系（P>0.05）。

论文的主要科学发现包括四个方面。（1）全球经历了快速的城市扩展过程且未来城市扩展过程仍将继续。全球城市土地面积从1992年的24.75万km²增长到2016年的62.10万km²，增长了34.64万km²，增加了1.4倍。对应的城市面积占比从0.20%增长到0.46%，增加0.26%。在各生物群区中，红树林群区城市扩展速度最快。2016-2050年，全球城市土地面积仍将增加39.42（28.76-48.73）万km²，对应城市面积占比将增长0.29%（0.21%-0.36%）。各生物群区中，仍然是红树林群区未来城市扩展速度最快。

（2）城市扩展过程导致自然生境面积损失。全球1992-2016年城市扩展过程导致自然生境面积减少12.34万km²，损失0.11%。各生物群区中，红树林群区城市扩展过程对生境面积的影响最大。全球2016-2050年城市扩展过程将导致自然生境面积减少14.76（10.91-19.05）万km²，减少0.13%（0.10%-0.17%）。各生物群区中，未来城市扩展过程对红树林群区的生境面积影响最大。城市扩展过程占用自然生境的类型以草地和林地为主。全球1992-2016年城市扩展过程占用了6.59万km²草地和林地，占全球同期城市扩展过程影响生境总面积的53.40%。2016-2050年，全球城市扩展过程将占用8.83（5.66-18.05）万km²草地和林地，占全球同期城市扩展过程影响生境总面积的59.74%（51.85%-65.03%）。

（3）城市扩展过程导致生境质量下降。全球1992-2016年城市扩展过程导致生境质量损失0.85%。各生物群区中，城市扩展过程对红树林群区生境质量的影响最大。全球2016-2050年城市扩展过程将导致生境质量下降0.51%（0.41%-0.62%）。各生物群区中，未来城市扩展过程对温带阔叶和混交林群区生境质量的影响最大。城市扩展过程对生境质量的间接影响远大于直接影响。全球1992-2016年城市扩展过程间接影响导致生境质量损失0.81%，直接影响导致生境质量损失0.04%，间接影响是直接影响的19.29倍。全球2016-2050年城市扩展过程间接影响导致生境质量损失0.50%（0.40%-0.60%），直接影响导致生境质量损失0.01%（0.01%-0.02%），间接影响是直接影响的36.72（29.59-53.71）倍。

（4）城市扩展过程造成的自然生境损失会影响全球超过40%的濒危物种，进而威胁2050年生物多样性保护目标的实现。全球1992-2016年城市扩展过程影响了1293种濒危物种，占全球濒危物种总数的43.14%。各生物群区中，热带亚热带湿润阔叶林群区城市扩展过程影响的濒危物种种类最多。全球2016-2050年城市扩展过程将影响1329（1301-1344）种濒危物种，占全球濒危物种总数的44.34%（43.41%-44.84%）。各生物群区中，热带亚热带湿润阔叶林群区未来城市扩展过程影响的濒危物种种类最多。城市扩展过程影响濒危物种会使得全球近一半的生态区（356个）在未来5种共享社会经济路径下都难以实现中等水平的生物多样性保护目标。在热带亚热带湿润阔叶林群区，接近60%的生态区（134个）在未来5种共享社会经济路径下都难以实现中等水平的生物多样性保护目标。

因此，论文建议关注城市扩展过程对自然生境的影响。首先，需要在2020年后全球生物多样性框架下，针对城市扩展过程对自然生境的影响设置本地化的生物多样性保护目标，并明确具体衡量指标。其次，提高城市土地利用效率，发展高密度城市，减轻城市蔓延对自然生境的影响。最后，通过科学合理的城市规划手段，避免城市扩展过程发生在高质量生境区域和濒危物种适宜生境集中区域，通过构建生态廊道和设置绿色隔离带等方式，减少城市扩展过程对自然生境的影响。

        Natural habitat refers to the natural environment which provides the living space for species persistence. Natural habitat is the foundation for maintaining biodiversity and promoting regional sustainable development. To achieve the 2050 Vision for Biodiversity of living in harmony with nature, the 15th Meeting of the Conference of the Parties to the Convention on Biological Diversity  was held in October 2021, and released the first draft of the post-2020 global biodiversity framework. This framework advocates galvanizing urgent and transformative actions by governments and all of society to protect the natural habitats to achieve the 2050 biodiversity conservation goals. However, urban expansion can directly impact natural habitats through the conversion of land to urban uses. It can also have indirect impacts on natural habitats, through disturbances emanating from the expanding urban land. Over the past few decades, urban expansion has progressed much faster than urban population growth in much of the world. Urban expansion has occupied 190 thousand km2 of natural habitats from 1992 to 2000, accounting for 16% of the total loss of natural habitats during the same period. Moreover, urban expansion will continue in the future. Therefore, it is of great significance to effectively evaluate the impacts of urban expansion on natural habitats globally. However, existing studies mainly evaluated the impacts of urban expansion on natural habitats from the perspective of the direct impacts or indirect impacts, and did not differentiate the direct and indirect impacts of urban expansion quantitatively. In addition, existing studies focused on the impacts of urban expansion before 2030, which remained inadequate to inform the policymaking relevant for the 2050 Biodiversity Conservation Goals.
       This dissertation aims to understand the impacts of urban expansion on natural habitats comprehensively. To achieve this purpose, this study conducts the research following the procedures of “dynamics - impact - sustainability”, and integrates the approaches of simulation and scenario analysis. Specifically, this study first simulates the global urban expansion from 2016 to 2050 by the Land Use Scenario Dynamics-urban (LUSD-urban) model under five shared socioeconomic pathways (SSPs) and analyses the dynamics of urban expansion from 1992 to 2016 and 2016 to 2050. Second, based on land use/cover data and urban land data, this study investigates the impacts of urban expansion on the area of habitats on multiple scales, and then discusses the types of natural habitats occupied by urban expansion and its impact on landscape patterns of the natural habitats. Third, the habitat quality model is improved to quantify the direct and indirect impacts of the global urban expansion on habitat quality from 1992 to 2016 and 2016 to 2050. And then this study discusses the differences between the direct and indirect impacts of urban expansion. Finally, coupling the range of natural habitats which were affected by urban expansion and the range of species-suitable natural habitats, this study assesses the impacts of the global urban expansion on the endangered species from 1992 to 2016 and 2016 to 2050 quantitatively. Then this study quantitatively assesses the potential future progress towards the 2050 biodiversity conservation goals under the post-2020 global biodiversity framework and provides support for the 2050 Vision for Biodiversity.
       The main innovation in the method is quantitatively evaluating the impacts of global urban expansion on natural habitats directly and indirectly based on the habitat quality index. Current studies usually verify the effectiveness of indicators in assessing the impacts of urban expansion on natural habitats by investigating its relationships with species richness. This study shows that the habitat quality index can be an useful proxy for evaluating the impacts of urban expansion on natural habitats. A statistically significant positive correlation (r=0.81, P<0.01) is found between habitat quality and species richness. By contrast, the correlations of natural habitats area and urban area, which are two commonly used indicators in existing studies, with the species richness are not statistically significant (P>0.05).
       There are four major findings in this dissertation. (1) The world has experienced a rapid urban expansion in the past several decades and the urban expansion will continue in the future. Urban land increased from 247.5 thousand km2 in 1992 to 621.0 thousand km2 in 2016, with an increase of 346.4 thousand km2. Correspondingly, the percentage of urban area increased from 0.20% to 0.46%, with an increase of 0.26%. Among the 14 biomes, urban areas in the Mangroves biome experienced the fastest expansion. The percentage of urban areas in the Mangroves biome increased from 0.73% in 1992 to 1.72 in 2016, with an increase of 0.99% and 3.81 times faster than the global average. From 2016 to 2050, urban land will increase by 394.2 (287.6 - 487.3) thousand km2 and the percentage of urban area will increase by 0.29% (0.21% - 0.36%). Among the 14 biomes, urban areas in the Mangroves biome will experience the fastest expansion. The percentage of urban areas in the Mangroves biome will increase to 2.84% (2.62% - 2.94%) in 2050, 3.9 (3.4 - 4.3) times faster than the global average.
       (2) Global urban expansion has and will result in the loss of habitat area. Urban expansion from 1992 to 2016 resulted in a loss of 123.4 thousand km2 of natural habitats, with a decrease of 0.11%. There was a clear disparity in terms of the impacts of urban expansion on natural habitats at the biome scale. Among biomes, urban expansion had the greatest impacts on habitats within the Mangroves biome (0.67%). The loss of natural habitats within the Mangroves biome was 5.9 times the mean loss across the world. Urban expansion from 2016 to 2050 will continue to lead to the loss of natural habitats. The area of natural habitats will decrease by 147.6 (109.1 - 190.5) thousand km2, a decrease of 0.13% (0.10% - 0.17%) of natural habitats in 2016. Among the biomes, urban expansion will also have the greatest impacts on natural habitats in the Mangroves biome. In this biome, urban expansion will result in a habitats loss of 1582 (1395 - 1706) km2, a decrease of 0.76% (0.67% - 0.81%). The decrease will be 5.6 (4.6 - 6.7) times the global average loss of natural habitats. Grasslands and forests are the main types of natural habitats, which are occupied by urban expansion. The global urban expansion from 1992 to 2016 occupied 65.9 thousand km2 of grasslands and forests, accounting for 53.40% of the total area of habitats loss caused by urban expansion during the same period. From 2016 to 2050, the urban expansion will occupy 88.3 (56.6 - 180.5) thousand km2 of grasslands and forests, accounting for 59.74% (51.85% - 65.03%) of the total area of habitats loss caused by urban expansion during the same period.
        (3) Global urban expansion has and will result in the loss of habitat quality. The average habitat quality decreased from 0.5161 in 1992 to 0.5117 in 2016, a decrease of 0.85%. Among the biomes, urban expansion had the greatest impacts on the habitat quality within the Mangroves biome. In this biome, the habitat quality decreased by 3.37% on average, which was 3.97 times the mean loss across the world. The average habitat quality will decrease to 0.5090 (0.5085 - 0.5096) in 2050, a decrease of 0.51% (0.41% - 0.62%). Among the biomes, urban expansion will have the greatest impacts on the habitat quality within the Temperate Broadleaf and Mixed Forests biome. In this biome, the average habitat quality will decrease 1.84% (1.12% - 2.57%), which will be 3.57 (2.75 - 4.14) times the mean loss across the world. Indirect impacts of urban expansion on natural habitats are orders of magnitude larger than its direct impacts. The indirect impacts of urban expansion from 1992 to 2016 resulted in a loss of habitat quality 19.29 times greater than that caused by its direct impacts (0.81% versus 0.04%, respectively). From 2016 to 2050, indirect impacts of urban expansion will decrease the habitat quality by 0.50% (0.40% - 0.60%), whereas the direct impacts will decrease the habitat quality by 0.01% (0.01% - 0.02%). The indirect impacts will be 36.72 (29.59 - 53.71) times the direct impacts.
         (4) The loss of natural habitats caused by urban expansion jeopardizes more than 40% of the endangered species worldwide, and then puts substantial obstacles to the 2050 Biodiversity Conservation Goals. Global urban expansion from 1992 to 2016 affected 1293 threatened species, which was 43.14% of endangered species. At the biome scale, urban expansion in the Tropical and Subtropical Moist Broadleaf Forests biome influenced the largest number of the threatened species. Urban expansion from 2016 to 2050 will encroach on the living spaces of 1329 (1301 - 1344) endangered species, accounting for 44.34% (43.41% - 44.84%) of the total endangered species worldwide. Among the biomes, urban expansion in the Tropical and Subtropical Moist Broadleaf Forests biome will influence the largest number of the threatened species. Impacts of urban expansion on the threatened species put substantial obstacles to the 2050 Biodiversity Conservation Goals. More than 40% of ecoregions (356) will fail to achieve the moderate target under any SSPs pathway. In the Tropical and Subtropical Moist Broadleaf Forests biome, nearly 60% of ecoregions (134) will fail to achieve the moderate target under any SSPs pathway.
         Hence, special attention and targeted measures should be paid to the impacts of urban expansion on natural habitats. First, it is necessary to set localized biodiversity conservation goals and clarify specific measurement indicators for the impacts of urban expansion on natural habitats under the post-2020 global biodiversity framework. Second, the efficiency of urban land should be improved and the high-density cities should be considered to reduce the impacts of urban sprawl on natural habitats. Finally, scientific and reasonable urban planning should be taken to prevent urban expansion within critical habitats and biodiversity hotspots. Certain environmental protection projects, such as the green belts and ecological corridors, can also be developed to mitigate the adverse impacts of urban expansion.