Global impacts of future urban expansion on terrestrial vertebrate diversity

The global population is projected to significantly increase by 2050, with a large proportion residing in urban areas. This rapid urbanization necessitates expanding urban land, which has already outpaced population growth. Although urban land covers a relatively small percentage of the global terrestrial surface, its expansion is a major driver of land use change, causing habitat conversion, degradation, fragmentation, and biodiversity loss. Past assessments show substantial losses in species richness and abundance in urbanized areas. Many biodiversity targets set by the Aichi Biodiversity Targets were not met by 2020, partly due to habitat loss and fragmentation from agricultural and urban land use changes. Future urban expansion is projected to be alarmingly rapid. To achieve the UN's Sustainable Development Goals (SDGs), particularly SDG 11 (sustainable cities) and SDG 15 (life on land), understanding the impacts of future urban expansion on biodiversity is crucial. This study addresses the gap in knowledge by integrating scenario analyses of future urban expansion with biodiversity data to quantify the impacts on global terrestrial biodiversity, focusing on habitat fragmentation, species richness loss, and species abundance loss across different scenarios and spatial resolutions.

Existing research highlights the negative impacts of urban expansion on biodiversity, often focusing on single aspects such as habitat loss or using single scenarios and coarse resolutions. While some studies have quantified the effects of urban expansion on biodiversity, they have often lacked the comprehensiveness of examining multiple dimensions of impact under various future scenarios. This study aims to address this gap in the literature by using a spatially explicit dataset of urban expansion projections based on the five Shared Socioeconomic Pathways (SSPs), which allows for a comprehensive assessment of impacts under different socioeconomic and environmental conditions. This provides a more nuanced understanding of the interplay between urban development and biodiversity loss than previously available.

This study integrates a comprehensive scenario analysis of future urban expansion with datasets on habitat and terrestrial biodiversity (amphibians, mammals, and birds). The spatially explicit urban expansion projection dataset (2020–2100, 1 km resolution) was based on the five Shared Socioeconomic Pathways (SSPs). Habitat loss and impacts on biodiversity prioritization areas were measured using spatial overlap analysis. The impacts of future urban expansion on landscape fragmentation of natural habitats near urban areas were assessed. The relative percentage changes in species richness and abundance on 1 km grids, and the potential mean absolute change in species richness numbers on 10 km grids were explored using model estimates from the PREDICTS database. The PREDICTS database provides biodiversity responses to different land use intensities. The study further investigated the effects of urban expansion on habitat fragmentation using metrics such as mean patch size (MPS), edge density (ED), and mean Euclidean nearest neighbor distance (ENN_MN). These analyses were performed at the global, ecoregional, and biome scales to understand the spatial heterogeneity of urban expansion’s impacts. Data sources included the ESA Climate Change Initiative (CCI) Land Cover product, the World Database on Protected Areas, biodiversity hotspot data, Global 200 ecoregions data, Last of the Wild areas data, and the PREDICTS database. The FLUS model was used to simulate spatially explicit urban expansion based on SSPs. Spatial overlap analyses were used to quantify habitat loss in different biomes and ecoregions. Landscape metrics were used to assess habitat fragmentation. Species richness and abundance data from the PREDICTS database were used to estimate biodiversity losses.

The study projects that 36–74 million hectares of land will be urbanized by 2100 across the five SSP scenarios, representing a substantial increase compared to 2015. Of this, 11–33 million hectares will be natural habitats. The patterns of habitat loss across different habitat types (forest, shrubland, grassland, wetland, and other) were generally consistent across SSP scenarios, with SSP3 projecting the greatest overall habitat loss and SSP1 the least. Spatial hot spots and cold spots of habitat loss varied across scenarios. Hot spots were concentrated in regions such as the U.S. coasts, Gulf of Guinea, Sub-Saharan Africa, and the Persian Gulf. Cold spots were concentrated in East Asia and South Asia. The temperate broadleaf and mixed forests biome was projected to experience the largest natural habitat loss. Urban expansion is projected to affect a substantial proportion of protected areas and biodiversity hotspots. By 2100, 13.2–19.8% of protected areas might be affected, with the WWF's Global 200 ecoregions experiencing a significant increase in urban land. Future urban expansion will lead to increased proximity of urban areas to natural habitats, increasing habitat fragmentation. This will be reflected in smaller mean patch sizes, increased edge density, and increased isolation of habitat patches. Urban land conversion will result in an average 34% loss in species richness and 52% loss in species abundance on a 1 km grid. The study projects an average loss of 7–9 species per 10 km grid cell, with losses concentrated in Sub-Saharan Africa, the United States, and Europe. The study also identified conservation priority ecoregions with high risks of habitat loss and small-ranged species loss due to future urban expansion, primarily in Latin America and Sub-Saharan Africa. The top 5% of 10 km grid cells with the highest species richness loss are often located in areas adjacent to urban areas but only partially covered by protected areas.

The findings highlight the significant and multifaceted impacts of future urban expansion on global terrestrial biodiversity. The magnitude and spatial extent of these impacts are substantial, emphasizing the need for urgent action. The study demonstrates that the specific development pathway chosen will influence the rate and distribution of biodiversity decline. The disproportionate impact on areas near urban edges, coupled with increased habitat fragmentation, underscores the importance of compact development and smart urban growth strategies. Although urban land conversion is a smaller scale than cropland change, its impact on biodiversity is significant, with the highest percentage losses occurring during the conversion from high-ecological-value land systems to built environments. The uneven distribution of species loss highlights the importance of targeted conservation efforts in specific ecoregions. The study emphasizes the need to re-evaluate and update biodiversity prioritization schemes to ensure that regions at high risk of biodiversity loss are adequately addressed, particularly in less developed countries that often lack resources for conservation.

This study provides a comprehensive assessment of the future impacts of urban expansion on global terrestrial biodiversity under different SSP scenarios. The findings highlight the significant and spatially heterogeneous impacts on habitat, fragmentation, and species richness and abundance. The results underscore the urgent need for sustainable urban planning and development strategies that balance urban growth with biodiversity conservation. Future research should focus on the indirect effects of urban expansion on biodiversity, including telecouplings and land-use displacement. Improved data availability and model refinement will further enhance our understanding of the complex interplay between urbanization and biodiversity.

This study primarily focuses on the direct effects of urban expansion on terrestrial biodiversity and does not fully account for indirect effects, such as telecouplings through supply chains or land-use displacement. Data limitations prevented a comprehensive assessment of the dynamic changes in habitat and biodiversity losses over time, as well as the effects of habitat fragmentation on biodiversity degradation. The use of macro-region data in projecting urban expansion may introduce uncertainties in the forecasts at the country and ecoregional levels. The assumption of continued urban expansion within protected areas without strict restrictions may overestimate the impacts on these areas. The study also does not consider the potential interactions between urban expansion, biodiversity change, and future climate change.