The accelerating expansion of urban areas around the globe poses significant challenges to biodiversity, but a particularly alarming trend, often overlooked until now, is the spread of urban development across hillside environments. These steep terrains, traditionally considered more difficult and less desirable for urban growth, are increasingly becoming focal points for human settlement and infrastructure. A groundbreaking study published in Nature Cities highlights the extensive and profound consequences of hillside urban expansion on terrestrial biodiversity, shedding new light on an urgent global environmental issue that threatens ecosystems, species, and conservation efforts worldwide.
Between 2000 and 2020, the world witnessed an unprecedented transformation of hillside landscapes, with an estimated 11.65 million hectares (Mha) converted from natural habitats to urban use. This area is enormous, comparable to the size of countries such as Cuba or Portugal, and the pace of this change has significant ecological ramifications that extend far beyond localized urban sprawl. Unlike urban expansion on flat terrain, which often occurs on already modified or less biologically rich land, hillside urbanization disproportionately consumes intact natural habitats — those crucial sanctuaries for a vast array of species. The fragility and ecological uniqueness of these hillside ecosystems are therefore placed under intensified threat.
What makes hillside urban expansion particularly alarming is its disproportionate impact on biodiversity hotspots. These globally recognized regions harbor extremely high numbers of endemic species and provide irreplaceable ecological services, but they are also increasingly subjected to human pressures. This study reveals that 35% of the global hillside urban expansion has taken place within such hotspots. These areas, already under severe pressure from human activity, stand to suffer profoundly from this kind of development, leading to further fragmentation and degradation of habitats critical to the survival of countless species.
Habitat fragmentation, a phenomenon whereby large, continuous habitats are broken into smaller, isolated units, is a central concern raised by this research. Fragmented habitats reduce the availability of resources, limit animal movement and dispersal, and increase vulnerability to environmental fluctuations. Hillside urban expansion intensifies this effect much more than flatland urbanization because steep terrains inherently support more unique and specialized ecological niches. This fragmentation leads to reduced genetic exchange and population decline, worsening the extinction risk for many terrestrial vertebrates.
A striking finding of this study is the direct impact on nearly 70% of the world’s globally threatened terrestrial vertebrate species. These species, which include mammals, birds, reptiles, and amphibians listed by the International Union for Conservation of Nature (IUCN), face mounting pressure as their habitats are increasingly appropriated by urban infrastructure. The ecological stability and long-term viability of these threatened species are at risk, and their decline not only affects ecosystem health but also undermines global biodiversity targets.
The global implications of hillside urban expansion extend into international policy frameworks focused on sustainability and biodiversity conservation such as the Sustainable Development Goals (SDGs) and the Global Biodiversity Framework (GBF). These initiatives rely on maintaining and restoring natural habitats and species populations, which is directly challenged by continued urban encroachment into hillside ecosystems. The findings of this study highlight a pressing need for policymakers and urban planners to reconsider current development patterns and to integrate conservation priorities more effectively.
Technological advances played a critical role in enabling this comprehensive analysis of hillside urban expansion. The study leveraged multi-source, high-resolution spatial data, including satellite imagery and land-use records, alongside detailed terrestrial vertebrate species datasets. This integration allowed for unprecedented granularity in mapping urban growth and understanding its intersections with biodiversity. Such a data-driven approach is vital for assessing ecological impacts at multiple scales—from local habitats to continental regions—and provides a robust foundation for forecasting future trends.
Beyond mapping land cover changes, this research incorporates species distribution models to understand how urban expansion intersects with the habitats of vulnerable species. By overlaying urbanization data with species occurrence records, researchers could precisely identify areas where hillside expansion directly threatens the survival of specific animal populations. This multiscale, multi-dimensional methodology sets a new standard for ecological risk assessment related to urban development.
The geography of hillside urban expansion reveals patterns influenced by socioeconomic, cultural, and environmental variables. Rapid urban growth in developing countries, driven by population pressures and economic development, often pushes settlements into marginal lands, including hillsides. The inclination to build in such areas is further exacerbated by land scarcity and limited flat terrain availability, especially near large metropolitan centers. In many regions, hillside building is also associated with informal settlements where planning and regulatory oversight are weak, posing even greater risks for biodiversity.
Environmental consequences of hillside urban expansion extend beyond direct habitat loss. The destabilization of slopes, increased surface runoff, and altered microclimates can degrade ecosystems over broader areas. Hillsides are often critical watershed catchment zones; therefore, urbanization in these regions can lead to soil erosion, sedimentation of water bodies, and decreased water quality. These ecological disturbances reduce habitat quality for many terrestrial species, compounding the negative effects of habitat fragmentation.
The study underscores the importance of adopting tailored land-use and urban planning strategies focused on biodiversity conservation in ecologically sensitive hillside regions. Conventional urban planning models, typically optimized for flat urban matrices, are insufficient for addressing the complex ecological dynamics of hillside environments. A nuanced approach that integrates terrain-specific ecological data, habitat connectivity, species vulnerability, and local socioeconomic factors is essential to curb biodiversity loss.
Implementing such conservation-oriented urban planning carries significant challenges and opportunities. It requires coordination among multiple stakeholders, including governments, urban developers, conservation organizations, and local communities. Moreover, innovative policies—such as green infrastructure, urban growth boundaries, and ecological restoration projects—must be prioritized for hillside areas to maintain ecosystem integrity while accommodating human development.
Looking forward, the research strongly advocates for enhanced regional and global monitoring systems focused on hillside urban growth and its ecological impacts. As urban areas continue to expand worldwide, real-time data acquisition and analysis could enable responsive management and policy interventions, minimizing damage to biodiversity. The integration of remote sensing technologies and biodiversity informatics will be pivotal in driving such sustainable urban transformations.
Crucially, the study also calls attention to the ethical dimension of hillside urban expansion. The loss of biodiversity is not only an environmental crisis but a moral one, touching upon humanity’s responsibility toward other species and future generations. Hillside regions often embody complex landscapes where cultural heritage, natural beauty, and ecological diversity intersect, reinforcing the need to protect them through inclusive, informed, and forward-looking urban governance.
In conclusion, the extensive threats posed by global hillside urban expansion demand an urgent and coordinated response. The evidence provided by this study offers a powerful clarion call for reimagining urban development strategies to prevent irreversible biodiversity loss. Without immediate action, the ongoing encroachment into hillside ecosystems could severely undermine global efforts to halt species extinctions and restore planetary health by mid-century.
By illuminating the intricate relationship between terrain, urban growth, and biodiversity, this pioneering research breaks new ground in environmental science. It equips policymakers and planners with the tools and knowledge needed to balance urbanization with nature conservation—a challenge central to humanity’s sustainable future. As urban landscapes rise on the hills, so too must our commitment to preserving the rich tapestry of life that these places harbor.
Subject of Research: Global hillside urban expansion and its impacts on terrestrial biodiversity.
Article Title: Extensive terrestrial biodiversity threats from global hillside urban expansion.
Article References:
Shi, K., Wu, Y., Sun, X. et al. Extensive terrestrial biodiversity threats from global hillside urban expansion. Nat Cities (2025). https://doi.org/10.1038/s44284-025-00316-9
Image Credits: AI Generated
