Human activities are most concentrated in lowlands; however, increasing evidence suggests significant expansion into Asian highlands. These highlands, encompassing one-third of Asia's land area and home to a substantial portion of global biodiversity, carbon sequestration, and freshwater resources, are ecologically fragile. While previous studies documented regional expansions (e.g., cultivated land in Southeast Asia, urban land in China), these were often limited in scope and impact assessment. A comprehensive understanding of highland human activity expansion is crucial for informing climate change models (used by organizations like FAO and IPCC), guiding ecological conservation efforts, and shaping regional sustainable development strategies. This study addresses two key questions: (1) the extent and location of human activity expansion in Asian highlands since 2000, and (2) the visible effects of this expansion on highland ecosystems. To answer these, we combined high-resolution satellite imagery (30m resolution) from 2000-2020 with socioeconomic data, focusing on cultivated land and artificial surface expansions as indicators of human activity.

Existing literature on highland expansion is fragmented, focusing on specific regions or impacts. Studies have noted cultivated land expansion in Southeast Asia and hillside urbanization in China, often linking these to forest loss. However, these studies lack comprehensive geographical coverage, considering only subsets of human activities and their impacts. Assumptions used in global climate change models, such as those by FAO, often underestimate highland expansion, neglecting the significant changes observed in regions like Southeast Asia. This research aims to bridge this gap by providing a comprehensive assessment of human activity expansion across Asian highlands, considering various ecological land types and socioeconomic factors.

This study employed a multi-faceted methodology combining satellite imagery analysis with socioeconomic data. First, Advanced Spaceborne Thermal Emission and Reflection Radiometer Global DEM (ASTER GDEM) data (30m resolution) was used to delineate highlands based on elevation and slope criteria. GlobeLand30 datasets, a global land cover product with 30m resolution covering the period 2000-2020, were utilized to identify changes in cultivated land and artificial surfaces. These datasets were selected for their fine spatial resolution, allowing detailed detection of human activities even in small hilly areas. The Python platform was used with a target-level change detection method to identify changes in land cover. Socioeconomic data, including population, economic level (from the World Bank), and high-resolution population density (from WorldPop), were integrated to assess the effects of highland expansion. The accuracy of the human activity expansion results was validated using approximately 22,000 high-resolution samples from various sources, including Google Earth, Landsat imagery, and in-situ measurements. The accuracy was over 90% for human activity expansions, and about 88% for ecological land loss, indicating the method's reliability. To assess the effects, we calculated ecological land loss, habitat fragmentation using a Habitat Fragmentation Index (HFI), the contribution rate of highland cultivated land net growth to total cultivated land conservation, and the population capacity supported by highland artificial surface expansion.

The analysis revealed widespread and heterogeneous human activity expansion in Asian highlands from 2000-2020. Approximately 23% of overall human activity growth occurred in these areas, with variations across different regions of Asia (West Asia showed the highest rate at 32.31%). Cultivated land expansion dominated (approximately 80%), followed by artificial surfaces (20%). Southeast and South Asia exhibited the highest proportions of cultivated land expansion, while West and East Asia showed higher artificial surface growth. The expansion impacted ecological lands significantly: 76% of the highland expansions came from converting ecological lands (forest, grassland, wetlands and shrublands), with Southeast Asia reaching 95%. This conversion led to substantial ecological land loss and increased habitat fragmentation, particularly in hilly areas. Habitat fragmentation increased from 2000 to 2020 in Asia. The degree of habitat fragmentation in hilly areas (~0.97) was significantly higher than that in mountainous areas (~0.65). Despite the ecological costs, highland development contributed significantly to overall Asian development. The net growth of cultivated land in highlands contributed approximately 54% to preventing the net loss of total cultivated land across Asia. The expansion of artificial surfaces in highlands provided living and working spaces for an estimated 40 million people. These positive impacts were more prominent in some regions, with South Asia showing a particularly high contribution to the conservation of cultivated land.

The findings highlight the dual nature of highland development in Asia. While contributing to food security and population growth, it carries substantial ecological costs, particularly in low and lower-middle income countries where ecological land loss was nearly double the rate observed in higher-income countries. This disparity may be attributed to the economic realities of these nations, where rapid economic growth often necessitates the exploitation of natural resources. International trade patterns could further exacerbate this problem, leading to environmental degradation in developing countries while developed countries benefit from resource consumption. The study underscores the urgent need for a balanced approach to highland development, integrating conservation efforts with economic growth. Strategies like ‘land sharing’ and ‘land sparing’ could facilitate sustainable agriculture while minimizing ecological damage. Additionally, improved transport infrastructure could enhance food production while reducing pressure on land resources.

This study provides the first comprehensive assessment of human activity expansion in Asian highlands, revealing widespread expansion with significant ecological impacts. The dual effects of highland development – contributing to economic growth while causing substantial ecological damage – necessitate a balanced approach for sustainable development. Integrating land sharing, land sparing strategies, and improved transport infrastructure can help mitigate negative consequences. Future research should refine methods to address uncertainties and explore specific impacts on biodiversity, considering local contexts and the trade-offs involved in balancing development and conservation.

The 30m spatial resolution of the satellite imagery might limit the precise delineation of expansion boundaries, especially in complex terrain. The accuracy of population capacity estimations depends on the accuracy of population density data, which can be influenced by errors in settlement mapping. The study uses a specific definition of highlands; alternative definitions could yield different results. Finally, while the study points to the need for a balanced approach, further research is needed to determine the optimal threshold for balancing development and ecological protection in various contexts.