Rapid tourism expansion and infrastructure growth in the Western Ghats are reshaping ecological stability in steep, topographically complex landscapes that still retain substantial forest cover. These changes are intensifying habitat fragmentation, degrading ecosystems, and placing increasing pressure on biodiversity-rich environments. Focusing on Idukki district in southern India, this study integrates the Urban Nature Access (UNA) and Habitat Risk Assessment (HRA) modules within the InVEST framework to quantify how accessibility-driven development influences habitat vulnerability between 2011 and 2025. Results reveal a widening spatial imbalance between population demand and accessible natural areas, with strongly negative urban nature balance values expanding across valley settlements and plantation corridors by 2025. Built-up land exhibits the highest mean habitat risk (R̄ = 0.42), followed by plantations (R̄ = 0.38) and croplands (R̄ = 0.34), while deciduous forests (R̄ = 0.22) and water bodies (R̄ = 0.05) remain comparatively less vulnerable. More than one-third of built-up and plantation landscapes fall within medium to high-risk categories. Spatial overlap between high-risk zones and documented landslide-affected areas during the 2018–2020 extreme monsoon events highlight the cumulative impact of tourism-driven development and associated forest conversion on steep slopes. The findings demonstrate that ecological vulnerability in Idukki is closely linked to localized forest loss and land-use transformation driven by concentrated human activities in hazard-prone terrain. These patterns highlight how steep and environmentally sensitive landscapes are increasingly affected by development pressures. This underscores the need to integrate ecosystem accessibility metrics with slope-sensitive land-use planning in tropical mountain systems.