This study quantitatively analyzed the ecological niche breadth (ENB) of 27 freshwater mollusk species (11,406 occurrence records) in Korea based on macroenvironmental gradients, including climate, topography, and land cover. We combined Levins’ index values (B: niche breadth, BA: equitability) with optimum-tolerance analysis (optimum value and tolerance range, t95-t05) to determine species-specific habitat strategies and environmental responses. The analysis revealed a distinct dichotomy in the habitat strategies of Korean freshwater mollusks, separating cold-water, lowdisturbance types in upstream areas from warm-water, disturbance-tolerant types in downstream areas, mainly structured by thermal and land-cover gradients. Species, such as Semisulcospira libertina, Radix auricularia, and the alien species Physa acuta, were identified as generalists, showing high B values but low BA values, suggesting that they are capable of inhabiting diverse environments but concentrate their presence in certain environmental conditions. Conversely, Koreoleptoxis nodifila and Clithon retropictum were classified as specialists with low B values and intermediate to high BA values, indicating stable and equitable distribution within restricted environmental limits. Urbanization appeared to be an influential limiting factor, showing low BA values for most species, whereas agricultural areas showed higher BA values, suggesting a comparatively broader habitat possibility. Notably, K. nodifila, a critically endangered and endemic species, showed optimal habitat conditions of low temperature (10.65°C), low urbanization (4.82%), and high forest cover (57.34%), confirming its status as a cold-water indicator species adapted to forested upstream systems. The study findings provide a quantitative framework to assess the environmental tolerance, vulnerability, and conservation priorities of Korean freshwater mollusks.

Abstract
1. INTRODUCTION
2. MATERIALS AND METHODS
    2.1. Compilation and processing of biologicaldata
    2.2. Environmental variables
    2.3. Data analysis
3. RESULTS
    3.1. 
    3.2. 
    3.3. Optimum and tolerance ranges alongmacro-environmental gradients
4. DISCUSSION
    4.1. Environmental gradients structuringfreshwater mollusk assemblages
    4.2. Generalist and specialist strategiesrevealed by Levins’ indices
    4.3. Optimum-tolerance patterns and keyenvironmental drivers
    4.4. Species-level differentiation andconservation implications
    4.5. Methodological significance and futureperspectives
CRediT authorship contribution statement
Declaration of Competing Interest
ACKNOWLEDGEMENTS
REFERENCES

• Mi-Jung Bae(Diversity Forecast & Evaluation Division, Nakdonggang National Institute of Biological Resources, Sangju 37242, Republic of Korea) Corresponding author
• Seo-Ha Kim(Diversity Forecast & Evaluation Division, Nakdonggang National Institute of Biological Resources, Sangju 37242, Republic of Korea)
• Dae-Seong Lee(Department of Biology, College of Science, Kyung Hee University, Seoul 02453, Republic of Korea)
• Da-Young Lee(Department of Biology, College of Science, Kyung Hee University, Seoul 02453, Republic of Korea)
• Young-Seuk Park(Department of Biology, College of Science, Kyung Hee University, Seoul 02453, Republic of Korea, Korea Institute of Ornithology, Kyung Hee University, Seoul 02453, Republic of Korea)