Spatial Dissimilarity and Taxon Contributions to Aquatic Insect Assemblages Across Selected Streams of Munnar, Western Ghats, India

Abstract: Aquatic insects are important bioindicators of freshwater ecosystem health, yet their spatial distribution and community composition in tropical streams of the Western Ghats remain insufficiently documented. Objectives: The present study was designed with the following objectives i) To quantify the taxonomic composition, abundance, richness, and diversity of aquatic insect assemblages, ii) To assess spatial dissimilarities in assemblage composition among streams using Bray–Curtis indices, iii) To examine patterns of assemblage similarity and spatial relationships through hierarchical clustering and non-metric multidimensional scaling (NMDS), iv) To evaluate taxon occupancy and site specificity across the study streams and v)To identify the key taxa contributing to community dissimilarities using similarity percentage (SIMPER) analysis. Methodology: Aquatic insects were sampled during 2023–2024 using kick-net technique in riffle habitats and identified morphologically. Taxonomic richness, Shannon diversity, Simpson diversity, and Pielou evenness were calculated. Bray–Curtis dissimilarity, hierarchical clustering, non-metric multidimensional scaling (NMDS), site-specificity, and similarity percentage (SIMPER) analyses were performed to assess spatial variation in assemblage composition. Results: A total of 5,892 individuals representing 40 taxa across seven orders were recorded. Ephemeroptera was predominant, followed by Trichoptera and Hemiptera. Hydropsyche sp. and Stenopsyche kodaikanalensis were dominant taxa. Sivanmalai recorded the highest taxon richness (38), Shannon diversity (3.400), and Simpson diversity (0.9575), whereas Kannimalai exhibited the lowest Shannon diversity (3.202) and Pielou evenness (0.9079). Multivariate analyses revealed distinct spatial patterns among streams. Aphelocheirus sp. was exclusive to Periyavarai, with 100% site specificity. SIMPER analysis identified Baetis sp. and Hydropsyche sp. as major contributors to inter-stream dissimilarity. Conclusion: Aquatic insect assemblages exhibited pronounced spatial heterogeneity across the studied streams. These findings provide baseline data for freshwater biodiversity monitoring and conservation in the Western Ghats. Further studies incorporating seasonal monitoring and environmental variables are recommended to identify the factors influencing these patterns.