Environmental Filtering Dominates Butterfly Community Assembly Along Himalayan Elevational Gradients

Abstract

Aim

Understanding the processes underlying biodiversity patterns and community assembly across mountain elevational gradients remains a central question in biogeography. We examined elevational patterns of butterfly taxonomic and functional diversity to evaluate the relative importance of environmental filtering, competition, and geometric constraints in structuring community assembly across two Himalayan valleys.

Location

Eastern Himalaya, India.

Taxon

Butterflies (Lepidoptera).

Methods

Butterfly communities were sampled along 43 elevational transects (350–4200 m) spanning the Teesta and Rangeet valleys in Sikkim, Eastern Himalaya. Taxonomic diversity, butterfly density, and complementary functional diversity indices were quantified using 16 life-history traits. Elevational responses were analysed using generalized linear models, information-theoretic model selection, model averaging, and mid-domain null models to evaluate community assembly processes and identify their environmental drivers.

Results

Species richness, estimated richness, butterfly density, functional richness (FRic), and Rao's quadratic entropy (RaoQ) declined consistently with elevation, supporting strong environmental filtering in the community assembly process. In contrast, different facets of functional diversity exhibited contrasting elevational responses, providing only partial support for trait convergence, whereas functional divergence showed no relationship with species richness, indicating limited support for competition-driven community assembly. Null-model analyses revealed increasingly negative departures from mid-domain expectations towards higher elevations, indicating that deterministic environmental processes outweigh geometric constraints. Environmental drivers were context-dependent, with vegetation structure and topographic variables predominating in the Teesta Valley, whereas precipitation seasonality and tree species richness exerted stronger influences in the Rangeet Valley.

Main Conclusions

Environmental filtering emerged as the predominant process structuring butterfly community assembly, whereas competition and geometric constraints played comparatively subsidiary roles. Contrasting responses among complementary functional diversity indices demonstrate that integrating trait-based functional diversity, information-theoretic environmental modelling, and null-model analyses provides a more comprehensive understanding of community assembly across tropical mountain systems under environmental change.