Do sub-groups of butterflies display different elevational distribution patterns in the Eastern Himalaya, India?

Abstract

Understanding the pattern of biodiversity along environmental gradients helps in identifying diversity hotspot areas that can be prioritized for conservation. While the elevational distribution of several taxa has been studied, responses of the subgroups within a taxon to elevation and its associated factors are not properly understood. Here we study species richness and butterfly density along an elevation gradient in Sikkim, Eastern Himalaya, India and explore the underlying causes of the patterns. We sampled butterflies using a fixed-width point count method in 16 elevational bands (150-200 m intervals), between a range of 300 and 3300 m a.s.l. We categorized butterflies into various subgroups based on family, range size, biogeographic affinity, and host-plant specialization. We recorded 3603 individuals and 253 species of butterflies after the completion of 1860 point counts. Overall, species richness in the majority of the subgroups (except for Riodinidae and Palearctic species) declines with elevation, as does the density of almost all the subgroups. From a selection of environmental factors, annual actual evapotranspiration has the strongest effect on the species richness pattern of butterflies as well as on the density of the overall butterfly community, especially the Lycaenidae family. The richness and density of butterfly groups display varied responses to the richness and density of trees and shrubs. The conducive climatic conditions and diverse habitats in the lower valleys of the Eastern Himalaya support a high diversity of butterflies (with majority of small range species) and thus warrants conservation attention. Highlights • The distribution of biodiversity along mountain elevational gradients has been well studied, but the disparity in patterns between various organisms and associated factors makes it difficult to develop a universal model for explaining the variation of biodiversity. • We report elevational patterns of species richness and density of butterflies in the Eastern Himalaya, both at whole group and subgroup levels. • A general decline in species richness and density with elevation is not supported for Riodinidae and for Palearctic species. • A measure of water energy balance, annual actual evapotranspiration, is the best correlated variable with species richness trends in butterflies. • Our paper highlights the importance of low elevation forests for butterfly diversity in the Eastern Himalaya.

Mountain biodiversity and elevational gradients

“…the plant species richness (native and alien) decreased with elevation, while endemic species richness increased. See also the paper on butterflies in Sikkim (Dewan et al 2021). However, different life forms may also have different responses, and in the Apuan Alps, the different Raunkiaer Life Forms show differentiated responses to the elevational gradient.…”

“…The other paper from the Himalayas (Sikkim) is an empirical analysis which also focuses on precipitation and other underlying mechanisms for butterfly species richness (Dewan et al 2021). In similar fashion to the analysis of the life form groups in the Mediterranean study (Apuan Alps), Dewan et al (2021) have divided total richness into smaller ecological defined subsets based on family, range size, biogeographic affinity, and host-plant specialization. All these groups respond differentially to environmental predictors, but species richness in the majority of the sub-groups declines monotonically with elevation.…”

“…This is partly because the studied range spans only c. 3000 m a.s.l.a feature shared also with the Apuan Alps study that similarly reported a linear decline (above). Dewan et al (2021) report that butterfly richness is in general best explained by annual actual evapotranspiration (AET) based on the Turc-formula. This version of AET estimation is in general highly influenced by precipitation, and the results thus confirm the importance of water availability for butterfly richness and composition.…”

Turnover in butterfly communities and traits along an elevational gradient in the eastern Himalaya, India

“…Due to the steep elevational gradient, the Himalayan region is a natural laboratory to test biogeographical hypotheses. In recent years, there has been an upsurge in biogeographical studies of multiple taxa, including butterflies, in various parts of the Himalaya (Acharya, Chettri, & Vijayan, 2011; Acharya, Sanders, et al, 2011; Acharya & Vijayan, 2015, 2017; Bhardwaj et al, 2012; Dewan et al, 2021; Rana et al, 2019). In the Himalayan region, species richness of butterflies declines with increasing elevation (Acharya & Vijayan, 2015; Bhardwaj et al, 2012) including in our own study in the Rangeet Valley, eastern Himalaya (see Dewan et al, 2021).…”

“…In recent years, there has been an upsurge in biogeographical studies of multiple taxa, including butterflies, in various parts of the Himalaya (Acharya, Chettri, & Vijayan, 2011; Acharya, Sanders, et al, 2011; Acharya & Vijayan, 2015, 2017; Bhardwaj et al, 2012; Dewan et al, 2021; Rana et al, 2019). In the Himalayan region, species richness of butterflies declines with increasing elevation (Acharya & Vijayan, 2015; Bhardwaj et al, 2012) including in our own study in the Rangeet Valley, eastern Himalaya (see Dewan et al, 2021). In that study, we found a significant influence of actual evapotranspiration and a suite of habitat variables in shaping the α‐diversity patterns (Dewan et al, 2021).…”