Bhoj Kumar Acharya

Elevational gradients in bird diversity in the Eastern Himalaya: an evaluation of distribution patterns and their underlying mechanisms.

Background Understanding diversity patterns and the mechanisms underlying those patterns along elevational gradients is critically important for conservation efforts in montane ecosystems, especially those that are biodiversity hotspots. Despite recent advances, consensus on the underlying causes, or even the relative influence of a suite of factors on elevational diversity patterns has remained elusive. Methods and Principal Findings We examined patterns of species richness, density and range size distribution of birds, and the suite of biotic and abiotic factors (primary productivity, habitat variables, climatic factors and geometric constraints) that governs diversity along a 4500-m elevational gradient in the Eastern Himalayan region, a biodiversity hotspot within the worlds tallest mountains. We used point count methods for sampling birds and quadrats for estimating vegetation at 22 sites along the elevational gradient. We found that species richness increased to approximately 2000 m, then declined. We found no evidence that geometric constraints influenced this pattern, whereas actual evapotranspiration (a surrogate for primary productivity) and various habitat variables (plant species richness, shrub density and basal area of trees) accounted for most of the variation in bird species richness. We also observed that ranges of most bird species were narrow along the elevation gradient. We find little evidence to support Rapoports rule for the birds of Sikkim region of the Himalaya. Conclusions and Significance This study in the Eastern Himalaya indicates that species richness of birds is highest at intermediate elevations along one of the most extensive elevational gradients ever examined. Additionally, primary productivity and factors associated with habitat accounted for most of the variation in avian species richness. The diversity peak at intermediate elevations and the narrow elevational ranges of most species suggest important conservation implications: not only should mid-elevation areas be conserved, but the entire gradient requires equal conservation attention.

Vertical stratification of birds in different vegetation types along an elevation gradient in the Eastern Himalaya, India

Abstract We examined the vertical stratification of birds in relation to foliage in different vegetation types along an elevation gradient in Sikkim, Eastern Himalaya, India. We used variable-width point count methods for sampling birds spread across 20 transects along an elevation gradient from 300 m to 3,800 m above mean sea level. We estimated species richness, abundance and Shannon-Weiner diversity (H′) of birds in seven height categories (0 m, 0–5 m, 5–10 m, 10–15 m, 15–20 m, 20–25 m and >25 m). Foliage structure and complexity of vegetation was assessed along all transects following Erdelen (1984) and Jayson and Mathew (2003). Birds displayed distinct vertical stratification in terms of species richness, abundance and diversity in Sikkim. Overall, maximum species richness (231) was observed at 0–5 m height followed by 5–10 m, 10–15 m and the ground layer (0 m). There was no significant difference in stratification pattern among elevation zones. Each height class harboured distinct species composition of birds with low similarity among height categories. We observed maximum foliage concentration within 10 m height from the ground, and the trend was consistent in all of the zones. Correlation analysis revealed significant positive relations between foliage abundance and species richness, abundance and diversity of birds. Results of this study have highlighted the significance of under-storey or sub-canopy vegetation in maintaining and conserving avifaunal diversity in the Eastern Himalaya.