title: Elevational gradients in bird diversity in the Eastern Himalaya: an evaluation of distribution patterns and their underlying mechanisms.
abstract: 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.
  
  
  RESULTS
  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 world's 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 Rapoport's rule for the birds of Sikkim region of the Himalaya.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Division of Conservation Ecology, Sàlim Ali Centre for Ornithology and Natural History, Anaikatty, Coimbatore, India. acharya2skm@gmail.com
authors.forename: Bhoj Kumar
authors.initials: BK
authors.lastname: Acharya
authors.forename: Nathan J
authors.initials: NJ
authors.lastname: Sanders
authors.forename: Lalitha
authors.initials: L
authors.lastname: Vijayan
authors.forename: Basundhara
authors.initials: B
authors.lastname: Chettri
mesh_terms: D000531:Altitude; D000818:Animals; D044822:Biodiversity; D001717:Birds / Q000502:physiology*; D005843:Geography; D016014:Linear Models; D008954:Models, Biological; D012819:Sikkim; D013045:Species Specificity