Determinants of Northerly Range Limits along the Himalayan Bird Diversity Gradient

Abstract

The primary explanation for the latitudinal gradient in species diversity must lie in why species fail to expand ranges across different climatic regimes. Theories of species gradients based in niche conservatism assume that whole clades are confined to particular climatic regimes because the traits they share limit adaptation to alternative regimes. We assess these theories in an analysis of the twofold decline in bird species richness along the Himalayas from the southeast to the northwest. The presence of fewer species in the northwest is entirely due to a steep decline in the number of forest species; species occupying more open habitats show a reversed gradient. Forest species numbers are exceptionally high at midelevations (1,000-2,000 m) in the southeast, which experience a warm, wet climate not present in the northwest, and a high proportion of these species fail to expand their range to the northwest. Despite this, many species do have populations or close relatives that straddle different climatic regimes along altitudinal gradients and/or the regional gradient, implying that climate-based niche conservatism per se does not strongly constrain range limits. We argue that climate- and competition-mediated resource distributions are important in setting northerly range limits and show that one measure of forest resources (foliage density) is lower in the northwest.

Out of the tropics, but how? Fossils, bridge species, and thermal ranges in the dynamics of the marine latitudinal diversity gradient

“…Bridge species are remarkably pervasive in marine bivalves and are also known in terrestrial clades (61,62); their evolutionary and biogeographic role deserves investigation. However, caution is needed when inferring process from present day species distributions, as species have almost certainly been repositioned by repeated, extensive reshuffling in response to Pleistocene climate fluctuations in both marine and terrestrial settings (63,64).…”

Comparative Phyloclimatic Analysis and Evolution of Ecological Niches in the Scimitar Babblers (Aves: Timaliidae: Pomatorhinus)

“…This indicates significant distinctions in habitat between the western and eastern Himalayas, perhaps explaining why species numbers in these two areas are so different (see [27]). Interestingly, there appears to be less ecological differentiation between species from the Eastern Himalayas and adjacent highlands such as Yunnan and Assam/Burma (e.g.…”

Evolutionary history of passerine birds (Aves: Passeriformes) from the Qinghai–Tibetan plateau: from a pre-Quarternary perspective to an integrative biodiversity assessment

“…Species richness patterns of Palearctic passerine birds almost perfectly reflect these four centres (Roselaar et al 2007; see review on passerine diversity of the Sino-Himalayas in Martens et al 2011). In the south, where Sino-Himalayan mountain forests constitute the greatest biodiversity hotspot of the region, avian species richness increases along a gradient from the drier Western Himalayas towards highest species richness in the southeast (Price et al 2003(Price et al , 2011358 species of Passeriformes alone were counted in the southeastern Indian Himalayas in the most recent study by Price et al 2014). The same differences of biodiversity indices among regions were found when comparing species richness hotspots of the Qilian and Hengduan mountains with those of the Western Himalayas and Central Asian mountains and the northern Chinese plains ; data for threatened birds only!).…”