Describing the pattern and variations in spatial pattern of biodiversity and revealing its underlying mechanisms remain a central focus in ecology. However, less attention was paid to the species range size, and few studies have explored the drivers of species range size and the relationship between species range size and species richness (rescue effect). Here, we use a comprehensive dataset of breeding birds collected from 2018 to 2019 along the elevational gradient in Lebu Valley, Eastern Himalayas of China to explore the a) species mean elevational range size pattern, b) drivers influencing species mean elevational range size, and c) rescue effect. We found that species mean elevational range size of birds in Lebu Valley was a hump-shaped pattern (species mean elevational range size was largest at middle elevations), and the annual temperature range and normalized vegetation index were the most important explanatory variables for the species mean elevational range size pattern. However, species mean elevational range size was negatively correlated with the annual temperature range and positively correlated with the normalized vegetation index, respectively. These results were contrary to the predictions of the climate variability hypothesis and the ambient energy hypothesis. In addition, the correlation between species mean elevational range size and habitat heterogeneity was weak, which indicated that the habitat heterogeneity hypothesis also failed to predict the breeding bird mean elevational range size pattern in Lebu Valley. Moreover, we found the hump-shaped species richness pattern, which could also be resulted from non-directional rescue effect. Given the uncertainty in mean elevational range size pattern and the fact that much of the previous research has rarely tested the relationship between species range size and richness patterns, hypotheses explaining the elevational range size and the underlying mechanisms should be tested in more studies of different taxa and regions.

How and why species range size varies along spatial gradients is fundamental yet controversial topics in biogeography. To advance our understanding on these questions and to provide insight into biological conservation, we assessed the elevational variations in vascular plants range size for different life form and biogeographical affinities, and explored the main drivers underlying above variations in the longest valley in China's Himalayas---the Gyirong Valley. Elevational range sizes of vascular plants were documented by 96 sampling plots along 12 elevational bands of 300-m ranging from 1800 to 5400 m above sea level. We assessed the elevational variations in range size by averaging the range size of all species within each elevational band. We then related range size to climate, disturbance, competition factors and the mid-domain effect, and explored the relative importance of aforementioned factors in explaining the range size variations using the Random Forest model. Total 545 vascular plants were documented by our sampling plots along the elevational gradient. Out of 545 plants, 158, 387, 337 and 112 were woody, herbaceous, temperate and tropical species respectively. Range size of each groups of vascular plants shown uniform increasing trends along the elevational gradient which are in accordance with the prediction of Rapoport's rule. Climate was the main driver for the increasing trends of vascular plants range size in the Gyirong Valley. Climate variability hypothesis and mean climate condition hypothesis were both supported to jointly explain such climate-range size relationship. Our results reinforce previous notion that Rapoport's rule applies to where the influence of climate is most pronounced, and call for close attention to the impact of climate change in order to prevent range contraction and even extinction under global warming.