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The relationship between species elevational range size of breeding birds, temperature range, primary productivity, habitat heterogeneity, and species richness: an empirical test of related environmental hypotheses and rescue effect in the Eastern Himalayas

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.