Non-directional rescue effect
The rescue effect emphasizes that the proximity of the range margins of potentially interacting species can impact species richness (Stevens, 1992). Our results detected a positive correlation between inflow intensity and species richness along the elevational gradient (Figure 4a), which indicated that the richness “hump” in the middle elevation of Lebu Valley should be “rescued” by the species from the lowland and highland simultaneously (non-directional rescue effect). Furthermore, the inflow intensity in the low elevations was higher than that in the high elevations, which indicated that the left-skewed species richness hump towards lower elevation in the Lebu Valley could be explained by the stronger inflow intensity in the low elevations.
Studies have shown that the hump-shaped pattern is one of the most common (about 50%) elevational species richness patterns (Rahbek, 1995). Particularly, most of the recent studies on terrestrial vertebrates in the Himalayas have discovered a consistent hump-shaped elevational richness pattern across taxa (e.g., Acharya, Sanders, Vijayan, & Chettri, 2011; Ding et al., 2019; Hu et al., 2017; Pan et al., 2016; Joshi & Bhatt, 2015). However, studies are often fascinated by the relationship between environments (e.g., climate and productivity) and biodiversity, and few studies have explored another form of explanation for species richness pattern, such as species range size and the interaction between species richness and species range size. To better understanding the relationship between species richness and species range size, the rescue effect should be tested in more regions.