3.1 Ecology niche analysis in environmental space

There were no novel components in the environment context comparison between study sites as shown by MOP, MESS and ExDet (see Figure S2.1-S2.2) indicated that the results of our subsequent niche analysis were reasonably comparable under similar conditions.
It can be seen in Niche A that the observed niche of two new distribution populations both overlapped with the native area, but some of the new northern area exceeded the native edge (Fig.1a). Analyzing the spatial relationships between niches (Fig.2a), we found only a small minority of low latitudes in China were analogous with Myanmar’s sedentary populations and India’s wintering populations in niche occupation (Fig.2b), and most of the population had no reference data because it was beyond the native niche (Fig.2d). In contrast, the niche occupation of the Malay Peninsula’s populations completely overlapped with Sri Lanka’s sedentary populations (Fig.2c).
The observed Malay Peninsula population niche was nearly totally within the original native range (Stability=99.42%), while the new northern population only partially overlapped with the native niche (Stability=22.39%). The results of Schoener’s D quantification showed that compared with the original native area, niche divergence of the new northern population was significantly higher than expected (P < 0.05), representing significant niche expansion. The niche equivalence and conservatism of the new southern population were significant (P < 0.05). In addition, the present population niches were still similar to the native area, and the expansion of the new northern population only accounted for 2.32% of the whole population (Fig.3) (Table 1). Nevertheless, when the three observed niches were compared with the original native niche, there was an incomplete niche match in overlap D value (the maximum value was 0.59) (Table 1), indicating that in recent dispersal years, the Asian openbill niche changed or resulted in an unbalanced regional distribution, especially for individuals in China.
A comparison of response range to occurrence within each environment variable between native and new northern populations revealed that five precipitation-related factors did not change, three of five temperature factors showed that the new northern population had a lower temperature tolerance than the native population, and the other two factors (Mean Temperature of Wettest Quarter and Mean Temperature of Warmest Quarter) were included in the native range. These results suggest that the climate conditions of the population in southwest China were similar to the native population during the high precipitation and temperature season, while temperatures in China sites were colder than native range’s in the dry winter season (Fig.4).