4.1 Tolerance niche expansion during
dispersal
The occurrence data used in this study were composed of breeding and
non-breeding populations, so the niches of several populations were
collectively referred to as the observed niche in our analysis. Compared
with the original native observed niche, the two new distribution sites
had different ecological niche characteristics. The Malay Peninsula had
niche conservatism, but southwest China had niche expansion. As opposed
to common niche expansion, however, the Asian openbill distributed in
China and the Malay Peninsula did not exhibit documented breeding
behavior and it lacked self-sustaining capacity (Han et al., 2016;
Zainul-Abidin et al., 2017). Therefore, the measured results should be
defined as the expansion of the tolerance niche.
Niche breadth has a close relationship with range size (Kambach et al.,
2019), so understanding niche breadth and physiological tolerances can
result in better estimates of range size (Austin et al., 2009; Bush et
al., 2018). The results of our measurements indicated that Asian
openbill in China had a stronger tolerance to low temperatures. Indeed,
Field surveys have found that some individuals in China can stand the
temperature below zero (Han et al., 2016; Lei et al., 2017).
Furthermore, native species ranges underestimate climate tolerance
(Bocsi et al., 2016), especially that endotherms able to endure a wide
range of environmental conditions (Strubbe et al., 2015). Hence abiotic
factors were not the direct cause of the dispersal of Asian openbill.
Despite the obvious niche expansion, the observed niche of the whole
population was still over a relatively short period, and niche
conservatism cannot be rejected. However, this does not mean that the
distribution of Asian openbill was in an equilibrium situation. At least
in the tolerance niche, it is not known whether the bird has completely
occupied the non-breeding space and presumably will continue to expand.