4.3 The potential distribution and dispersal trend of Asian
openbill
The model predicted that the potential distribution of the Asian
openbill has not increased significantly in the new distribution area.
The newly added suitable areas in China included the south of Sichuan
province in the southwest, and the coastal areas of Taiwan island,
Hainan island and Leizhou Peninsula in the south. It is worth mentioning
that the occurrence points, which were filtered during the previous data
processing, such as records from southern Sichuan and Leizhou Peninsula,
were also included in the prediction results. Outside the present
distribution area, the suitable area increased significantly in the
low-latitude islands, such as Kalimantan, New Guinea, and northern
Australia. In fact, in recent years, the number of Asian openbills that
that spread outwards were quite erratic, especially the Malay Peninsula
population, which declined (Zainul-Abidin et al., 2017). Even if the
model predicted that there were potential suitable climatic conditions,
it was still difficult to determine whether the storks would continue to
spread. Analyzing the reasons for native population dispersal can help
us better understand species dispersal trends.
Generally, the factors that promote species invasion are density of the
native population and reproduction pressure (Lockwood et al., 2005), the
shortage of food resources (Davis et al., 2000), the lack of natural
enemies in the invaded area (Keane & Crawley, 2002), or a species with
high environmental adaptability and physiological tolerance (Blackburn
et al., 2009; Marchetti et al., 2004). Previous research has suggested
that Asian openbill residential situation were most influenced by
precipitation factors. Low et al. (2013) speculated that the spread of
Asian openbill into the Malay Peninsula was probably caused by a large
drought in Thailand. In addition, in Thailand and the Raiganj Reserve of
India, water levels also affect food abundance and are directly
proportional to the number of Asian openbill observed (Jinnath et al.,
2016; Sawangproh et al., 2012; Sharma, 2007). In terms of food, the
large-scale invasion of the golden apple snail (Pomacea
canaliculata ) in southeast Asia led to the rapid growth of the Asian
openbill population (Zainul-Abidin et al., 2017). As one of the major
invasive species in China (Lv et al., 2009), the golden apple snail is
also one of attracting factors for the Asian openbill (Han et al.,
2016). In terms of population density, Asian openbill demographics in
India, Cambodia, Thailand have shown that native colonies have increased
rapidly in the past decade (Han et al., 2016; Roy & Sah, 2013; Willcox
et al., 2016). The above reasons provide an external driving force for
Asian openbill dispersal.
On the other hand, the Asian openbill is able to migrate long distances
for forage and environment adaptation (Blanford, 1898). It has been
documented that the Thailand population migrates north to central and
northern Thailand, northeast India, and Bangladesh during the
non-breeding season (McClure, 1998); In other words, one source
population in Thailand has had a life history trait of migrating
northward. Considering that the Chinese population increased from one to
thousands in recent years, the intra-species composition is
predominantly subadult and juvenile with irregular population dynamics
(Han et al., 2016; Liu et al., 2015), so the Asian openbill is
considered to be a vagrant bird in China (Zheng, 2017). In general, the
migration of juvenile and subadult birds is variable and unstable, and
their fidelity to wintering and breeding grounds is lower than the adult
birds (Bentzen & Powell, 2015). Perhaps “over-migration” occurred in
Thailand when they initially migrated north, bringing a few migrants to
locations out of their normal migratory range as vagrants (Ralph &
Wolfe, 2018), and arriving in southwest China. Being far from the
fundamental niche centroid accelerated the dispersal rate (Ingenloff et
al., 2017), resulting in a wide distribution range that was extremely
unstable in the early stage of species dispersal, such as occasional
records in Shangri-La, Leizhou Peninsula, and Poyang lake in China. Then
a relatively stable distribution pattern was formed after several years
of dispersal.
In the Malay Peninsula, studies have suggested that food and paddy flood
level may contributed to Asian openbill southward dispersal
(Zainul-Abidin et al., 2017). At present, there is still a lack of
information on the migration ecology and continuous population dynamic
monitoring of Asian openbill in the Malay Peninsula. In summary, an
understanding of the model results needs to be considered with actual
information. We believe that the behavior of Asian openbill dispersal to
China during the non-breeding season become stable and the range size is
likely to continue expand to the north or coastal areas. More
fundamental data are needed to support whether the more suitable
low-latitude islands predicted by the model have sufficient natural
dispersal potential.