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.