4.2 The role of new distribution areas in species evolution

A more reasonable explanation for the assumption of niche conservatism is that over a short-to-moderate time span and at lower taxonomic ranks (genus and species), niche conservatism dominates, but stability will decline over a longer time span and at higher taxonomic ranks (Peterson et al., 2011). According to the present niche occupation characteristics of various populations, it is important to discuss the future evolutionary trend of the Asian openbill.
Niche occupancy in the small area in southern China was similar to northern India, northeastern Myanmar, and northern Laos. The Malay Peninsula population niche was similar to that in southern Thailand and Sri Lanka. Although there were few recorded sites that we could reference in these similar native regions (Fig.2), many records of Asian openbill distributed around these locations. Given the potential sampling intensity bias and species dispersal capabilities (Peterson & Raghavan, 2017), in fact, there probably have more occurrences in these native regions. Although the Asian openbill distribution map may have been inaccurate due to rapid spread and data incompleteness, as a reference we could properly infer the residential types of the species (Elliott et al., 2020). The analysis found that similar areas in India and Laos were non-breeding areas of Asian openbill, and Myanmar and Sri Lanka were breeding areas. This meant that under different combinations of temperature and precipitation, the partial niche of China, which overlapped with the native niche, included some breeding and non-breeding sites with the possibility of breeding sites in southwest China Xishuangbanna and western Guangxi. The Malay Peninsula niche was similar to Sri Lanka according to breeding conditions. The expanded tolerance niche had no reference population residential information (Fig. 2).
The Asian openbill exhibited different niche changes in different directions, but they may play similar roles in the evolution of the entire population. The populations in the new distribution regions had peaks and troughs throughout the year but no breeding behavior was found. We believe that this phenomenon conformed to the source-sink theory (Dias, 1996; Pulliam, 1988), where new distribution regions serve as a sink habitat. Although the suitability or occurrence probability of these sink habitats was low, it was still higher than the expected value based on a random distribution. Therefore, the source of the new population should to be determined by further studies on bird migration. Fortunately, the population in China has a ring record that indicates that at least part of it is from central Thailand (Yang et al., 2019). The breeding time of the population in Thailand is generally around January in the local dry season (Elliott et al., 2020), and the non-breeding period is migration or dispersal, which is consistent with the characteristics of the population in China with more in summer and less in winter. Therefore, it is speculated that the spread of the population into a new area over a short time may be more dependent on the dynamic changes of the source population.
According to the continuous records of the past decade and the comparative analysis of the several population probabilities and random expectations, we believe that dispersal in two directions has begun, but the establishment success of the species in a new distribution area depends on propagule pressure (Lockwood et al., 2005), conspecific density (Taylor & Hastings, 2005) and species life-history traits (Blackburn et al., 2009). Historically, there have been many birds that have spread to low and high latitudes at the same time and successfully established colonies, such as the cattle egret (Lovette & Fitzpatrick, 2016), the ring-necked parakeet (Strubbe et al., 2015), the Egyptian goose and the Common waxbill (Blackburn et al., 2009). Even if the Asian openbill have not yet colonized in new distribution areas, combined with the low temperature tolerance, similar environmental requirements, and established migration corridors and niche expansion potential, a new breeding population with low temperature tolerance may appear in China. However, the southern dispersal populations have a higher reproductive potential and could form the Malay Peninsula or even Australian population.