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.