2.1 Occurrence data
The occurrence data on the Asian openbill consists of internet sources
and field surveys. Data were downloaded from the Global Biodiversity
Information Facility (GBIF,www.gbif.org), which contained 26905
original distribution records from 1970 to 2018. Field data consisted of
43 records obtained from our surveys in southwest China in 2016 and
2017. Firstly, 26948 records were obtained by combining the above two
datasets to make up the present data on Asian openbill. Then, based on
the initial dispersal event time, we extracted data before 2006 as the
original native data. Finally, according to the current distribution
pattern, the Chinese data were extracted from the present data as the
new northern population, and data from Malaysia and Singapore
represented the new southern population of the Malay Peninsula.
Without sampling bias is a critical assumptions for ENM (Peterson et
al., 2011), but the occurrence data tend to be affected by spatial
sampling bias (Bystriakova et al., 2012). Hence, we removed obvious
erroneous data, such as duplicates, poor precision records, and
conspicuously inaccurate data. Considering the relative independence of
the data and the activity range of the species at a small scale, only
one data point was kept for each 10km. The above steps were implemented
using the Wallace package in R v3.5.1 (Kass et al., 2018), and the
present dataset was finally reduced to 2,181 records.
We calculated Asian openbill distribution density in 12 recorded
countries. If the distribution data were concentrated in a limited
range, such as China, we excluded the other absence areas. Then we
selected Bangladesh as the reference density (3.654 records
10-8 km -2) in combination with the
actual situations. The density values for countries higher than the
reference value were randomly deleted to reduce density; otherwise all
country data was retained. All processes were carried out using QGIS,
version 2.18.20. Finally, we obtained balanced data as the present data,
including 264 original native records, 41 new northern records, and 10
new southern records (8 records in Malaysia and 2 records in Singapore)
(Fig.1).