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).