Modelling Procedure
To build the model, the authors referred to the protocols suggested by
various studies (Bradie & Leung,2017; Halvorsen 2013; Kaky et al.,2020;
Zhang et al., 2019), and selected several basic environmental factors,
including terrain, vegetation, human disturbance variables, and climate
data (Table 1), which are widely available for many locations around the
world. MaxENT was adopted to develop the models (Elith et al., 2011;
Phillips & Dudík, 2008), as it is a widely applied method to predict
species distributions and proven to be reliable for numerous taxa (Kumar
& Stohlgren, 2009; Onojeghuo et al., 2015; Khadka et al., 2017; Thapa
et al., 2018). Before running MaxENT, all relevant accessible factors in
the dataset were given full consideration and transformed into raster
format (90m × 90m), using UTM coordination system. A total 19 climate
variables, 2 terrain variables (elevation, slope), vegetation (forest),
2 disturbance variables (distance to towns, density of main roads) were
included in the analysis. A jack-knife method based on 10 repetitions
was used to evaluate model performance, using the AUC value to evaluate
the performance of the models. Finally, we adopted the average results
of the 10 repetitions. The threshold value to divide the suitability
into suitable habitat or unsuitable habitat was based on the Maximum
Youden index (Fluss et al., 2005).
To improve the accuracy of takin distribution map, a suitable habitat
distribution map based on the final model was created. The raw takin
habitat map was created after converting the resulting raster file into
a shape file, which was overlayed with 5 kmx5km polygons with the
resulting habitat suitability value. All known takin occurrence
locations were used to generate a raster map based the euclidean
distances to the locations and divided the raster value into 5 classes
using 10km intervals. Based on the takin distribution map and euclidean
distance map, polygons at the edge of suitable habitat were identified.
The edge was selected where habitat present or distance to nearest
location less than 20km, producing a species distribution polygon with
jagged edges. To smooth the shape and cover as much potential habitat in
the distribution range, the edge angles were modified to those less than
90 degrees or equal to 90 degrees by connecting consecutive corners with
a straight line, making all angles larger than 90 degrees. After
smoothing, this map was overlapped with CDR of takin from the IUCN and
considered the overlapped area as the updated distribution range (UDR).